THE HUMAN BODY IS ARGUABLY THE MOST AMAZING ORGANISM ON THE PLANET. IT IS COMPLEX AND AN ALMOST PERFECT COLLECTION OF THE CULMINATION OF HUNDREDS OF MILLIONS OF YEARS OF EVOLUTION THROUGHOUT THE LIFE OF OUR PLANET EARTH. OUR BODIES MAY BE VERY SIMILAR, HOWEVER WE ARE ALL UNIQUE, ALMOST SEVEN BILLION OF US AND YET NO TWO FINGERPRINTS ARE THE SAME, NO TWO IRISES, NO TWO VOICES. THIS MAKE EACH OF US VERY SPECIAL, AND THE MORE WE UNDERSTAND ABOUT OURSELVES THE BETTER WE ARE TO HAVE A HAPPY AND HEALTHY LIFE.
CONTENT OF BODY SYSTEMS
THE SKELETAL SYSTEM.- THE DIGESTIVE SYSTEM.- THE MUSCULAR SYSTEM._ THE LYMPHATIC SYSTEM.- THE ENDOCRINE SYSTEM.- THE NERVOUS SYSTEM.- THE CARDIOVASCULAR SYSTEM.- THE MALE REPRODUCTIVE SYSTEM.- THE FEMALE REPRODUCTIVE SYSTEM.- THE URINARY SYSTEM.
The Skeletal System
The Skeletal System serves many important functions; it provides the shape and form for our bodies in addition to supporting, protecting, allowing bodily movement, producing blood for the body, and storing minerals.
Its 206 bones form a rigid framework to which the softer tissues and organs of the body are attached.
Vital organs are protected by the skeletal system. The brain is protected by the surrounding skull as the heart and lungs are encased by the sternum and rib cage.
Bodily movement is carried out by the interaction of the muscular and skeletal systems. For this reason, they are often grouped together as the musculo-skeletal system. Muscles are connected to bones by tendons. Bones are connected to each other by ligaments. Where bones meet one another is typically called a joint. Muscles which cause movement of a joint are connected to two different bones and contract to pull them together. An example would be the contraction of the biceps and a relaxation of the triceps. This produces a bend at the elbow. The contraction of the triceps and relaxation of the biceps produces the effect of straightening the arm.
Blood cells are produced by the marrow located in some bones. An average of 2.6 million red blood cells are produced each second by the bone marrow to replace those worn out and destroyed by the liver.
Bones serve as a storage area for minerals such as calcium and phosphorus. When an excess is present in the blood, buildup will occur within the bones. When the supply of these minerals within the blood is low, it will be withdrawn from the bones to replenish the supply.
Divisions of the Skeleton
The human skeleton is divided into two distinct parts:
The axial skeleton consists of bones that form the axis of the body and support and protect the organs of the head, neck, and trunk.
The appendicular skeleton is composed of bones that anchor the appendages to the axial skeleton.
Types of Bone
The bones of the body fall into four general categories: long bones, short bones, flat bones, and irregular bones. Long bones are longer than they are wide and work as levers. The bones of the upper and lower extremities (ex. humerus, tibia, femur, ulna, metacarpals, etc.) are of this type. Short bones are short, cube-shaped, and found in the wrists and ankles. Flat bones have broad surfaces for protection of organs and attachment of muscles (ex. ribs, cranial bones, bones of shoulder girdle). Irregular bones are all others that do not fall into the previous categories. They have varied shapes, sizes, and surfaces features and include the bones of the vertebrae and a few in the skull.
Bones are composed of tissue that may take one of two forms. Compact, or dense bone, and spongy, or cancellous, bone. Most bones contain both types. Compact bone is dense, hard, and forms the protective exterior portion of all bones. Spongy bone is inside the compact bone and is very porous (full of tiny holes). Spongy bone occurs in most bones. The bone tissue is composed of several types of bone cells embedded in a web of inorganic salts (mostly calcium and phosphorus) to give the bone strength, and collagenous fibers and ground substance to give the bone flexibility
On this page:
The digestive system is made up of the digestive tract—a series of hollow organs joined in a long, twisting tube from the mouth to the anus—and other organs that help the body break down and absorb food (see figure).
Organs that make up the digestive tract are the mouth, esophagus, stomach, small intestine, large intestine—also called the colon—rectum, and anus. Inside these hollow organs is a lining called the mucosa. In the mouth, stomach, and small intestine, the mucosa contains tiny glands that produce juices to help digest food. The digestive tract also contains a layer of smooth muscle that helps break down food and move it along the tract.
Two “solid” digestive organs, the liver and the pancreas, produce digestive juices that reach the intestine through small tubes called ducts. The gallbladder stores the liver’s digestive juices until they are needed in the intestine. Parts of the nervous and circulatory systems also play major roles in the digestive system.
When you eat foods—such as bread, meat, and vegetables—they are not in a form that the body can use as nourishment. Food and drink must be changed into smaller molecules of nutrients before they can be absorbed into the blood and carried to cells throughout the body. Digestion is the process by which food and drink are broken down into their smallest parts so the body can use them to build and nourish cells and to provide energy.
Digestion involves mixing food with digestive juices, moving it through the digestive tract, and breaking down large molecules of food into smaller molecules. Digestion begins in the mouth, when you chew and swallow, and is completed in the small intestine.
The large, hollow organs of the digestive tract contain a layer of muscle that enables their walls to move. The movement of organ walls can propel food and liquid through the system and also can mix the contents within each organ. Food moves from one organ to the next through muscle action called peristalsis. Peristalsis looks like an ocean wave traveling through the muscle. The muscle of the organ contracts to create a narrowing and then propels the narrowed portion slowly down the length of the organ. These waves of narrowing push the food and fluid in front of them through each hollow organ.
The first major muscle movement occurs when food or liquid is swallowed. Although you are able to start swallowing by choice, once the swallow begins, it becomes involuntary and proceeds under the control of the nerves.
Swallowed food is pushed into the esophagus, which connects the throat above with the stomach below. At the junction of the esophagus and stomach, there is a ringlike muscle, called the lower esophageal sphincter, closing the passage between the two organs. As food approaches the closed sphincter, the sphincter relaxes and allows the food to pass through to the stomach.
The stomach has three mechanical tasks. First, it stores the swallowed food and liquid. To do this, the muscle of the upper part of the stomach relaxes to accept large volumes of swallowed material. The second job is to mix up the food, liquid, and digestive juice produced by the stomach. The lower part of the stomach mixes these materials by its muscle action. The third task of the stomach is to empty its contents slowly into the small intestine.
Several factors affect emptying of the stomach, including the kind of food and the degree of muscle action of the emptying stomach and the small intestine. Carbohydrates, for example, spend the least amount of time in the stomach, while protein stays in the stomach longer, and fats the longest. As the food dissolves into the juices from the pancreas, liver, and intestine, the contents of the intestine are mixed and pushed forward to allow further digestion.
Finally, the digested nutrients are absorbed through the intestinal walls and transported throughout the body. The waste products of this process include undigested parts of the food, known as fiber, and older cells that have been shed from the mucosa. These materials are pushed into the colon, where they remain until the feces are expelled by a bowel movement.
The digestive glands that act first are in the mouth—the salivary glands. Saliva produced by these glands contains an enzyme that begins to digest the starch from food into smaller molecules. An enzyme is a substance that speeds up chemical reactions in the body.
The next set of digestive glands is in the stomach lining. They produce stomach acid and an enzyme that digests protein. A thick mucus layer coats the mucosa and helps keep the acidic digestive juice from dissolving the tissue of the stomach itself. In most people, the stomach mucosa is able to resist the juice, although food and other tissues of the body cannot.
After the stomach empties the food and juice mixture into the small intestine, the juices of two other digestive organs mix with the food. One of these organs, the pancreas, produces a juice that contains a wide array of enzymes to break down the carbohydrate, fat, and protein in food. Other enzymes that are active in the process come from glands in the wall of the intestine.
The second organ, the liver, produces yet another digestive juice—bile. Bile is stored between meals in the gallbladder. At mealtime, it is squeezed out of the gallbladder, through the bile ducts, and into the intestine to mix with the fat in food. The bile acids dissolve fat into the watery contents of the intestine, much like detergents that dissolve grease from a frying pan. After fat is dissolved, it is digested by enzymes from the pancreas and the lining of the intestine.
Most digested molecules of food, as well as water and minerals, are absorbed through the small intestine. The mucosa of the small intestine contains many folds that are covered with tiny fingerlike projections called villi. In turn, the villi are covered with microscopic projections called microvilli. These structures create a vast surface area through which nutrients can be absorbed. Specialized cells allow absorbed materials to cross the mucosa into the blood, where they are carried off in the bloodstream to other parts of the body for storage or further chemical change. This part of the process varies with different types of nutrients.
Carbohydrates. The Dietary Guidelines for Americans 2005 recommend that 45 to 65 percent of total daily calories be from carbohydrates. Foods rich in carbohydrates include bread, potatoes, dried peas and beans, rice, pasta, fruits, and vegetables. Many of these foods contain both starch and fiber.
The digestible carbohydrates—starch and sugar—are broken into simpler molecules by enzymes in the saliva, in juice produced by the pancreas, and in the lining of the small intestine. Starch is digested in two steps. First, an enzyme in the saliva and pancreatic juice breaks the starch into molecules called maltose. Then an enzyme in the lining of the small intestine splits the maltose into glucose molecules that can be absorbed into the blood. Glucose is carried through the bloodstream to the liver, where it is stored or used to provide energy for the work of the body.
Sugars are digested in one step. An enzyme in the lining of the small intestine digests sucrose, also known as table sugar, into glucose and fructose, which are absorbed through the intestine into the blood. Milk contains another type of sugar, lactose, which is changed into absorbable molecules by another enzyme in the intestinal lining.
Fiber is undigestible and moves through the digestive tract without being broken down by enzymes. Many foods contain both soluble and insoluble fiber. Soluble fiber dissolves easily in water and takes on a soft, gel-like texture in the intestines. Insoluble fiber, on the other hand, passes essentially unchanged through the intestines.
Protein. Foods such as meat, eggs, and beans consist of giant molecules of protein that must be digested by enzymes before they can be used to build and repair body tissues. An enzyme in the juice of the stomach starts the digestion of swallowed protein. Then in the small intestine, several enzymes from the pancreatic juice and the lining of the intestine complete the breakdown of huge protein molecules into small molecules called amino acids. These small molecules can be absorbed through the small intestine into the blood and then be carried to all parts of the body to build the walls and other parts of cells.
Fats. Fat molecules are a rich source of energy for the body. The first step in digestion of a fat such as butter is to dissolve it into the watery content of the intestine. The bile acids produced by the liver dissolve fat into tiny droplets and allow pancreatic and intestinal enzymes to break the large fat molecules into smaller ones. Some of these small molecules are fatty acids and cholesterol. The bile acids combine with the fatty acids and cholesterol and help these molecules move into the cells of the mucosa. In these cells the small molecules are formed back into large ones, most of which pass into vessels called lymphatics near the intestine. These small vessels carry the reformed fat to the veins of the chest, and the blood carries the fat to storage depots in different parts of the body.
Vitamins. Another vital part of food that is absorbed through the small intestine are vitamins. The two types of vitamins are classified by the fluid in which they can be dissolved: water-soluble vitamins (all the B vitamins and vitamin C) and fat-soluble vitamins (vitamins A, D, E, and K). Fat-soluble vitamins are stored in the liver and fatty tissue of the body, whereas water-soluble vitamins are not easily stored and excess amounts are flushed out in the urine.
Water and salt. Most of the material absorbed through the small intestine is water in which salt is dissolved. The salt and water come from the food and liquid you swallow and the juices secreted by the many digestive glands.
How is the digestive process controlled?
The major hormones that control the functions of the digestive system are produced and released by cells in the mucosa of the stomach and small intestine. These hormones are released into the blood of the digestive tract, travel back to the heart and through the arteries, and return to the digestive system where they stimulate digestive juices and cause organ movement.
The main hormones that control digestion are gastrin, secretin, and cholecystokinin (CCK):
Additional hormones in the digestive system regulate appetite:
Both of these hormones work on the brain to help regulate the intake of food for energy. Researchers are studying other hormones that may play a part in inhibiting appetite, including glucagon-like peptide-1 (GPL-1), oxyntomodulin (+ ), and pancreatic polypeptide.
Two types of nerves help control the action of the digestive system.
Extrinsic, or outside, nerves come to the digestive organs from the brain or the spinal cord. They release two chemicals, acetylcholine and adrenaline. Acetylcholine causes the muscle layer of the digestive organs to squeeze with more force and increase the “push” of food and juice through the digestive tract. It also causes the stomach and pancreas to produce more digestive juice. Adrenaline has the opposite effect. It relaxes the muscle of the stomach and intestine and decreases the flow of blood to these organs, slowing or stopping digestion.
The intrinsic, or inside, nerves make up a very dense network embedded in the walls of the esophagus, stomach, small intestine, and colon. The intrinsic nerves are triggered to act when the walls of the hollow organs are stretched by food. They release many different substances that speed up or delay the movement of food and the production of juices by the digestive organs.
Together, nerves, hormones, the blood, and the organs of the digestive system conduct the complex tasks of digesting and absorbing nutrients from the foods and liquids you consume each day.
Over 600 skeletal muscles function for body movement through contraction and relaxation of voluntary, striated muscle fibers. These muscles are attached to bones, and are typically under conscious control for locomotion, facial expressions, posture, and other body movements. Muscles account for approximately 40 percent of body weight. The metabolism that occurs in this large mass-produces heat essential for the maintenance of body temperature.
Cardiac muscle is only in the heart and makes up the atria and ventricles (heart walls). Like skeletal muscle, cardiac muscle contains striated fibers. Cardiac muscle is called involuntary muscle because conscious thought does not control its contractions. Specialized cardiac muscle cells maintain a consistent heart rate.
Smooth muscle is throughout the body, including in visceral (internal) organs, blood vessels, and glands. Like cardiac muscle, smooth muscle is involuntary. Unlike skeletal and cardiac muscle, smooth muscle is nonstriated (not banded). Smooth muscle, which is extensively within the walls of digestive tract organs, causes peristalsis (wave-like contractions) that aids in food digestion and transport.
Except the heart, any action that the body performs without conscious thought is done by smooth muscle contractions. This includes diverse activities such as constricting (closing) the bronchioles (air passages) of the lungs or pupils of the eye or causing goosebumps in cold conditions.
A skeletal muscle has regular, ordered groups of fascicles, muscle fibers, myofibrils, and myofilaments. Epimysium (thick connective tissue) binds groups of fascicles together. A fascicle has muscle fibers; perimysium (connective tissue) envelops the fascicle. Endomysium (connective tissue) surrounds the muscle fibers.
A muscle fiber divides into even smaller parts. Within each fiber are strands of myofibrils. These long cylindrical structures appear striped due to strands of tiny myofilaments. Myofilaments have two types of protein: actin (thin myofilaments) and myosin (thick myofilaments).
The actin and myosin myofilaments align evenly, producing dark and light bands on the myofibril. Each dark band depicts an area where the myofilaments overlap, causing the striated appearance of skeletal muscle.
All dark and light bands of the myofilaments have names. At the Z-line, actin strands interweave. The region between two Z-lines is a sarcomere, the functional unit of skeletal muscle. Muscle contraction occurs when overlapping actin and myosin myofilaments overlap further and shorten the muscle cell. The myofilaments keep their length. The overlapping of myofilaments is the basis for the sliding filament theory of contraction.
Skeletal muscle is a system of pairs that relax and contract to move a joint. For example, when front leg muscles contract, the knee extends (straightens) while back leg muscles relax. Conversely, to flex (bend) the knee, back leg muscles contract while front leg muscles relax. Some muscles are named for their ability to extend or flex a joint; for example, extensor carpiradialis longus muscle and flexor digitorum brevis muscle.
Tendons attach most skeletal muscles to bones. Tendons are strong sheets of connective tissue that are identical with ligaments. Tendons and ligaments differ in function only: tendons attach muscle to bone and ligaments attach bone to bone. Physical exercise strengthens the attachment of tendons to bones.
Skeletal muscles have muscle tone (remain partly contracted), which helps maintain body posture. Ongoing signals from the nervous system to the muscle cells help maintain tone and readiness for physical activity.
Skeletal muscle aids in heat generation. During muscle contractions, muscle cells expend much energy, most of which is converted to heat. To prevent overheating, glands in the skin produce sweat to cool the skin; and, the body radiates heat from the blood and tissues through the skin. When the body is chilly, shivering causes quick muscle contractions that generate heat.
Skeletal muscles have two types of muscle fibers: fast-twitch and slow-twitch. Anaerobic exercise uses fast-twitch fibers. Such exercise includes activities that are fleeting and require brief high-energy expenditure. Weightlifting, sprinting, and push-ups are examples of anaerobic exercise. Because all cells require oxygen to produce energy, anaerobic exercise depletes oxygen reserves in the muscle cells quickly. The result is an oxygen debt. To repay the debt, humans breathe deeply and rapidly, which restores the oxygen level. Anaerobic exercise creates excess lactic acid (a waste product). By increasing oxygen intake, the liver cells can convert the excess lactic acid into glucose, the primary food molecule used in cellular metabolism.
Aerobic exercise uses slow-twitch muscle fibers. Such exercise includes activities that are prolonged and require constant energy. Long distance running and cycling are examples of aerobic exercise. In aerobic exercise, the muscle cell requires the same amount of oxygen that the body supplies. The oxygen debt is slashed and lactic acid is not formed.
The treatment of lymphedema is based on the structures and functions of the lymphatic system. This article presents basic information about the lymphatic system as it relates to the lymphedema care provided by patients as self-care and by caregivers who are aiding lymphedema patients. A qualified lymphedema therapist must have a more extensive understanding of this body system that is presented here.
Functions of the Lymphatic System
The lymphatic system works in close cooperation with other body systems to perform these important functions:
In lymphedema affected tissues, the lymph is unable to drain properly. Instead within these swollen tissues the protein-rich lymph becomes stagnant. When bacteria enter this fluid through a break in the skin, they thrive on this protein-rich fluid. It is for this reason that lymphedema affected tissues are prone to infections.
Blood capillaries allow fluid to leave,
The Origin of Lymph
Lymph originates as plasma, which is the fluid portion of blood. The arterial blood that flows out of the heart slows as it moves through a capillary bed (see figure above). This slowing allows some plasma to leave the arterioles and flow into the tissues where it becomes tissue fluid.
Blood Flow Compared with Lymphatic Flow
Lymph returning to the
The bloodstream is pumped by the heart. It circulates throughout the body and is cleansed by being filtered by the kidneys. The lymphatic system does not have a pump to aid in its flow, instead this system is designed so that lymph only flows upward through the body traveling from the extremities (feet and hands) and upward through the body toward the neck.
As it travels through the body, lymph passes through lymph nodes where it is filtered. At the base of the neck, the lymph enters the subclavian veins and once again becomes plasma in the bloodstream.
In order to leave the tissues, the lymph must enter the lymphatic system through specialized lymphatic capillaries. Approximately 70 percent of these are superficial capillaries that are located near, or just under, the skin. The remaining 30 percent, which are known as deep lymphatic capillaries, surround most of the body’s organs.
Lymphatic capillaries begin as blind-ended tubes that are only a single cell in thickness. These cells are arranged in a slightly overlapping pattern, much like the shingles on a roof. Each of these individual cells is fastened to nearby tissues by an anchoring filament.
As shown in the animation below, pressure from the fluid surrounding the capillary forces these cells to separate for a moment to allow lymph to enter the capillary. Then the cells of the wall close together. This does not allow the lymph to leave the capillary. Instead it is forced to move forward.
Lymph entering a lymph capillary. (Courtesy of John Ross).
The lymphatic capillaries gradually join together to form a mesh-like network of tubes that are located deeper in the body. As they become larger, these structures are known as lymphatic vessels.
A functioning lymphangion
There are between 600-700 lymph nodes present in the average human body. It is the role of these nodes to filter the lymph before it can be returned to the circulatory system. Although these nodes can increase or decrease in size throughout life, any nodes that has been damaged or destroyed, does not regenerate.
Lymph nodes kill pathogens and cancer cells.
Lymphatic Drainage Areas
Lymphatic drainage is organization into two separate and very unequal drainage areas. These are the right and left drainage areas and normally lymph does not drain across the invisible lines that separate these areas. Structures within each area carry lymph to its destination, which is to return to the circulatory system.
Right Drainage Area
Left Drainage Area
Why This Information is so Important
The nervous system coordinates rapid and precise responses to stimuli using action potentials. The endocrine system maintains homeostasis and long-term control using chemical signals. The endocrine system works in parallel with the nervous system to control growth and maturation along with homeostasis.
The endocrine system is a collection of glands that secrete chemical messages we call hormones. These signals are passed through the blood to arrive at a target organ, which has cells possessing the appropriate receptor. Exocrine glands (not part of the endocrine system) secrete products that are passed outside the body. Sweat glands, salivary glands, and digestive glands are examples of exocrine glands.
The roles of hormones in selecting target cells and delivering the hormonal message. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
Hormones are grouped into three classes based on their structure:
Steroids are lipids derived from cholesterol. Testosterone is the male sex hormone. Estradiol, similar in structure to testosterone, is responsible for many female sex characteristics. Steroid hormones are secreted by the gonads, adrenal cortex, and placenta.
Structure of some steroid hormones and their pathways of formation. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
Peptides are short chains of amino acids; most hormones are peptides. They are secreted by the pituitary, parathyroid, heart, stomach, liver, and kidneys. Amines are derived from the amino acid tyrosine and are secreted from the thyroid and the adrenal medulla. Solubility of the various hormone classes varies.
Steroid hormones are derived from cholesterol by a biochemical reaction series. Defects along this series often lead to hormonal imbalances with serious consequences. Once synthesized, steroid hormones pass into the bloodstream; they are not stored by cells, and the rate of synthesis controls them.
Peptide hormones are synthesized as precursor molecules and processed by the endoplasmic reticulum and Golgi where they are stored in secretory granules. When needed, the granules are dumped into the bloodstream. Different hormones can often be made from the same precursor molecule by cleaving it with a different enzyme.
Amine hormones (notably epinephrine) are stored as granules in the cytoplasm until needed.
Most animals with well-developed nervous and circulatory systems have an endocrine system. Most of the similarities among the endocrine systems of crustaceans, arthropods, and vertebrates are examples of convergent evolution. The vertebrate endocrine system consists of glands (pituitary, thyroid, adrenal), and diffuse cell groups scattered in epithelial tissues.
More than fifty different hormones are secreted. Endocrine glands arise during development for all three embryologic tissue layers (endoderm, mesoderm, ectoderm). The type of endocrine product is determined by which tissue layer a gland originated in. Glands of ectodermal and endodermal origin produce peptide and amine hormones; mesodermal-origin glands secrete hormones based on lipids.
The endocrine system uses cycles and negative feedback to regulate physiological functions. Negative feedback regulates the secretion of almost every hormone. Cycles of secretion maintain physiological and homeostatic control. These cycles can range from hours to months in duration.
Negative feedback in the thyroxine release reflex. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
The endocrine system acts by releasing hormones that in turn trigger actions in specific target cells. Receptors on target cell membranes bind only to one type of hormone. More than fifty human hormones have been identified; all act by binding to receptor molecules. The binding hormone changes the shape of the receptor causing the response to the hormone. There are two mechanisms of hormone action on all target cells.
Nonsteroid hormones (water soluble) do not enter the cell but bind to plasma membrane receptors, generating a chemical signal (second messenger) inside the target cell. Five different second messenger chemicals, including cyclic AMP have been identified. Second messengers activate other intracellular chemicals to produce the target cell response.
The action of nonsteroid hormones. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
The second mechanism involves steroid hormones, which pass through the plasma membrane and act in a two step process. Steroid hormones bind, once inside the cell, to the nuclear membrane receptors, producing an activated hormone-receptor complex. The activated hormone-receptor complex binds to DNA and activates specific genes, increasing production of proteins.
The pituitary gland (often called the master gland) is located in a small bony cavity at the base of the brain. A stalk links the pituitary to the hypothalamus, which controls release of pituitary hormones. The pituitary gland has two lobes: the anterior and posterior lobes. The anterior pituitary is glandular.
The endocrine system in females and males. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
The hypothalamus contains neurons that control releases from the anterior pituitary. Seven hypothalamic hormones are released into a portal system connecting the hypothalamus and pituitary, and cause targets in the pituitary to release eight hormones.
The location and roles of the hypothalamus and pituitary glands. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
Growth hormone (GH) is a peptide anterior pituitary hormone essential for growth. GH-releasing hormone stimulates release of GH. GH-inhibiting hormone suppresses the release of GH. The hypothalamus maintains homeostatic levels of GH. Cells under the action of GH increase in size (hypertrophy) and number (hyperplasia). GH also causes increase in bone length and thickness by deposition of cartilage at the ends of bones. During adolescence, sex hormones cause replacement of cartilage by bone, halting further bone growth even though GH is still present. Too little or two much GH can cause dwarfism or gigantism, respectively.
Hypothalamus receptors monitor blood levels of thyroid hormones. Low blood levels of Thyroid-stimulating hormone (TSH) cause the release of TSH-releasing hormone from the hypothalamus, which in turn causes the release of TSH from the anterior pituitary. TSH travels to the thyroid where it promotes production of thyroid hormones, which in turn regulate metabolic rates and body temperatures.
Gonadotropins and prolactin are also secreted by the anterior pituitary. Gonadotropins (which include follicle-stimulating hormone, FSH, and luteinizing hormone, LH) affect the gonads by stimulating gamete formation and production of sex hormones. Prolactin is secreted near the end of pregnancy and prepares the breasts for milk production. .
The posterior pituitary stores and releases hormones into the blood. Antidiuretic hormone (ADH) and oxytocin are produced in the hypothalamus and transported by axons to the posterior pituitary where they are dumped into the blood. ADH controls water balance in the body and blood pressure. Oxytocin is a small peptide hormone that stimulates uterine contractions during childbirth.
Each kidney has an adrenal gland located above it. The adrenal gland is divided into an inner medulla and an outer cortex. The medulla synthesizes amine hormones, the cortex secretes steroid hormones. The adrenal medulla consists of modified neurons that secrete two hormones: epinephrine and norepinephrine. Stimulation of the cortex by the sympathetic nervous system causes release of hormones into the blood to initiate the "fight or flight" response. The adrenal cortex produces several steroid hormones in three classes: mineralocorticoids, glucocorticoids, and sex hormones. Mineralocorticoids maintain electrolyte balance. Glucocorticoids produce a long-term, slow response to stress by raising blood glucose levels through the breakdown of fats and proteins; they also suppress the immune response and inhibit the inflammatory response.
The structure of the kidney as relates to hormones. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
The thyroid gland is located in the neck. Follicles in the thyroid secrete thyroglobulin, a storage form of thyroid hormone. Thyroid stimulating hormone (TSH) from the anterior pituitary causes conversion of thyroglobulin into thyroid hormones T4 and T3. Almost all body cells are targets of thyroid hormones.
Thyroid hormone increases the overall metabolic rate, regulates growth and development as well as the onset of sexual maturity. Calcitonin is also secreted by large cells in the thyroid; it plays a role in regulation of calcium.
The pancreas contains exocrine cells that secrete digestive enzymes into the small intestine and clusters of endocrine cells (the pancreatic islets). The islets secrete the hormones insulin and glucagon, which regulate blood glucose levels.
After a meal, blood glucose levels rise, prompting the release of insulin, which causes cells to take up glucose, and liver and skeletal muscle cells to form the carbohydrate glycogen. As glucose levels in the blood fall, further insulin production is inhibited. Glucagon causes the breakdown of glycogen into glucose, which in turn is released into the blood to maintain glucose levels within a homeostatic range. Glucagon production is stimulated when blood glucose levels fall, and inhibited when they rise.
Diabetes results from inadequate levels of insulin. Type I diabetes is characterized by inadequate levels of insulin secretion, often due to a genetic cause. Type II usually develops in adults from both genetic and environmental causes. Loss of response of targets to insulin rather than lack of insulin causes this type of diabetes. Diabetes causes impairment in the functioning of the eyes, circulatory system, nervous system, and failure of the kidneys. Diabetes is the second leading cause of blindness in the US. Treatments involve daily injections of insulin, monitoring of blood glucose levels and a controlled diet.
Prostaglandins are fatty acids that behave in many ways like hormones. They are produced by most cells in the body and act on neighboring cells.
Pheromones are chemical signals that travel between organisms rather than between cells within an organism. Pheromones are used to mark territory, signal prospective mates, and communicate. The presence of a human sex attractant/pheromone has not been established conclusively.
Biological cycles ranging from minutes to years occur throughout the animal kingdom. Cycles involve hibernation, mating behavior, body temperature and many other physiological processes.
Rhythms or cycles that show cyclic changes on a daily (or even a few hours) basis are known as circadian rhythms. Many hormones, such as ACTH-cortisol, TSH, and GH show circadian rhythms.
The menstrual cycle is controlled by a number of hormones secreted in a cyclical fashion. Thyroid secretion is usually higher in winter than in summer. Childbirth is hormonally controlled, and is highest between 2 and 7 AM.
Internal cycles of hormone production are controlled by the hypothalamus, specifically the suprachiasmic nucleus (SCN). According to one model, the SCN is signaled by messages from the light-detecting retina of the eyes.The SCN signals the pineal gland in the brain to signal the hypothalamus, etc.
Nervous tissue is composed of two main cell types: neurons and glial cells. Neurons transmit nerve messages. Glial cells are in direct contact with neurons and often surround them.
Nerve Cells and Astrocyte (SEM x2,250). This image is copyright Dennis Kunkel at www.DennisKunkel.com, used with permission.
The neuron is the functional unit of the nervous system. Humans have about 100 billion neurons in their brain alone! While variable in size and shape, all neurons have three parts. Dendrites receive information from another cell and transmit the message to the cell body. The cell body contains the nucleus, mitochondria and other organelles typical of eukaryotic cells. The axon conducts messages away from the cell body.
Structure of a typical neuron. The above image is from http://eleceng.ukc.ac.uk/~sd5/pics/research/big/neuron.gif.
Three types of neurons occur. Sensory neurons typically have a long dendrite and short axon, and carry messages from sensory receptors to the central nervous system. Motor neurons have a long axon and short dendrites and transmit messages from the central nervous system to the muscles (or to glands). Interneurons are found only in the central nervous system where they connect neuron to neuron.
Structure of a neuron and the direction of nerve message transmission. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
Some axons are wrapped in a myelin sheath formed from the plasma membranes of specialized glial cells known as Schwann cells. Schwann cells serve as supportive, nutritive, and service facilities for neurons. The gap between Schwann cells is known as the node of Ranvier, and serves as points along the neuron for generating a signal. Signals jumping from node to node travel hundreds of times faster than signals traveling along the surface of the axon. This allows your brain to communicate with your toes in a few thousandths of a second.
Cross section of myelin sheaths that surround axons (TEM x191,175). This image is copyright Dennis Kunkel at www.DennisKunkel.com, used with permission.
The plasma membrane of neurons, like all other cells, has an unequal distribution of ions and electrical charges between the two sides of the membrane. The outside of the membrane has a positive charge, inside has a negative charge. This charge difference is a resting potential and is measured in millivolts. Passage of ions across the cell membrane passes the electrical charge along the cell. The voltage potential is -65mV (millivolts) of a cell at rest (resting potential). Resting potential results from differences between sodium and potassium positively charged ions and negatively charged ions in the cytoplasm. Sodium ions are more concentrated outside the membrane, while potassium ions are more concentrated inside the membrane. This imbalance is maintained by the active transport of ions to reset the membrane known as the sodium potassium pump. The sodium-potassium pump maintains this unequal concentration by actively transporting ions against their concentration gradients.
Transmission of an action potential. The above image is from http://eleceng.ukc.ac.uk/~sd5/pics/research/big/actpot.gif.
Changed polarity of the membrane, the action potential, results in propagation of the nerve impulse along the membrane. An action potential is a temporary reversal of the electrical potential along the membrane for a few milliseconds. Sodium gates and potassium gates open in the membrane to allow their respective ions to cross. Sodium and potassium ions reverse positions by passing through membrane protein channel gates that can be opened or closed to control ion passage. Sodium crosses first. At the height of the membrane potential reversal, potassium channels open to allow potassium ions to pass to the outside of the membrane. Potassium crosses second, resulting in changed ionic distributions, which must be reset by the continuously running sodium-potassium pump. Eventually enough potassium ions pass to the outside to restore the membrane charges to those of the original resting potential.The cell begins then to pump the ions back to their original sides of the membrane.
The action potential begins at one spot on the membrane, but spreads to adjacent areas of the membrane, propagating the message along the length of the cell membrane. After passage of the action potential, there is a brief period, the refractory period, during which the membrane cannot be stimulated. This prevents the message from being transmitted backward along the membrane.
The junction between a nerve cell and another cell is called a synapse. Messages travel within the neuron as an electrical action potential. The space between two cells is known as the synaptic cleft. To cross the synaptic cleft requires the actions of neurotransmitters. Neurotransmitters are stored in small synaptic vessicles clustered at the tip of the axon.
Excitatory Synapse from the Central Nervous System (TEM x27,360). This image is copyright Dennis Kunkel at www.DennisKunkel.com, used with permission.
Arrival of the action potential causes some of the vesicles to move to the end of the axon and discharge their contents into the synaptic cleft. Released neurotransmitters diffuse across the cleft, and bind to receptors on the other cell's membrane, causing ion channels on that cell to open. Some neurotransmitters cause an action potential, others are inhibitory.
Neurotransmitters tend to be small molecules, some are even hormones. The time for neurotransmitter action is between 0,5 and 1 millisecond. Neurotransmitters are either destroyed by specific enzymes in the synaptic cleft, diffuse out of the cleft, or are reabsorbed by the cell. More than 30 organic molecules are thought to act as neurotransmitters. The neurotransmitters cross the cleft, binding to receptor molecules on the next cell, prompting transmission of the message along that cell's membrane. Acetylcholine is an example of a neurotransmitter, as is norepinephrine, although each acts in different responses. Once in the cleft, neurotransmitters are active for only a short time. Enzymes in the cleft inactivate the neurotransmitters. Inactivated neurotransmitters are taken back into the axon and recycled.
Diseases that affect the function of signal transmission can have serious consequences. Parkinson's disease has a deficiency of the neurotransmitter dopamine. Progressive death of brain cells increases this deficit, causing tremors, rigidity and unstable posture. L-dopa is a chemical related to dopamine that eases some of the symptoms (by acting as a substitute neurotransmitter) but cannot reverse the progression of the disease.
The bacterium Clostridium tetani produces a toxin that prevents the release of GABA. GABA is important in control of skeletal muscles. Without this control chemical, regulation of muscle contraction is lost; it can be fatal when it effects the muscles used in breathing.
Clostridium botulinum produces a toxin found in improperly canned foods. This toxin causes the progressive relaxation of muscles, and can be fatal. A wide range of drugs also operate in the synapses: cocaine, LSD, caffeine, and insecticides.
Multicellular animals must monitor and maintain a constant internal environment as well as monitor and respond to an external environment. In many animals, these two functions are coordinated by two integrated and coordinated organ systems: the nervous system and the endocrine system. Click here for a diagram of the Nervous System.
Three basic functions are prformed by nervous systems:
Receptors are parts of the nervous system that sense changes in the internal or external environments. Sensory input can be in many forms, including pressure, taste, sound, light, blood pH, or hormone levels, that are converted to a signal and sent to the brain or spinal cord.
In the sensory centers of the brain or in the spinal cord, the barrage of input is integrated and a response is generated. The response, a motor output, is a signal transmitted to organs than can convert the signal into some form of action, such as movement, changes in heart rate, release of hormones, etc.
Some animals have a second control system, the endocrine system. The nervous system coordinates rapid responses to external stimuli. The endocrine system controls slower, longer lasting responses to internal stimuli. Activity of both systems is integrated.
The nervous system monitors and controls almost every organ system through a series of positive and negative feedback loops.The Central Nervous System (CNS) includes the brain and spinal cord. The Peripheral Nervous System (PNS) connects the CNS to other parts of the body, and is composed of nerves (bundles of neurons).
Not all animals have highly specialized nervous systems. Those with simple systems tend to be either small and very mobile or large and immobile. Large, mobile animals have highly developed nervous systems: the evolution of nervous systems must have been an important adaptation in the evolution of body size and mobility.
Coelenterates, cnidarians, and echinoderms have their neurons organized into a nerve net. These creatures have radial symmetry and lack a head. Although lacking a brain or either nervous system (CNS or PNS) nerve nets are capable of some complex behavior.
Nervous systems in radially symmetrical animals. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
Bilaterally symmetrical animals have a body plan that includes a defined head and a tail region. Development of bilateral symmetry is associated with cephalization, the development of a head with the accumulation of sensory organs at the front end of the organism. Flatworms have neurons associated into clusters known as ganglia, which in turn form a small brain. Vertebrates have a spinal cord in addition to a more developed brain.
Some nervous systems in bilaterally symmetrical animals. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
Chordates have a dorsal rather than ventral nervous system. Several evolutionary trends occur in chordates: spinal cord, continuation of cephalization in the form of larger and more complex brains, and development of a more elaborate nervous system. The vertebrate nervous system is divided into a number of parts. The central nervous system includes the brain and spinal cord. The peripheral nervous system consists of all body nerves. Motor neuron pathways are of two types: somatic (skeletal) and autonomic (smooth muscle, cardiac muscle, and glands). The autonomic system is subdivided into the sympathetic and parasympathetic systems.
The Peripheral Nervous System (PNS)contains only nerves and connects the brain and spinal cord (CNS) to the rest of the body. The axons and dendrites are surrounded by a white myelin sheath. Cell bodies are in the central nervous system (CNS) or ganglia. Ganglia are collections of nerve cell bodies. Cranial nerves in the PNS take impulses to and from the brain (CNS). Spinal nerves take impulses to and away from the spinal cord. There are two major subdivisions of the PNS motor pathways: the somatic and the autonomic.
Two main components of the PNS:
Most sensory input carried in the PNS remains below the level of conscious awareness. Input that does reach the conscious level contributes to perception of our external environment.
The Somatic Nervous System (SNS) includes all nerves controlling the muscular system and external sensory receptors. External sense organs (including skin) are receptors. Muscle fibers and gland cells are effectors. The reflex arc is an automatic, involuntary reaction to a stimulus. When the doctor taps your knee with the rubber hammer, she/he is testing your reflex (or knee-jerk). The reaction to the stimulus is involuntary, with the CNS being informed but not consciously controlling the response. Examples of reflex arcs include balance, the blinking reflex, and the stretch reflex.
Sensory input from the PNS is processed by the CNS and responses are sent by the PNS from the CNS to the organs of the body.
Motor neurons of the somatic system are distinct from those of the autonomic system. Inhibitory signals, cannot be sent through the motor neurons of the somatic system.
The Autonomic Nervous System is that part of PNS consisting of motor neurons that control internal organs. It has two subsystems. The autonomic system controls muscles in the heart, the smooth muscle in internal organs such as the intestine, bladder, and uterus. The Sympathetic Nervous System is involved in the fight or flight response. The Parasympathetic Nervous System is involved in relaxation. Each of these subsystems operates in the reverse of the other (antagonism). Both systems innervate the same organs and act in opposition to maintain homeostasis. For example: when you are scared the sympathetic system causes your heart to beat faster; the parasympathetic system reverses this effect.
Motor neurons in this system do not reach their targets directly (as do those in the somatic system) but rather connect to a secondary motor neuron which in turn innervates the target organ.
Click here for a diagram of the Autonomic Nervous System.
Areas of the brain. The above image is from http://www.prs.k12.nj.us/schools/PHS/Science_Dept/APBio/pic/brain.gif.
The medulla oblongata is closest to the spinal cord, and is involved with the regulation of heartbeat, breathing, vasoconstriction (blood pressure), and reflex centers for vomiting, coughing, sneezing, swallowing, and hiccuping. The hypothalamus regulates homeostasis. It has regulatory areas for thirst, hunger, body temperature, water balance, and blood pressure, and links the Nervous System to the Endocrine System. The midbrain and pons are also part of the unconscious brain. The thalamus serves as a central relay point for incoming nervous messages.
The cerebellum is the second largest part of the brain, after the cerebrum. It functions for muscle coordination and maintains normal muscle tone and posture. The cerebellum coordinates balance.
The conscious brain includes the cerebral hemispheres, which are are separated by the corpus callosum. In reptiles, birds, and mammals, the cerebrum coordinates sensory data and motor functions. The cerebrum governs intelligence and reasoning, learning and memory. While the cause of memory is not yet definitely known, studies on slugs indicate learning is accompanied by a synapse decrease. Within the cell, learning involves change in gene regulation and increased ability to secrete transmitters.
During embryonic development, the brain first forms as a tube, the anterior end of which enlarges into three hollow swellings that form the brain, and the posterior of which develops into the spinal cord. Some parts of the brain have changed little during vertebrate evolutionary history. Click here to view an diagram of the brain, and here for a clickable map of the brain.
Parts of the brain as seen from the middle of the brain. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
Vertebrate evolutionary trends include
The brain stem is the smallest and from an evolutionary viewpoint, the oldest and most primitive part of the brain. The brain stem is continuous with the spinal cord, and is composed of the parts of the hindbrain and midbrain. The medulla oblongata and pons control heart rate, constriction of blood vessels, digestion and respiration.
The midbrain consists of connections between the hindbrain and forebrain. Mammals use this part of the brain only for eye reflexes.
The cerebellum is the third part of the hindbrain, but it is not considered part of the brain stem. Functions of the cerebellum include fine motor coordination and body movement, posture, and balance. This region of the brain is enlarged in birds and controls muscle action needed for flight.
The forebrain consists of the diencephalon and cerebrum. The thalamus and hypothalamus are the parts of the diencephalon. The thalamus acts as a switching center for nerve messages. The hypothalamus is a major homeostatic center having both nervous and endocrine functions.
The cerebrum, the largest part of the human brain, is divided into left and right hemispheres connected to each other by the corpus callosum. The hemispheres are covered by a thin layer of gray matter known as the cerebral cortex, the most recently evolved region of the vertebrate brain. Fish have no cerebral cortex, amphibians and reptiles have only rudiments of this area.
The cortex in each hemisphere of the cerebrum is between 1 and 4 mm thick. Folds divide the cortex into four lobes: occipital, temporal, parietal, and frontal. No region of the brain functions alone, although major functions of various parts of the lobes have been determined.
The occipital lobe (back of the head) receives and processes visual information. The temporal lobe receives auditory signals, processing language and the meaning of words. The parietal lobe is associated with the sensory cortex and processes information about touch, taste, pressure, pain, and heat and cold. The frontal lobe conducts three functions:
Most people who have been studied have their language and speech areas on the left hemisphere of their brain. Language comprehension is found in Wernicke's area. Speaking ability is in Broca's area. Damage to Broca's area causes speech impairment but not impairment of language comprehension. Lesions in Wernicke's area impairs ability to comprehend written and spoken words but not speech. The remaining parts of the cortex are associated with higher thought processes, planning, memory, personality and other human activities.
Parts of the cerebral cortex and the relative areas that are devoted to controlling various body regions. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
The spinal cord runs along the dorsal side of the body and links the brain to the rest of the body. Vertebrates have their spinal cords encased in a series of (usually) bony vertebrae that comprise the vertebral column.
The gray matter of the spinal cord consists mostly of cell bodies and dendrites. The surrounding white matter is made up of bundles of interneuronal axons (tracts). Some tracts are ascending (carrying messages to the brain), others are descending (carrying messages from the brain). The spinal cord is also involved in reflexes that do not immediately involve the brain.
Some neurotransmitters are excitory, such as acetylcholine, norepinephrine, serotonin, and dopamine. Some are associated with relaxation, such as dopamine and serotonin. Dopamine release seems related to sensations of pleasure. Endorphins are natural opioids that produce elation and reduction of pain, as do artificial chemicals such as opium and heroin. Neurological diseases, for example Parkinson's disease and Huntington's disease, are due to imbalances of neurotransmitters. Parkinson's is due to a dopamine deficiency. Huntington's disease is thought to be cause by malfunctioning of an inhibitory neurotransmitter. Alzheimer's disease is associated with protein plaques in the brain.
Drugs are stimulants or depressants that block or enhance certain neurotransmitters. Dopamine is thought involved with all forms of pleasure. Cocaine interferes with uptake of dopamine from the synaptic cleft. Alcohol causes a euphoric "high" followed by a depression.
Marijuana, material from the Indian hemp plant (Cannabis sativa), has a potent chemical THC (tetrahydracannibinol) that in low, concentrations causes a euphoric high (if inhaled, the most common form of action is smoke inhalation). High dosages may cause severe effects such as hallucinations, anxiety, depression, and psychotic symptoms.
Cocaine is derives from the plant Erthoxylon coca. Inhaled, smoked or injected. Cocaine users report a "rush" of euphoria following use. Following the rush is a short (5-30 minute) period of arousal followed by a depression. Repeated cycle of use terminate in a "crash" when the cocaine is gone. Prolonged used causes production of less dopamine, causing the user to need more of the drug.
Heroin is a derivative of morphine, which in turn is obtained from opium, the milky secretions obtained from the opium poppy, Papaver somniferum. Heroin is usually injected intravenously, although snorting and smoking serve as alternative delivery methods. Heroin binds to ophioid receptors in the brain, where the natural chemical endorphins are involved in the cessation pain. Heroin is physically addictive, and prolonged use causes less endorphin production. Once this happens, the euphoria is no longer felt, only dependence and delay of withdrawal symptoms.
Input to the nervous system is in the form of our five senses: pain, vision, taste, smell, and hearing. Vision, taste, smell, and hearing input are the special senses. Pain, temperature, and pressure are known as somatic senses. Sensory input begins with sensors that react to stimuli in the form of energy that is transmitted into an action potential and sent to the CNS.
Mechanoreceptors vary greatly in the specific type of stimulus and duration of stimulus/action potentials. The most adaptable vertebrate mechanoreceptor is the hair cell. Hair cells are present in the lateral line of fish. In humans and mammals hair cells are involved with detection of sound and gravity and providing balance.
Hearing involves the actions of the external ear, eardrum, ossicles, and cochlea. In hearing, sound waves in air are converted into vibrations of a liquid then into movement of hair cells in the cochlea. Finally they are converted into action potentials in a sensory dendrite connected to the auditory nerve. Very loud sounds can cause violent vibrations in the membrane under hair cells, causing a shearing or permanent distortion to the cells, resulting in permanent hearing loss.
Orientation and gravity are detected at the semicircular canals. Hair cells along three planes respond to shifts of liquid within the cochlea, providing a three-dimensional sense of equilibrium. Calcium carbonate crystals can shift in response to gravity, providing sensory information about gravity and acceleration.
The human eye can detect light in the 400-700 nanometer (nm) range, a small portion of the electromagnetic spectrum, the visible light spectrum. Light with wavelengths shorter than 400 nm is termed ultraviolet (UV) light. Light with wavelengths longer than 700 nm is termed infrared (IR) light.
In the eye, two types of photoreceptor cells are clustered on the retina, or back portion of the eye. These receptors, rods and cones, apparently evolved from hair cells. Rods detect differences in light intensity; cones detect color. Rods are more common in a circular zone near the edge of the eye. Cones occur in the center (or fovea centralis) of the retina.
Light reaching a photoreceptor causes the breakdown of the chemical rhodopsin, which in turn causes a membrane potential that is transmitted to an action potential. The action potential transfers to synapsed neurons that connect to the optic nerve. The optic nerve connects to the occipital lobe of the brain.
Humans have three types of cones, each sensitive to a different color of light: red, blue and green. Opsins are chemicals that bind to cone cells and make those cells sensitive to light of a particular wavelength (or color). Humans have three different form of opsins coded for by three genes on the X chromosome. Defects in one or more of these opsin genes can cause color blindness, usually in males.
The cardiovascular system supplies oxygen from the lungs to the tissues around the body. It also transports carbon dioxide, a waste product, from the body to the lungs. Breathing out removes carbon dioxide from the body.
Your cardiovascular system is your:
Oxygen makes up about a fifth of the atmosphere. You breathe air through your mouth and nose and it travels to your lungs. Oxygen from the air is absorbed into your bloodstream through your lungs. Your heart then pumps oxygen-rich ('oxygenated') blood through a network of blood vessels – the arteries – to tissues including your organs, muscles and nerves, all around your body.
When blood reaches the capillaries in your tissues it releases oxygen, which cells use to make energy. These cells release waste products, such as carbon dioxide and water, which your blood absorbs and carries away.
The used (or 'deoxygenated') blood then travels along your veins and back towards your heart. Your heart pumps the deoxygenated blood back to your lungs, where it absorbs fresh oxygen, and the cycle starts again.
Your heart is roughly the size of a clenched fist and weighs about 300g. It lies just to the left in your chest, surrounded by a protective membrane called the pericardium.
Your heart is a pump, divided into left and right sides. It has walls, made of muscle, which squeeze (contract) to pump blood into the blood vessels and around your body. You have around 8 pints of blood in your body, and in an average day your heart beats 100,000 times to keep the blood moving around your body.
Your veins deliver deoxygenated blood to the right side of your heart. Your heart pumps this blood back to your lungs, where it absorbs more oxygen. This oxygenated blood then returns to the left side of your heart, which pumps it out to the rest of your body through the arteries. The muscle on the left side of your heart is slightly larger because it has more work to do than the right: the right side only pumps blood to your lungs, the left side pumps blood around your body.
Each side of your heart is divided into an upper chamber called an atrium and a larger, lower chamber, called a ventricle. Blood flows from each atrium to the ventricle below, through a one-way valve.
The main organs, arteries and veins in the cardiovascular system
Your lungs are on either side of your heart in your chest (thorax) and consist of spongy tissue with a rich blood supply.
Your diaphragm is a sheet of muscle that separates your chest from your abdominal cavity and forms the floor of your thorax. Movement of your diaphragm as you breathe in makes your lungs inflate.
Air passes from your nose and mouth into your trachea (windpipe) and into each lung, through two airways called the bronchi. These divide into smaller airways, called bronchioles, which repeatedly divide and end in tiny sacs called alveoli. These are air sacs with walls just one cell thick. It's here that oxygen and carbon dioxide filter into and out of your blood. In this process, known as gaseous exchange, molecules of oxygen and carbon dioxide bind to the haemoglobin, a protein in your red blood cells.
There are about 300 million alveoli in each lung, which provide a vast surface area for gaseous exchange – around the size of a tennis court if it could be spread out.
In an average day, you breathe 10,000 litres of air in and out of your lungs.
Blood carrying oxygen and nutrients is pumped around your body by your heart. The blood is under pressure as a result of the pumping action of your heart and the size and flexibility of your arteries. This blood pressure is an essential part of the way your body works.
When blood pressure is measured, the result is expressed as two numbers, such as 120/80mmHg (one hundred and twenty over eighty millimetres of mercury).
The first figure – the systolic blood pressure – is a measure of the pressure when your heart muscle is contracted and pumping blood. This is the maximum pressure in your blood vessels.
The second figure – the diastolic blood pressure – is the pressure between heart beats when your heart is resting and filling with blood. This is the minimum pressure in your blood vessels.
The lower your blood pressure, the better for your health, although very low blood pressure can make you feel dizzy or faint. Doctors recommend that blood pressure is kept below 140/85. If you have diabetes, kidney disease or cardiovascular disease, your blood pressure should be lower than this – ideally less than 130/80.
Your lifestyle plays an essential part in maintaining your long-term cardiovascular health. A healthy diet, moderate drinking, plenty of exercise, and not smoking can all help to maintain a healthy cardiovascular system.
THE MALE REPRODUCTIVE SYSTEM
The organs of the male reproductive system are specialized for the following functions:
The anatomy of male reproductive system has internal and external structures or form.
What are the external reproductive structures?
The external structure of the male reproductive system is mostly found outside the body of male humans. These are the penis, the scrotum and the testicles.
Penis — This is the organ used by male for copulation or sexual intercourse. It is composed of three parts, namely, the root of the penis, which is attached to the abdominal walls; the body of the penis, or sometimes called shaft, which is composed of fleshy structure which becomes hard when filled with blood during erection; and the glans, or the cone-shaped structure attached to the end of the penis. The glans is also called the head of the penis. This is covered with foreskin—a layer of skin which is sometimes removed during circumcision procedure. Found at the tip of the glans of the penis is a tiny opening of the urethra—the tube which functions as a transport system for semen and urine. Due to the large concentration of nerve endings located at the glans of the penis, this provides for the added sensitivity of the said structure.
The body of the penis is somewhat shaped in a cylindrical manner. It has three internal chambers. The composition of these internal chambers is of special and sponge-like tissues necessary during erection. Erection occurs when, the male is excited or aroused sexually, the erectile tissues becomes filled with blood thereby making it hard and erect, which makes it possible for penetration of the vaginal during sexual intercourse. The outer skin covering the erectile tissues are loose, flexible and elastic thus making it possible to adjust to the changes in shape and size during erection.
Semen , this is the fluid which carries sperm during ejaculation. Ejaculation occurs then the man reaches orgasm or the peak of sexual activity or sometimes refered to as climax. During erection, urine cannot flow because it is blocked by the urethra and only the semen is allowed to flow during orgasm or ejaculation.
Scrotum — This is the loose and pouch-like sac of skin which hangs posterior to the penis. Inside the scrotum are the testicles, also called testes, bunch of nerve endings, and blood vessels. The function of the scrotum is to serve as climate control system for the testes which is sensitive to extreme temperatures as this could affect the life of the sperm produced therein. The main reason why the testes hang outside the body is to keep the sperm in a temperature which is lower than that of the human body to keep them alive and healthy. When extreme cold temperature is detected, the scrotum contract which draws the testes closer to the body to provide the necessary warmth for the sperm, and upon the other hand, when a hot temperature is detected in the body, the scrotum relaxes to keep the testes away from the body to provide cold temperature.
Testicles (testes) — These are the oval-shaped organs found inside the scrotum. They are about the size of large olives and are secured at either end by what is termed as the spermatic cord. Normally, men have two testes. The primary function of these organs is to produce testosterone, which is the primary male sex hormone, and produce sperm cells. Inside the testes are tubes masses which are coiled or bundled together. They are called as the seminiferous tubules. These tubules function by producing sperm cells in a process called spermatogenesis.
Epididymis — This is the long, coiled tube-like structure which is located on the posterior of each testicle. Its primary function is to transport and store sperm cells which are produced inside the testes. Another function of the epididymis is to nurture the sperm until it matures. This is because the sperm produced in the testes are basically immature and not capable of fertilization. During the arousal stage of sexual activity, the contractions of the genitals force the sperm which are contained in the fluid-like semen into the vas deferens.
What are the internal reproductive organs?
After the discussion of the external internal organs of the male reproductive system, what then are the internal organs of the male reproductive system? These internal organs are also called accessory organs. They include the following:
Vas deferens — This is a long and muscular tube which connects the epididymis to the pelvic cavity which is at the posterior of the bladder. The main function of the vas deferens is to carry mature sperm cells via the urethra in anticipation for the ejaculation process.
Ejaculatory ducts — These organs are the outcome of the fusion of the vas deferens and the seminal vesicles. These ducts empty into the urethra.
Urethra — This is the tube-like structure which is the passage of the urine from the bladder to be discharged outside the body. In male reproductive system, it has an additional function of ejaculating semen during climax or orgasm. During erection, the flow of urine is hindered or blocked from the urethra, thus allowing only semen to be expelled outside.
Seminal vesicles — These are the sac-like pouches which are attached to the vas deferens located proximal to the base of the bladder. The main function of the seminal vesicles is to produce fructose, a fluid rich in sugar which is needed to provide the sperm a source of energy and help in adding motility or the ability to move or propel once it is lodged inside the female reproductive tract during copulation. The fluid produced makes up the most concentrate of the male's ejacule.
Prostate gland — This gland is the walnut-sized structure which is inferior to the urinary bladder and anterior of the rectum. The main function of this gland is to provide additional fluid to the ejaculate, provide nourishment to the sperm. The center of the prostate gland is where the urethra, carrying the ejaculate during orgasm, runs through.
Bulbourethral glands — The bulbourethral glands, or Cowper's glands are the structures just lateral the urethra and inferior to the prostate gland. The main function of these glands is to produce a clear, slippery fluid that passes directly through the urethra. The said fluid acts as a lubricant and neutralizer to the acidity which may be produced by droplets or residues of urine passing through the urethra.
How does the male reproductive system function?
The male reproductive system depends on hormones. Hormones are the chemicals that regulate or stimulate body functions. The main hormones in the proper functioning of the male reproductive system are the follicle-stimulating hormone (FSH), luteinizing hormone (LH), and the testosterone.
The follicle-stimulating hormone (FSH) and the luteinizing hormone (LH) are produced by the brain, specifically by its pituitary gland which is located at the base of the brain. FSH is important for the production of the sperm cells or the process called spermatogenesis. The LH is responsible of the stimulation of the production of the testosterone. This is necessary to continue the process of spermatogenesis. Testosterone, upon the other hand, is necessary in the development of male characteristics. Some examples of male characteristics are muscle mass, strength, fat distribution, bone mass, and sex drive.
The female reproductive system or genitalia has two main parts, namely: the uterus and the ovaries. The uterus houses the fetus during its development stage. It also produces secretions in the vagina and the uterus as well. It is also where the sperm cells during copulation pass in order to reach the fallopian tubes to fertilize the egg cell. The ovaries, upon the other hand, produce the egg cells. The aforementioned parts are internal in the female reproductive system. As regards the external parts, these are the vagina with the vulva in the external region with the labia minora and majora, the clitoris and the urethra. The cervix connects the vagina to the uterus, while the fallopian tubes attach the uterus to the ovaries. During periods or intervals, the ovaries release an ovum through the fallopian tube. If during such release, the sperm meets the ovum, fertilization occurs. Fertilization occurs when the sperm impregnates the ovum and unites with it in the process. In fertilization, the sperm and egg meet, most commonly, in the oviducts or the fallopian tube. This process may also occur within the uterus. Then the fertilized egg or zygote is implanted in the walls of the uterus to commence further changes or processes such as embryogenesis and morphogenesis. After a period of about nine months, and when the embryo is well developed in order to survive apart from the womb, the cervix dilates and contractions occur in the uterus, which pushes the fetus through the vagina. This is commonly known as childbirth. However, if fertilization does not occur, the ova, is released approximately every month, a process commonly known as menstruation, or a process of oogenesis wherein the mature ovum is passed through the fallopian tube in anticipation of fertilization.
The internal reproductive organs of human female are composed of the ovaries, the fallopian tubes, the cervix, and the vagina.
In human female, the vagina is composed of a fibro muscular tube-like component that leads from the uterus to the external part of the body. This is true to all female mammals, in some reptiles, and to the cloaca in female birds. In female insects and other invertebrates, vagina is the terminal part of their fallopian tubes.
In ejaculation or the climax of coitus or commonly known as sexual intercourse, the semen from the male is deposited in the vagina of the female. Pubic hairs which grow around the outer part of the vagina serve as protection against infection during puberty.
This is the lower and narrow part of the uterus. Sometimes the cervix is called the neck of the uterus. It connects the end portion of the vagina inside to the uterus. Its shape is like a cone and cylindrical protruding through the upper anterior portion of the vaginal wall. Almost half of its entire length may be visible but the rest is lying above the vagina and are hidden from view. Outside of the vagina, there is a thick layer of skin where the baby comes out during delivery.
Uterus, commonly known as the womb, is the major organ for reproduction of female humans. This organ protects the developing embryo, provides nutrition to it, and supports waste removal of the embryo (during the first until the eighth week) and fetus (eighth until delivery). During birth the uterus contracts. These contractions are responsible in pushing the fetus outward during delivery. There are three suspensory ligaments within the uterus which stabilizes its position and restricts its movements. The uterosacral ligaments restricts the anterior and inferior body movements. The posterior movement is restricted by the round ligaments, while the inferior movements are restricted too by the cardinal ligaments. The shape of the uterus is pear-shaped. It is a muscular organ which houses the fertilized ovum and develops the same until childbirth. When fertilization does not occur, the egg which is not embedded on the uterine wall is flushed from the uterus, a process called menstruation. If the egg is fertilized, it is implanted into the endometrium or the walls of the uterus. The nourishment of the embryo is passed through the blood vessels connected to it. From a fertilized ovum, it becomes an embryo, then develops into a fetus and is carried in the womb, a process called gestation, until child birth or delivery.
The pair of tubes connecting the ovaries to the uterus in female mammals is called the fallopian tubes. When the egg cell or ovum matures, the follicle and the walls of the ovaries shall break down. This rupture shall cause the ovum to be released into the fallopian tube then traveling to the uterus which is made possible by the cilia or hair-like structures inside the uterus. Its trip can take hours or days. When it meets a sperm and is fertilized, it implants itself in the endometrium upon reaching the uterus. This is the beginning of pregnancy.
These are the pair of organs which are located proximal to the lateral walls of the pelvic cavity. The ovaries produce the egg cells or ova and secrete hormones. Ovulation is the process where an egg cell is released and made available for fertilization. The ovulation period is periodic in nature and affects the length of the menstrual cycle. With the ovulation, the ovum is transported via the oviduct or commonly known as the fallopian tube to the uterus. Usually, the ovum is fertilized in the oviduct. Then the fertilized egg is moved to the uterus by the action of the hair-like structures called cilia which are abundant in the fallopian tubes.
THE URINARY SYSTEM
The principal function of the urinary system is to maintain the volume and composition of body fluids within normal limits. One aspect of this function is to rid the body of waste products that accumulate as a result of cellular metabolism. Other aspects of its function include regulating the concentrations of various electrolytes in the body fluids and maintaining normal pH of the blood.
In addition to maintaining fluid homeostasis in the body, the urinary system controls red blood cell production by secreting the hormone erythropoietin. The urinary system also plays a role in maintaining normal blood pressure by secreting the enzyme renin.
The urinary system consists of the kidneys, ureters, urinary bladder, and urethra. The kidneys form the urine and account for the other functions attributed to the urinary system. The ureters carry the urine away from kidneys to the urinary bladder, which is a temporary reservoir for the urine. The urethra is a tubular structure that carries the urine from the urinary bladder to the outside.
The kidneys are the primary organs of the urinary system. The kidneys are the organs that filter the blood, remove the wastes, and excrete the wastes in the urine. They are the organs that perform the functions of the urinary system. The other components are accessory structures to eliminate the urine from the body. The paired kidneys are located between the twelfth thoracic and third lumbar vertebrae, one on each side of the vertebral column. The right kidney usually is slightly lower than the left because the liver displaces it downward. The kidneys protected by the lower ribs, lie in shallow depressions against the posterior abdominal wall and behind the parietal peritoneum. This means they are retroperitoneal. Each kidney is held in place by connective tissue, called renal fascia, and is surrounded by a thick layer of adipose tissue, called perirenal fat, which helps to protect it. A tough, fibrous, connective tissue renal capsule closely envelopes each kidney and provides support for the soft tissue that is inside.
In the adult, each kidney is approximately 3 cm thick, 6 cm wide, and 12 cm long. It is roughly bean-shaped with an indentation, called the hilum, on the medial side. The hilum leads to a large cavity, called the renal sinus, within the kidney. The ureter and renal vein leave the kidney, and the renal artery enters the kidney at the hilum.
The outer, reddish region, next to the capsule, is the renal cortex. This surrounds a darker reddish-brown region called the renal medulla. The renal medulla consists of a series of renal pyramids, which appear striated because they contain straight tubular structures and blood vessels. The wide bases of the pyramids are adjacent to the cortex and the pointed ends, called renal papillae, are directed toward the center of the kidney. Portions of the renal cortex extend into the spaces between adjacent pyramids to form renal columns. The cortex and medulla make up the parenchyma, or functional tissue, of the kidney.
The central region of the kidney contains the renal pelvis, which is located in the renal sinus and is continuous with the ureter. The renal pelvis is a large cavity that collects the urine as it is produced. The periphery of the renal pelvis is interrupted by cuplike projections called calyces. A minor calyx surrounds the renal papillae of each pyramid and collects urine from that pyramid. Several minor calyces converge to form a major calyx. From the major calyces the urine flows into the renal pelvis and from there into the ureter.
Each kidney contains over a million functional units, called nephrons, in the parenchyma (cortex and medulla). A nephron has two parts: a renal corpuscle and a renal tubule. The renal corpuscle consists of a cluster of capillaries, called the glomerulus, surrounded by a double-layered epithelial cup, called the glomerular capsule. An afferent arteriole leads into the renal corpuscle and an efferent arteriole leaves the renal corpuscle. Urine passes from the nephrons into collecting ducts then into the minor calyces.
The juxtaglomerular apparatus, which monitors blood pressure and secretes renin, is formed from modified cells in the afferent arteriole and the ascending limb of the nephron loop.
Each ureter is a small tube, about 25 cm long, that carries urine from the renal pelvis to the urinary bladder. It descends from the renal pelvis, along the posterior abdominal wall, behind the parietal peritoneum, and enters the urinary bladder on the posterior inferior surface.
The wall of the ureter consists of three layers. The outer layer, the fibrous coat, is a supporting layer of fibrous connective tissue. The middle layer, the muscular coat, consists of inner circular and outer longitudinal smooth muscle. The main function of this layer is peristalsis to propel the urine. The inner layer, the mucosa, is transitional epithelium that is continuous with the lining of the renal pelvis and the urinary bladder. This layer secretes mucus which coats and protects the surface of the cells.
The urinary bladder is a temporary storage reservoir for urine. It is located in the pelvic cavity, posterior to the symphysis pubis, and below the parietal peritoneum. The size and shape of the urinary bladder varies with the amount of urine it contains and with pressure it receives from surrounding organs.
The inner lining of the urinary bladder is a mucous membrane of transitional epithelium that is continuous with that in the ureters. When the bladder is empty, the mucosa has numerous folds called rugae. The rugae and transitional epithelium allow the bladder to expand as it fills.
The second layer in the walls is the submucosa that supports the mucous membrane. It is composed of connective tissue with elastic fibers.
The next layer is the muscularis, which is composed of smooth muscle. The smooth muscle fibers are interwoven in all directions and collectively these are called the detrusor muscle. Contraction of this muscle expels urine from the bladder. On the superior surface, the outer layer of the bladder wall is parietal peritoneum. In all other regions, the outer layer is fibrous connective tissue.
There is a triangular area, called the trigone, formed by three openings in the floor of the urinary bladder. Two of the openings are from the ureters and form the base of the trigone. Small flaps of mucosa cover these openings and act as valves that allow urine to enter the bladder but prevent it from backing up from the bladder into the ureters. The third opening, at the apex of the trigone, is the opening into the urethra. A band of the detrusor muscle encircles this opening to form the internal urethral sphincter.
The final passageway for the flow of urine is the urethra, a thin-walled tube that conveys urine from the floor of the urinary bladder to the outside. The opening to the outside is the external urethral orifice. The mucosal lining of the urethra is transitional epithelium. The wall also contains smooth muscle fibers and is supported by connective tissue.
The internal urethral sphincter surrounds the beginning of the urethra, where it leaves the urinary bladder. This sphincter is smooth (involuntary) muscle. Another sphincter, the external urethral sphincter, is skeletal (voluntary) muscle and encircles the urethra where it goes through the pelvic floor. These two sphincters control the flow of urine through the urethra.
In females, the urethra is short, only 3 to 4 cm (about 1.5 inches) long. The external urethral orifice opens to the outside just anterior to the opening for the vagina.
In males, the urethra is much longer, about 20 cm (7 to 8 inches) in length, and transports both urine and semen. The first part, next to the urinary bladder, passes through the prostate gland and is called the prostatic urethra. The second part, a short region that penetrates the pelvic floor and enters the penis, is called the membranous urethra. The third part, the spongy urethra, is the longest region. This portion of the urethra extends the entire length of the penis, and the external urethral orifice opens to the outside at the tip of the penis.