Membrane

Membrane (Latin membrana,”parchment”), in biology, any thin layer of connective tissue coating individual cells and organs of the body, or lining the joints and the ducts and tracts that open to the exterior of the body. The membrane surrounding single-celled animals and plants and individual cells in multicellular organisms is important in the nutritive, respiratory, and excretory processes of these cells. Such cell membranes are semipermeable; that is, they allow the passage of small molecules, such as those of sugars and salts, but not large molecules, such as those of proteins. Structures inside cells, such as the nucleus, may also have membranes.

Each organ in the animal body is surrounded by a membrane, extensions of which often anchor the organ to the body wall. Three membranes, known as meninges, surround the brain and spinal cord; the outermost is known as the dura mater, the middle layer as the arachnoid, and the innermost as the pia mater. Each lung is coated with a membrane known as a visceral pleura. The visceral pleurae anchor the lungs to the wall of the pleural cavity by extensions, known as the parietal pleurae, which line the cavity. The abdominal cavity is lined by a large membrane called the peritoneum, which is attached to the mesenteries—the membranes coating the abdominal organs. A double membrane from the stomach, known as the omentum, hangs like an apron in the abdominal cavity and is interlaced with fat; the omentum is one of the major fat-storage areas of the body. The articular surfaces of bones making up a joint are lined with lubricating membranes. Small membrane sacs, or bursae, occur in the space between the bones of most joints. The hollow tracts, such as the respiratory and alimentary tracts, and the blood vessels and glandular ducts are lined with membranes. The membranes lining body cavities and coating organs are generally known as serous membranes because the cavities usually contain a serumlike fluid; the membranes lining joints are known as synovial membranes because they secrete synovial lubricating fluid; and the membranes lining the hollow tracts are known as mucous membranes because they secrete mucus. Inflammations of the membranes are assigned names by adding the suffix -itis to their anatomic name.

Tissue

Tissue, group of associated, similarly structured cells that perform specialized functions for the survival of the organism (see Physiology). Animal tissues, to which this article is limited, take their first form when the blastula cells, arising from the fertilized ovum, differentiate into three germ layers: the ectoderm, mesoderm, and endoderm (see Embryology). Through further cell differentiation, or histogenesis, groups of cells grow into more specialized units to form organs made up, usually, of several tissues of similarly performing cells. Animal tissues are classified into four main groups.

EPITHELIAL TISSUES

These tissues include the skin and the inner surfaces of the body, such as those of the lungs, stomach, intestines, and blood vessels. Because its primary function is to protect the body from injury and infection, epithelium is made up of tightly packed cells with little intercellular substance between them.

About 12 kinds of epithelial tissue occur. One kind is stratified squamous tissue found in the skin and the linings of the esophagus and vagina. It is made up of thin layers of flat, scalelike cells that form rapidly above the blood capillaries and are pushed toward the tissue surface, where they die and are shed. Another is simple columnar epithelium, which lines the digestive system from the stomach to the anus; these cells stand upright and not only control the absorption of nutrients but also secrete mucus through individual goblet cells. Glands are formed by the inward growth of epithelium—for example, the sweat glands of the skin and the gastric glands of the stomach. Outward growth results in hair, nails, and other structures. See Epithelium.

CONNECTIVE TISSUES

These tissues, which support and hold parts of the body together, comprise the fibrous and elastic connective tissues, the adipose (fatty) tissues, and cartilage and bone. In contrast to epithelium, the cells of these tissues are widely separated from one another, with a large amount of intercellular substance between them. The cells of fibrous tissue, found throughout the body, connect to one another by an irregular network of strands, forming a soft, cushiony layer that also supports blood vessels, nerves, and other organs. Adipose tissue has a similar function, except that its fibroblasts also contain and store fat. Elastic tissue, found in ligaments, the trachea, and the arterial walls, stretches and contracts again with each pulse beat. In the human embryo, the fibroblast cells that originally secreted collagen for the formation of fibrous tissue later change to secrete a different form of protein called chondrion, for the formation of cartilage; some cartilage later becomes calcified by the action of osteoblasts to form bones. Blood and lymph are also often considered connective tissues. See Bone; Connective Tissue.

MUSCLE TISSUES

These tissues, which contract and relax, comprise the striated, smooth, and cardiac muscles. Striated muscles, also called skeletal or voluntary muscles, include those that are activated by the somatic, or voluntary, nervous system. They are joined together without cell walls and have several nuclei. The smooth, or involuntary muscles, which are activated by the autonomic nervous system, are found in the internal organs and consist of simple sheets of cells. Cardiac muscles, which have characteristics of both striated and smooth muscles, are joined together in a vast network of interlacing cells and muscle sheaths. See Muscle.

NERVE TISSUES

These highly complex groups of cells, called ganglia, transfer information from one part of the body to another. Each neuron, or nerve cell, consists of a cell body with branching dendrites and one long fiber, or axon. The dendrites connect one neuron to another; the axon transmits impulses to an organ or collects impulses from a sensory organ. See Nervous System; Neurophysiology.

Pulse

Pulse (physiology), rhythmic expansion of the arteries resulting from passage of successive surges of blood, produced by continuing contractions of the heart. The arteries resemble elastic tubes, and at each contraction of the heart, 30 to 60 g (2 to 4 oz) of blood are forced into the already-filled vessels. The consequent distension passes along the arterial system at a rate of about 7 m (about 23 ft) a second until it reaches the capillaries, in which it is lost because of peripheral resistance to blood flow and lack of elasticity in the vessel walls.

The pulse may be felt wherever an artery passes over a solid structure, such as a bone or cartilage. The crest of the pulse wave represents the systolic pressure; the trough, the diastolic (see Blood Pressure). The rate of the pulse varies from 150 beats per minute in the embryo, to about 60 in the aged. Autosuggestion and certain training programs may alter the rate substantially (see Biofeedback; Consciousness, States of; Sports Medicine). In disease, the pulse rate usually varies in direct ratio to the body temperature; this correspondence is so regular that an experienced physician can approximate the temperature of a patient from observation of the rate of the pulse. The pulse is commonly taken at the wrist, and changes in its rate, rhythm, and strength alert the specialist to existing or impending disease. A pulse may sometimes be observed in the large veins; it is usually twice as fast as the arterial pulse, and is caused by variations in pressure in the left auricle by variations in pressure in the left atrium. See Heart.

Cellular Respiration

Cellular Respiration, process in which cells produce the energy they need to survive. In cellular respiration, cells use oxygen to break down the sugar glucose and store its energy in molecules of adenosine triphosphate (ATP). Cellular respiration is critical for the survival of most organisms because the energy in glucose cannot be used by cells until it is stored in ATP. Cells use ATP to power virtually all of their activities—to grow, divide, replace worn out cell parts, and execute many other tasks. Cellular respiration provides the energy required for an amoeba to glide toward food, the Venus fly trap to capture its prey, or the ballet dancer to execute stunning leaps. Cellular respiration occurs within a cell constantly, day and night, and if it ceases, the cell—and ultimately the organism—dies.

Two critical ingredients required for cellular respiration are glucose and oxygen. The glucose used in cellular respiration enters cells in a variety of ways. Plants, algae, and certain bacteria make their own glucose through photosynthesis, the process by which plants use light to convert carbon dioxide and water into sugar. Animals obtain glucose by eating plants, and fungi and bacteria absorb glucose as they break down the tissues of plants and animals. Regardless of how they obtain it, cells must have a steady supply of glucose so that ATP production is continuous.

Oxygen is present in the air, and also is found dissolved in water. It either diffuses into cells—as in bacteria, fungi, plants, and many aquatic animals, such as sponges and fish—or it is inhaled—as in more complex animals, including humans. Cellular respiration sometimes is referred to as aerobic respiration, meaning that it occurs in the presence of oxygen.

Cellular respiration transfers about 40 percent of the energy of glucose to ATP. The rest of the energy from glucose is released as heat, which warm-blooded organisms use to maintain body temperature, and cold-blooded organisms release to the atmosphere. Cellular respiration is strikingly efficient compared to other energy conversion processes, such as the burning of gasoline, in which only about 25 percent of the energy is used and about 75 percent is released as heat.

While most organisms carry out cellular respiration to produce ATP, some cannot produce ATP through this process because they live in anaerobic environments, or environments that lack oxygen. These organisms, typically bacteria, rely on anaerobic processes such as fermentation to generate their ATP.

See also: Converting Food to Usable Energy

Respiratory System

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Respiratory System, in anatomy and physiology, organs that deliver oxygen to the circulatory system for transport to all body cells. Oxygen is essential for cells, which use this vital substance to liberate the energy needed for cellular activities. In addition to supplying oxygen, the respiratory system aids in removing of carbon dioxide, preventing the lethal buildup of this waste product in body tissues. Day-in and day-out, without the prompt of conscious thought, the respiratory system carries out its life-sustaining activities. If the respiratory system’s tasks are interrupted for more than a few minutes, serious, irreversible damage to tissues occurs, followed by the failure of all body systems, and ultimately, death.

While the intake of oxygen and removal of carbon dioxide are the primary functions of the respiratory system, it plays other important roles in the body. The respiratory system helps regulate the balance of acid and base in tissues, a process crucial for the normal functioning of cells. It protects the body against disease-causing organisms and toxic substances inhaled with air. The respiratory system also houses the cells that detect smell, and assists in the production of sounds for speech.

The respiratory and circulatory systems work together to deliver oxygen to cells and remove carbon dioxide in a two-phase process called respiration. The first phase of respiration begins with breathing in, or inhalation. Inhalation brings air from outside the body into the lungs. Oxygen in the air moves from the lungs through blood vessels to the heart, which pumps the oxygen-rich blood to all parts of the body. Oxygen then moves from the bloodstream into cells, which completes the first phase of respiration. In the cells, oxygen is used in a separate energy-producing process called cellular respiration, which produces carbon dioxide as a byproduct. The second phase of respiration begins with the movement of carbon dioxide from the cells to the bloodstream. The bloodstream carries carbon dioxide to the heart, which pumps the carbon dioxide-laden blood to the lungs. In the lungs, breathing out, or exhalation, removes carbon dioxide from the body, thus completing the respiration cycle.

Topics:

• Stucture of the Respiratory System
• Regulation
• Diseases and Disorders

Related topic: Respiratory Systems in Other Animals

Nervous System

Nervous System, those elements within the animal organism that are concerned with the reception of stimuli, the transmission of nerve impulses, or the activation of muscle mechanisms.

Nervous System Organization

The nervous system is composed of the central nervous system and the peripheral nervous system. The central nervous system, which includes the brain and spinal cord, processes and coordinates all incoming sensory information and outgoing motor commands, and it is also the seat of complex brain functions such as memory, intelligence, learning, and emotion. The peripheral nervous system includes all neural tissue outside of the central nervous system. It is responsible for providing sensory, or afferent, information to the central nervous system and carrying motor, or efferent, commands out to the body’s tissues. Voluntary motor commands, such as moving muscles to walk or talk, are controlled by the somatic nervous system, while involuntary motor commands, such as digestion and heart beat, are controlled by the autonomic nervous system. The autonomic nervous system is further divided into two systems. The sympathetic nervous system, sometimes called the “fight or flight” system, increases alertness, stimulates tissue, and prepares the body for quick responses to unusual situations. In contrast, the parasympathetic nervous system, sometimes called the “rest and repose” system, conserves energy and controls sedentary activities, such as digestion.

Topics:

• Anatomy and Function of the Nervous System
• Disorders of the Nervous System

Urinary System

Urinary System, system of organs that produces and excretes urine from the body. Urine is a transparent yellow fluid containing unwanted wastes, mostly excess water, salts, and nitrogen compounds. The major organs of the urinary system are the kidneys, a pair of bean-shaped organs that continuously filter substances from the blood and produce urine. Urine flows from the kidneys through two long, thin tubes called ureters. With the aid of gravity and wavelike contractions, the ureters transport the urine to the bladder, a muscular vessel. The normal adult bladder can store up to about 0.5 liter (1 pt) of urine, which it excretes through the tubelike urethra.

An average adult produces about 1.5 liters (3 pt) of urine each day, and the body needs, at a minimum, to excrete about 0.5 liter (1 pint) of urine daily to get rid of its waste products. Excessive or inadequate production of urine may indicate illness and doctors often use urinalysis (examination of a patient’s urine) as part of diagnosing disease. For instance, the presence of glucose, or blood sugar, in the urine is a sign of diabetes mellitus; bacteria in the urine signal an infection of the urinary system; and red blood cells in the urine may indicate cancer of the urinary tract.

Topics:

• Structure and Function of the Urinary System
• Disoders of the Urinary System

Related topic: Urinary System of the Animals

Kidney

Kidney, paired organ whose functions include removing waste products from the blood and regulating the amount of fluid in the body. The basic units of the kidneys are microscopically thin structures called nephrons, which filter the blood and cause wastes to be removed in the form of urine. Together with the bladder, two ureters, and the single urethra, the kidneys make up the body’s urinary system. Human beings, as well as members of all other vertebrate species, typically have two kidneys.

Like kidney beans, the body’s kidneys are dark red in color and have a shape in which one side is convex, or rounded, and the other is concave, or indented. The kidneys of adult humans are about 10 to 13 cm (4 to 5 in) long and about 5 to 7.5 cm (2 to 3 in) wide—about the size of a computer mouse.

The kidneys lie against the rear wall of the abdomen, on either side of the spine. They are situated below the middle of the back, beneath the liver on the right and the spleen on the left. Each kidney is encased in a transparent, fibrous membrane called a renal capsule, which helps protect it against trauma and infection. The concave part of the kidney attaches to two of the body’s crucial blood vessels—the renal artery and the renal vein—and the ureter, a tubelike structure that carries urine to the bladder.

A primary function of kidneys is the removal of poisonous wastes from the blood. Chief among these wastes are the nitrogen-containing compounds urea and uric acid, which result from the breakdown of proteins and nucleic acids. Life-threatening illnesses occur when too many of these waste products accumulate in the bloodstream. Fortunately, a healthy kidney can easily rid the body of these substances.

STRUCTURE OF THE KIDNEY

The outermost layer of the kidney is called the cortex. Beneath the cortex lies the medulla, an area that contains between 8 and 18 cone-shaped sections known as pyramids, which are formed almost entirely of bundles of microscopic tubules. The tips of these pyramids point toward the center of the kidney. The cortex extends into the spaces between the pyramids, forming structures called renal columns. At the center of the kidney is a cavity called the renal pelvis.

The task of cleaning, or filtering, the blood is performed by millions of nephrons, remarkable structures that extend between the cortex and the medulla. Under magnification, nephrons look like tangles of tiny vessels or tubules, but each nephron actually has an orderly arrangement that makes possible filtration of wastes from the blood. The primary structure in this filtering system is the glomerulus, a network of extremely thin blood vessels called capillaries. The glomerulus is contained in a cuplike structure called Bowman’s capsule, from which extends a narrow vessel, called the renal tubule. This tube twists and turns until it drains into a collecting tubule that carries urine toward the renal pelvis. Part of the renal tubule, called Henle’s loop, becomes extremely narrow, extending down away from Bowman’s capsule and then back up again in a U shape. Surrounding Henle’s loop and the other parts of the renal tubule is a network of capillaries, which are formed from a small blood vessel that branches out from the glomerulus.

Heart

Heart, in anatomy, hollow muscular organ that pumps blood through the body. The heart, blood, and blood vessels make up the circulatory system, which is responsible for distributing oxygen and nutrients to the body and carrying away carbon dioxide and other waste products. The heart is the circulatory system’s power supply. It must beat ceaselessly because the body’s tissues—especially the brain and the heart itself—depend on a constant supply of oxygen and nutrients delivered by the flowing blood. If the heart stops pumping blood for more than a few minutes, death will result.

The human heart is shaped like an upside-down pear and is located slightly to the left of center inside the chest cavity. About the size of a closed fist, the heart is made primarily of muscle tissue that contracts rhythmically to propel blood to all parts of the body. This rhythmic contraction begins in the developing embryo about three weeks after conception and continues throughout an individual’s life. The muscle rests only for a fraction of a second between beats. Over a typical life span of 76 years, the heart will beat nearly 2.8 billion times and move 169 million liters (179 million quarts) of blood.

STRUCTURE OF THE HEART

The human heart has four chambers. The upper two chambers, the right and left atria, are receiving chambers for blood. The atria are sometimes known as auricles. They collect blood that pours in from veins, blood vessels that return blood to the heart. The heart’s lower two chambers, the right and left ventricles, are the powerful pumping chambers. The ventricles propel blood into arteries, blood vessels that carry blood away from the heart.

A wall of tissue separates the right and left sides of the heart. Each side pumps blood through a different circuit of blood vessels: The right side of the heart pumps oxygen-poor blood to the lungs, while the left side of the heart pumps oxygen-rich blood to the body. Blood returning from a trip around the body has given up most of its oxygen and picked up carbon dioxide in the body’s tissues. This oxygen-poor blood feeds into two large veins, the superior vena cava and inferior vena cava, which empty into the right atrium of the heart.

The right atrium conducts blood to the right ventricle, and the right ventricle pumps blood into the pulmonary artery. The pulmonary artery carries the blood to the lungs, where it picks up a fresh supply of oxygen and eliminates carbon dioxide. The blood, now oxygen-rich, returns to the heart through the pulmonary veins, which empty into the left atrium. Blood passes from the left atrium into the left ventricle, from where it is pumped out of the heart into the aorta, the body’s largest artery. Smaller arteries that branch off the aorta distribute blood to various parts of the body.

See also Coronary Heart Disease.

Histamine

Histamine, also histamine phosphate, an amine (beta-imidazolyl-ethylamine, ergamine, or ergotidime) that is a normal constituent of almost all animal body cells.

Histamine is also found in minute quantities in ergot and putrefied meat products and is produced synthetically for medicinal purposes. In the body, it is synthesized in a type of leukocyte (see Blood) called a basophil or mast cell. In response to certain stimuli these cells release histamine, which immediately effects a dilation of the blood vessels. This dilation is accompanied by a lowering of blood pressure and an increased permeability of the vessel walls, so that fluids escape into the surrounding tissues. This reaction may result in a general depletion of vascular fluids, causing a condition known as histamine poisoning or histamine shock. Allergic reactions in which histamine is released, resulting in the swelling of body tissue, show similarities to histamine poisoning; the two may be basically allied, and the two conditions are treated similarly. The release of histamine might also be partly responsible for difficult breathing during an asthma attack.

Histamine also causes contraction of involuntary muscles, especially of the genital tract and gastrointestinal canal, with an accompanying secretion by associated glands. Because histamine stimulates the flow of gastric juices, it is used diagnostically in patients with gastric disturbances. One drug effective in treating gastric ulcers acts by antagonizing the action of histamine. The ability of the body to localize infections may be due to the secretion of histamine and the subsequent increased local blood supply and increased permeability of the blood vessels.

Stomach

Stomach, organ of the digestive system. Most animals, like humans, have a single stomach, but birds and ruminants have digestive organs composed of two or more chambers. The outer surface of the stomach is smooth; the inner surface is folded into numerous complex ridges, which assist in the mixing of food with digestive juices and channel this material through the stomach into the intestines. Only water, alcohol, and certain drugs seem to be absorbed from the stomach; most food absorption takes place in the small intestine.

In humans the stomach is situated in the upper part of the abdominal cavity (see Abdomen), mostly to the left of the midline. The large, domed end of the stomach, the fundus, lies in the left vault of the diaphragm; the esophagus enters the upper side, or lesser curvature, a short distance below the fundus. The region immediately below the fundus is called the body. The upper part of the stomach, spoken of as the cardiac portion, includes the fundus and body. The lower, or pyloric, portion curves downward, forward, and to the right and includes the antrum and pyloric canal. The latter is continuous with the upper part of the small intestine, the duodenum.

The tissues of the stomach include an outer fibrous coat derived from the peritoneum and, beneath this, a coat of smooth muscle fibers arranged in diagonal, longitudinal, and circular layers. At the junction of the esophagus and stomach the circular muscles are much enlarged, forming the esophageal sphincter. Contraction of this muscle prevents the regurgitation of gastric contents into the esophagus. A similar structure, the pyloric sphincter, is found at the junction of the pylorus and the duodenum. Another layer of the stomach, the submucosa, is made up of loose connective tissue in which are found numerous blood and lymph vessels (see Circulatory System) and nerves of the autonomic nervous system. The innermost layer, the mucosa, contains secretory cells.

Muscle

Muscle, tissue or organ of the animal body characterized by the ability to contract, usually in response to a stimulus from the nervous system. The basic unit of all muscle is the myofibril, a minute, threadlike structure composed of complex proteins. Each muscle cell, or fiber, contains several myofibrils, which are composed of regularly arranged myofilaments of two types, thick and thin. Each thick myofilament contains several hundred molecules of the protein myosin. Thin filaments contain two strands of the protein actin. The myofibrils are made up of alternating rows of thick and thin myofilaments with their ends interleaved. During muscular contractions, these interdigitated rows of filaments slide along each other by means of cross bridges that act as ratchets. The energy for this motion is generated by densely packed mitochondria that surround the myofibrils.

Three types of muscular tissue are recognized:

1. smooth muscle -> Human smooth muscle is composed of slender, spindle-shaped cells, each with a single nucleus. Smooth muscle cells contract in rhythmic waves to propel food through the digestive tract and provide tension in the urinary bladder, blood vessels, uterus, and other internal organs.
2. skeletal or striated mucle tissue -> Skeletal muscle enables the voluntary movement of bones. Skeletal muscle consists of densely packed groups of elongated cells known as muscle fibers. Within these fibers, the alternation of thick and thin myofilaments gives skeletal muscles a striated, or striped, appearance.
3. cardiac muscle -> Cardiac muscle, found only in the heart, drives blood through the circulatory system. Cardiac muscle cells connect to each other by specialized junctions called intercalated disks. Without a constant supply of oxygen, cardiac muscle will die, and heart attacks occur from the damage caused by insufficient blood supply to cardiac muscle.

Joint

Joints, in anatomy, regions of union between bones or cartilages in the skeleton. Synarthroses are rigid, immovable joints, such as the connections between the bones of the skull; symphyses are slightly movable joints, such as the junction of the bones making up the front of the pelvis; and diarthroses are movable joints, such as the meeting of the bones of the limbs with those of the trunk.

Immovable joints are held together by actual intergrowth of bone or by strong fibrous cartilage. Slightly movable joints are held together by elastic cartilage. Typical movable joints consist of an external layer of fibrous cartilage giving rise to strong ligaments that support the separate bones. The bones of movable joints are covered with smooth cartilage and are lubricated by a thick fluid, called synovial fluid, produced between the bones in membranous sacs, known as bursae. Bursitis, or inflammation of the bursae, is a common painful condition of movable joints. See also Arthritis.

The human body has several types of movable joints. Ball-and-socket joints, which allow free movement in all directions, are found in the hip and shoulder. Hinge joints, allowing movement in one plane only, are found in the elbows, knees, and fingers. Pivot joints, permitting rotation only, are found between the first two vertebrae; the head rotates from side to side on a joint of this type called the axis. Gliding joints, in which the surfaces of the bones move a short distance over each other, are found between the various bones of the wrist and ankle.

Lung

Lung, either of a pair of elastic, spongy organs used in breathing and respiration. Lungs are present in all mammals, birds, and reptiles. Most amphibians and a few species of fish also have lungs.

In humans the lungs occupy a large portion of the chest cavity from the collarbone down to the diaphragm. The right lung is divided into three sections, or lobes. The left lung, with a cleft to accommodate the heart, has only two lobes. The two branches of the trachea, called bronchi, subdivide within the lobes into smaller and smaller air vessels known as bronchioles. Bronchioles terminate in alveoli, tiny air sacs surrounded by capillaries. When the alveoli inflate with inhaled air, oxygen diffuses into the blood in the capillaries to be pumped by the heart to the tissues of the body. At the same time carbon dioxide diffuses out of the blood into the lungs, where it is exhaled.

Air travels to the lungs through a series of air tubes and passages. It enters the body through the nostrils or the mouth, passing down the throat to the larynx, or voice box, and then to the trachea, or windpipe. In the chest cavity the trachea divides into two branches, called the right and left bronchi or bronchial tubes, that enter the lungs.

Circulatory System

Circulatory System, or cardiovascular system, in humans, the combined function of the heart, blood, and blood vessels to transport oxygen and nutrients to organs and tissues throughout the body and carry away waste products. Among its vital functions, the circulatory system increases the flow of blood to meet increased energy demands during exercise and regulates body temperature. In addition, when foreign substances or organisms invade the body, the circulatory system swiftly conveys disease-fighting elements of the immune system, such as white blood cells and antibodies, to regions under attack. Also, in the case of injury or bleeding, the circulatory system sends clotting cells and proteins to the affected site, which quickly stop bleeding and promote healing.

Topics:

• Components of the Circulatory System
• Operation and Function
• Disorders of the Circulatory System

Blood

Blood, vital fluid found in humans and other animals that provides important nourishment to all body organs and tissues and carries away waste materials. Sometimes referred to as “the river of life,” blood is pumped from the heart through a network of blood vessels collectively known as the circulatory system.

An adult human has about 5 to 6 liters (1 to 2 gal) of blood, which is roughly 7 to 8 percent of total body weight. Infants and children have comparably lower volumes of blood, roughly proportionate to their smaller size. The volume of blood in an individual fluctuates. During dehydration, for example while running a marathon, blood volume decreases. Blood volume increases in circumstances such as pregnancy, when the mother’s blood needs to carry extra oxygen and nutrients to the baby.

Blood carries oxygen from the lungs to all the other tissues in the body and, in turn, carries waste products, predominantly carbon dioxide, back to the lungs where they are released into the air. When oxygen transport fails, a person dies within a few minutes. Food that has been processed by the digestive system into smaller components such as proteins, fats, and carbohydrates is also delivered to the tissues by the blood. These nutrients provide the materials and energy needed by individual cells for metabolism, or the performance of cellular function. Waste products produced during metabolism, such as urea and uric acid, are carried by the blood to the kidneys, where they are transferred from the blood into urine and eliminated from the body. In addition to oxygen and nutrients, blood also transports special chemicals, called hormones, that regulate certain body functions. The movement of these chemicals enables one organ to control the function of another even though the two organs may be located far apart. In this way, the blood acts not just as a means of transportation but also as a communications system.

The blood is more than a pipeline for nutrients and information; it is also responsible for the activities of the immune system, helping fend off infection and fight disease. In addition, blood carries the means for stopping itself from leaking out of the body after an injury. The blood does this by carrying special cells and proteins, known as the coagulation system, that start to form clots within a matter of seconds after injury.

See Composition of Blood.

Diuretic

Diuretic, chemical compound that increases the flow of urine and thus eliminates accumulations of water in cells, tissues, blood, and organs. The retention of excess water may occur following injury, as when water accumulates in the knee; in congestive heart failure, when the heart pumps insufficient blood to eliminate a normal volume of fluid; and in a variety of other disabilities, including hypertension, cirrhosis of the liver, and kidney diseases. Heart stimulants, such as digitalis, produce a diuretic effect by increasing blood pressure and thus increasing the flow of blood through the kidneys. Certain alkaloids found in coffee and tea (caffeine, theobromine, and theophylline, in increasing order of strength) increase urine output by counteracting the tendency of blood proteins to prevent the removal of water from the blood by the kidneys. A class of drug known as loop diuretics, such as furosemide, are extremely effective when taken orally. All diuretics may have severe side effects, may cause abnormal excretion of sodium, potassium, and chloride, and may become ineffective after repeated use.

Neurotransmitter

Neurotransmitter, chemical made by neurons, or nerve cells. Neurons send out neurotransmitters as chemical signals to activate or inhibit the function of neighboring cells.

Within the central nervous system, which consists of the brain and the spinal cord, neurotransmitters pass from neuron to neuron. In the peripheral nervous system, which is made up of the nerves that run from the central nervous system to the rest of the body, the chemical signals pass between a neuron and an adjacent muscle or gland cell.

Nine chemical compounds—belonging to three chemical families—are widely recognized as neurotransmitters. In addition, certain other body chemicals, including adenosine, histamine, enkephalins, endorphins, and epinephrine, have neurotransmitterlike properties. Experts believe that there are many more neurotransmitters as yet undiscovered.

The first of the three families is composed of amines, a group of compounds containing molecules of carbon, hydrogen, and nitrogen. Among the amine neurotransmitters are acetylcholine, norepinephrine, dopamine, and serotonin. Acetylcholine is the most widely used neurotransmitter in the body, and neurons that leave the central nervous system (for example, those running to skeletal muscle) use acetylcholine as their neurotransmitter; neurons that run to the heart, blood vessels, and other organs may use acetylcholine or norepinephrine. Dopamine is involved in the movement of muscles, and it controls the secretion of the pituitary hormone prolactin, which triggers milk production in nursing mothers.

The second neurotransmitter family is composed of amino acids, organic compounds containing both an amino group (NH2) and a carboxylic acid group (COOH). Amino acids that serve as neurotransmitters include glycine, glutamic and aspartic acids, and gamma-amino butyric acid (GABA). Glutamic acid and GABA are the most abundant neurotransmitters within the central nervous system, and especially in the cerebral cortex, which is largely responsible for such higher brain functions as thought and interpreting sensations.

The third neurotransmitter family is composed of peptides, which are compounds that contain at least 2, and sometimes as many as 100 amino acids. Peptide neurotransmitters are poorly understood, but scientists know that the peptide neurotransmitter called substance P influences the sensation of pain.

In general, each neuron uses only a single compound as its neurotransmitter. However, some neurons outside the central nervous system are able to release both an amine and a peptide neurotransmitter.

Insulin

Insulin, hormone, produced in the islets of Langerhans of the pancreas, that regulates the metabolism of carbohydrates, fats, and starches in the body. Like other proteins, insulin is partially digested if administered orally and hence must be injected into a muscle when used clinically. In the treatment of diabetes mellitus, which is caused by a deficiency of insulin production or by inhibition of its action on cells, insulin is often combined with protamine, which prolongs the period of absorption of the hormone. Insulin crystallized from the pancreas contains zinc, which also lengthens absorption. A preparation called protamine zinc insulin extends the hormone's action still further.

Insulin was first extracted from the pancreatic tissue of dogs in 1921 by the Canadian physiologists Sir Frederick Grant Banting and Charles Herbert Best and the British physiologist John James Rickard Macleod. The Canadian biochemist James Bertram Collip then produced it in sufficiently pure form to be injected into humans. The molecular structure of insulin was determined in 1955 by the British biochemist Frederick Sanger; it was the first protein to be deciphered. Human insulin, the first human protein to be synthesized, was made in 1965. In 1981 insulin made in bacteria by genetic engineering became the first human hormone obtained in this way to be used to treat human disease. For the biochemistry of insulin, see Sugar Metabolism.

Hormone

Hormone, chemical that transfers information and instructions between cells in animals and plants. Often described as the body’s chemical messengers, hormones regulate growth and development, control the function of various tissues, support reproductive functions, and regulate metabolism (the process used to break down food to create energy). Unlike information sent by the nervous system, which is transmitted via electronic impulses that travel quickly and have an almost immediate and short-term effect, hormones act more slowly, and their effects typically are maintained over a longer period of time.

Hormones are made by specialized glands or tissues that manufacture and secrete these chemicals as the body needs them. The majority of hormones are produced by the glands of the endocrine system, such as the pituitary, thyroid, adrenal glands, and the ovaries or testes. These endocrine glands produce and secrete hormones directly into the bloodstream. However, not all hormones are produced by endocrine glands. The mucous membranes of the small intestine secrete hormones that stimulate secretion of digestive juices from the pancreas. Other hormones are produced in the placenta, an organ formed during pregnancy, to regulate some aspects of fetal development.

Hormones are classified into two basic types based on their chemical makeup. The majority of hormones are peptides, or amino acid derivatives that include the hormones produced by the anterior pituitary, thyroid, parathyroid, placenta, and pancreas. Peptide hormones are typically produced as larger proteins. When they are called into action, these peptides are broken down into biologically active hormones and secreted into the blood to be circulated throughout the body. The second type of hormones are steroid hormones, which include those hormones secreted by the adrenal glands and ovaries or testes. Steroid hormones are synthesized from cholesterol (a fatty substance produced by the body) and modified by a series of chemical reactions to form a hormone ready for immediate action.

Steroids

Steroids, large group of naturally occurring and synthetic lipids, or fat-soluble chemicals, with a great diversity of physiological activity. Included among the steroids are certain alcohols (sterols), bile acids, many important hormones, some natural drugs, and the poisons found in the skin of some toads (see Digitalis; Hormone). Various sterols found in the skin of human beings are transformed into vitamin D when they are exposed to the ultraviolet rays of the sun (see Vitamin: Vitamin D). Cholesterol, a major contributor to arteriosclerosis, is a sterol. Steroid hormones, which are similar to but not identical with sterols, include the adrenal cortical steroids hydrocortisone, cortisone, aldosterone, and progesterone; and the female and male sex hormones (see Estrogen; Testosterone). Most oral contraceptives are synthetic steroids consisting of female sex hormones that inhibit ovulation (see Birth Control). Perhaps the most widely used steroids in medicine are cortisone and various synthetic derivatives of this substance. Such steroids are prescription drugs used for a variety of skin ailments, rheumatoid arthritis, asthma and allergies, and various eye diseases, and in cases of adrenal insufficiency, or the malfunctioning of the adrenal cortex (see Adrenal Gland; Endocrine System).

Dermis

Unlike the epidermis, the dermis or lower layer of the skin is richly supplied with blood vessels and sensory nerve endings. The dermis also contains relatively few cells compared to the epidermis—instead, it is made up mainly of fibrous proteins and other large molecules.

The main structural component of the dermis is a protein called collagen. Bundles of collagen molecules pack together throughout the dermis, accounting for three-fourths of the dry weight of skin. Collagen is also responsible for the skin’s strength. Another protein in the dermis, elastin, is the main component of elastic fibers. These protein bundles give skin its elasticity—the ability to return to its original shape after stretching. Collagen and elastin are produced by cells called fibroblasts, which are found scattered throughout the dermis.

The upper part of the dermis is known as the papillary layer. It is characterized by dermal papillae, tiny, fingerlike projections of tissue that indent into the epidermis above. In the thick skin on the palms and soles, the epidermis conforms to the shape of the underlying dermal papillae, forming ridges and valleys that we know as fingerprints. These ridges provide traction that helps people grasp objects and surfaces.

Epidermis

About 90 percent of the cells in the epidermis are keratinocytes, named because they produce a tough, fibrous protein called keratin. This protein is the main structural protein of the epidermis, and it provides many of the skin’s protective properties. Keratinocytes in the epidermis are arranged in layers, with the youngest cells in the lower layers and the oldest cells in the upper layers. The old keratinocytes at the surface of the skin constantly slough off. Meanwhile, cells in the lower layers of the epidermis divide continually, producing new keratinocytes to replace those that have sloughed off. As keratinocytes push up through the layers of the epidermis, they age and, in the process, produce keratin. By the time the cells reach the uppermost layer of the epidermis, they are dead and completely filled with the tough protein. Healthy epidermis replaces itself in a neatly orchestrated way every month.

Scattered among the keratinocytes in the epidermis are melanocytes, cells that produce a dark pigment called melanin. This pigment gives color to the skin and protects it from the sun’s ultraviolet rays. After being produced in the melanocytes, packets of melanin called melanosomes transfer to the keratinocytes. There they are arranged to protect the deoxyribonucleic acid (DNA), or genetic material, of the keratinocytes.

Hair

Hair, collective term for slender, threadlike outgrowths of the epidermis of mammals, forming a characteristic body covering. No animals other than mammals have true hair, and all mammals have hair. Even such apparently hairless mammals as the rhinoceros, elephant, and armadillo have hairs around the snout, at the tip of the tail, and behind each scale, respectively. (Whales and manatees have hair only in the embryonic state.) When the individual hairs are fine and closely spaced, the coat of hair is called fur; when soft, kinked, and matted together, the coat is called wool. Coarse, stiff hairs are called bristles. When bristles are also pointed, as in the hedgehog and porcupine, they are called spines or quills.

In humans the development of the hair begins in the embryo, and by the sixth month the fetus is covered by a growth of fine hair, the lanugo. In the first few months of infancy the lanugo is shed and is replaced by hair, relatively coarse over the cranium and the eyebrows, but fine and downy over the rest of the body. At puberty coarse hair develops in the armpits and over the pubic region in both sexes; in males facial hair begins to grow coarse to form the beard. The rate of growth of the hair varies with the age of the person and with the length of the hair. When a hair is short, its rate of growth averages about 2 cm per month; by the time the hair is a foot long, the rate of growth is reduced by one-half. The fastest growth is found in women from 16 to 24 years of age.

Skin

Skin, outer body covering of an animal. The term skin is commonly used to describe the body covering of any animal but technically refers only to the body covering of vertebrates (animals that have a backbone). The skin has the same basic structure in all vertebrates, including fish, reptiles, birds, and humans and other mammals. This article focuses primarily on human skin.

The skin is made up of two layers, the epidermis and the dermis. The epidermis, the upper or outer layer of the skin, is a tough, waterproof, protective layer. The dermis, or inner layer, is thicker than the epidermis and gives the skin its strength and elasticity. The two layers of the skin are anchored to one another by a thin but complex layer of tissue, known as the basement membrane. This tissue is composed of a series of elaborately interconnecting molecules that act as ropes and grappling hooks to hold the skin together. Below the dermis is the subcutaneous layer, a layer of tissue composed of protein fibers and adipose tissue (fat). Although not part of the skin itself, the subcutaneous layer contains glands and other skin structures, as well as sensory receptors involved in the sense of touch.

See also: Diseases and Disorders of the Skin

Reproduction

Reproduction, process whereby all living organisms produce offspring. Reproduction is one of the essential functions of plants, animals, and single celled organisms, as necessary for the preservation of the species as eating is for the preservation of the individual.

In almost all animal organisms, reproduction occurs during or after the period of maximum growth. In plants, which continue to grow throughout their lifetime, the relationship between growth and reproduction is more complex. Individual plants have growth limitations imposed by inherited characteristics and environmental conditions; if the plant grows excessively, any of a number of reproductive processes may be stimulated (see Plant Propagation). Environmental conditions also play some part in the reproduction of higher animals, but hormonal elements are more important.

Human Sexuality

Human Sexuality, general term referring to various sexually related aspects of human life, including physical and psychological development, and behaviors, attitudes, and social customs associated with the individual's sense of gender, relationships, sexual activity, mate selection, and reproduction. Sexuality permeates many areas of human life and culture, thereby setting humans apart from other members of the animal kingdom, in which the objective of sexuality is more often confined to reproduction. This article discusses the sexual anatomy, development, physiology, and behavior of human beings.

Topics:

• Human Sexual Characteristics
• Sexual Development
• Physiology of Sex
• Sexual Risks
• Sexual Dysfunctions

Estrogen

Estrogen is any of a group of female sex hormones that stimulate the appearance of secondary female sex characteristics in girls at puberty. Estrogens control growth of the lining of the uterus during the first part of the menstrual cycle, cause changes in the breast during pregnancy, and regulate various metabolic processes. Among the better known estrogens are estrone, ethinyl estradiol, and estriol, all produced primarily in the ovaries. Stilbestrol and ethinyl estradiol, two synthetic estrogens, are respectively five and ten times as potent as estrone; their activity is similar to that of natural estrogens. They are used to treat various conditions, including estrogen deficiencies in women (most commonly after menopause) and inflammation of the vagina. They may be used to stimulate lactation following childbirth and in the treatment, but not cure, of advanced and even disseminated cancer of the prostate gland in men.

Menstruation

Menstruation, periodic vaginal discharge in humans and other mammals, consisting of blood and cells shed from the endometrium, or lining of the uterus (see Reproductive System). Menstruation accompanies a woman's childbearing years, usually beginning between the ages of 10 and 16, at puberty, and most often ceasing between the ages of 45 and 50, at menopause. Menstruation is part of the process that prepares a woman for pregnancy. Each month the lining of the uterus thickens; if pregnancy does not occur, this lining breaks down and is discharged through the vagina. The three to seven days that menstruation lasts is called the menstrual period.

In most women the menstrual cycle is about 28 days, but it can vary considerably even from one month to another. The cycle is initiated by hormones in the blood that stimulate the ovaries (the two female organs that produce ova, or eggs). Each month, hormones cause an egg in one of the two ovaries to mature (to become capable of being fertilized and develop into a fetus). The ovaries also produce hormones of their own, primarily estrogen, which cause the endometrium to thicken. About midway through the menstrual cycle, 14 to 15 days before the next period, the ovary releases the mature egg in a process called ovulation. The egg passes through the fallopian tube to the uterus. If the egg unites with a sperm on its way to the uterus, fertilization occurs and pregnancy ensues.

Types of Human Teeth

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Adult humans typically have 32 teeth—16 in the upper jaw and 16 in the lower jaw—that fit together and work in concert to chew food. Teeth on the right side of each jaw are usually identical to the teeth on the left side and matching teeth on opposite sides are referred to as sets, or pairs. Humans are heterodonts—that is, they have teeth of different sizes and shapes that serve different functions, such as tearing and grinding. In contrast, the homodont teeth found in many animals are all the same size and shape, and perform the same function.

Humans have four types of teeth, each with a specific size, shape, and function. Adult humans have eight incisors, located at the front of the mouth—four in the upper jaw and four in the lower jaw. Incisors have a sharp edge that is used to cut food. On either side of the incisors are the canines, named for their resemblance to the pointy fangs of dogs. The upper canines are sometimes called eyeteeth. There are two canines in each jaw, and their primary role is to tear food. Behind the canines are the bicuspids, or premolars, flat teeth with pronounced cusps that grind and mash food. There are two sets, or four bicuspids, in each jaw. Behind the bicuspids are the molars, where the most vigorous chewing occurs. There are twelve molars—three sets in each jaw—referred to as the first, second, and third molars. Third molars are often called wisdom teeth; they developed thousands of years ago when human diets consisted of mostly raw and unprocessed foods that required the extra chewing and grinding power of a third set of molars. Today wisdom teeth are not needed for chewing and, because they can crowd other teeth, are often removed.

Teeth

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Teeth, hard, bony structures in the mouths of humans and animals used primarily to chew food, but also for gnawing, digging, fighting, and catching and killing prey. Teeth are the body’s hardest, most durable organ—long after bones and flesh have dissolved, archaeologists find well-preserved teeth from humans and other animals that lived thousands of years ago.

Structure of a Tooth
The sensitive nerves and blood vessels at the center of each tooth are protected by several layers of tissue, the outermost (the enamel) being the hardest substance in the body. Under the enamel, surrounding the pulp from crown to root, lies a layer of bonelike dentin. A hard tissue called cementum separates the root from the periodontal ligament, which in turn holds the root in place and cushions the tooth against the gum and jaw during the grinding, jarring activity of chewing.

Human teeth are made of four distinct types of tissue: enamel, dentin, pulp, and cementum. Enamel, the clear outer layer of the tooth above the gum line, is the hardest substance in the human body. In human teeth, the enamel layer is about 0.16 cm (about 0.06 in) thick and protects the inner layers of the teeth from harmful bacteria and changes in temperature from hot or cold food. Directly beneath the enamel is dentin, a hard, mineral material that is similar to human bone, only stronger. Dentin surrounds and protects the pulp, or core of the tooth. Pulp contains blood vessels, which carry oxygen and nutrients to the tooth, and nerves, which transmit pain and temperature sensations to the brain. The outer layer of the tooth that lies below the gum line is cementum, a bonelike substance that anchors the tooth to the jawbone.

The visible portion of the tooth is called the crown. Projections on the top of each crown, used primarily for chewing and grinding, are called cusps. The portion of the tooth that lies beneath the gum line is the root. The periodontal ligament anchors the tooth in place with small elastic fibers that connect the cementum in the root to a special socket in the jawbone called the alveolus.

Topics:

• Types of Human Teeth
• Tooth Development
• Disorders of Human Teeth

Salivary Glands

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Salivary Glands, glands that secrete saliva, a somewhat alkaline fluid that moistens the mouth, softens food, and aids in digestion. The submaxillary glands are located around the mouth under the lower jaw, the sublingual glands are located beneath the tongue, and the parotid glands are found in front of each ear. The buccal glands, in the cheeks near the front of the mouth, also secrete saliva. The saliva of the parotid gland contains enzymes called amylases, one of which, known as ptyalin, aids in the digestion of carbohydrates.

In human beings the salivary glands, especially the parotid, are affected by the disease called mumps, a viral disease most common among children, although adults can also be infected.

Palate

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Palate, roof of the mouth, separating the mouth from the nasal cavities. The palate consists of two portions: the hard palate in front and the soft palate behind. The hard palate is formed of periosteum, a bony plate covered by mucous membrane, and arches over to meet the gums in front and on either side. The soft palate is a movable fold of mucous membrane enclosing muscular fibers. Its sides blend with the pharynx (throat), but its lower border is free. It is suspended from the rear of the hard palate so as to form a wall or division between the mouth and the pharynx. During swallowing, this wall is raised to close the entrance to the nasal passages. A small cone-shaped structure, the uvula, hangs from the lower border of the soft palate.

The condition called cleft palate is a birth defect that results from incomplete development of the palate. It is characterized by a hole or gap in the palate that may extend from behind the teeth to the nasal cavity.

Mouth

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Mouth, opening in an animal's body used for taking in food. Mouths are also typically used for making sounds, such as barks, chirps, howls, and in humans, speech. In most animals, the mouth is found on the face, near the eyes and nose.

Lips, which form the mouth's muscular opening, are an especially familiar part of the body for humans. Lips help hold food in the mouth and are used to form words during speech. They also help form facial expressions, such as smiling and frowning. Lips open wide during a yawn and squeeze together during a whistle. Lips are darker than the surrounding skin because of the many extremely small blood vessels, called capillaries, that show through the skin.

The cheeks form the sides of the mouth. They are composed of muscle tissue that is covered on the outside by skin. Like the lips, the cheeks help hold food and they also play a role in speech.

Inside the mouth is the large, muscular tongue. This extremely flexible muscle is used for eating and swallowing and also for talking. It is attached to the floor, or bottom, of the mouth. Its upper surface is covered with tiny projections, called papillae, that give the tongue a somewhat rough texture. The papillae contain tiny pores that are the site of taste buds, the receptor cells responsible for our sense of taste. There are four kinds of taste buds that are grouped together on certain areas of the tongue’s surface—those that are sensitive to sweet, salty, sour, and bitter flavors.

The roof, or top, of the mouth is called the palate. It separates the mouth from the nasal passages above it. The front part of the palate—the part closer to the lips—is made of bone covered with moist tissue, called mucous membrane. This part of the mouth is known as the hard palate. Behind the hard palate is the soft palate, a small area composed mainly of muscle tissue. During swallowing, the soft palate presses against the back of the throat, preventing food or liquid from moving upward into the nasal passages.

Teeth are used for biting into and chewing food. Their interaction with the lips and tongue helps a person speak clearly. Children have 20 primary teeth, which begin to erupt, or break through the gums, at about six months of age. At six years of age, the primary teeth start to fall out, as permanent teeth replace them. The number of permanent teeth is 32. The crown, or top, of each tooth is covered with enamel, the hardest substance in the human body.

The mouth also contains three pairs of salivary glands. These glands secrete a watery fluid called saliva, which moistens food and the tissues of the mouth. Saliva contains amylase, a digestive enzyme that starts to break down carbohydrates in food even before it is swallowed. Saliva also contains a specialized protein, or enzyme, called lysozyme, which fights bacteria.

Despite the presence of saliva, many kinds of bacteria live in the warm, moist environment of the mouth. Caring for the mouth, called oral hygiene, helps keep these bacteria from multiplying and causing illness. Daily brushing of the teeth and tongue, flossing between the teeth, and regular checkups with a dentist help keep the mouth clean and the teeth and gums healthy (see Dentistry).

The most common ailment of the mouth is tooth decay. Other disorders affecting the mouth include gingivitis, a condition marked by inflamed, infected gums; trench mouth, a severe form of gingivitis that causes bleeding ulcers in the mouth; and thrush, a fungal infection characterized by white sores in the mouth. Oral cancer is a risk for individuals who smoke or chew tobacco or who drink alcohol excessively. A small lump or thickened tissue in the mouth may indicate cancer. It should be checked by a doctor or dentist without delay, as many oral cancers can be cured if treated early.

Epiglottis

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Epiglottis, thin, lidlike flap of cartilage attached to the base of the tongue of terrestrial vertebrates. The epiglottis is normally pointed upward, but during the passage of solids and liquids from the mouth into the esophagus, the epiglottis is folded down over the glottis, the opening between the vocal cords, to prevent food from passing into the trachea.

Intestine

Intestine, also bowels, in higher animals, the portion of the digestive tract between the stomach and anus. In humans the intestine is divided into two major sections: the small intestine, which is about 6 m (20 ft) long, where the most extensive part of digestion occurs and where most food products are absorbed; and the large intestine, which has a larger diameter and is about 1.5 m (5 ft) long, where water is absorbed and from which solid waste material is excreted (see Digestive System; Feces).

The small intestine, which is coiled in the center of the abdominal cavity (see Abdomen), is divided into three sections. The upper portion includes the pylorus, the opening at the lower part of the stomach, through which the contents of the stomach pass into the duodenum. The duodenum is a horseshoe-shaped section surrounding part of the pancreas and the pancreatic duct, as well as ducts from the liver and gall bladder that open into it. The middle part of the small intestine, extending from the duodenum to the ileum, is called the jejunum, and the terminal portion is the ileum, which leads into the side of the first part of the large intestine, the cecum. The lining membrane, or mucosa, of the small intestine is especially suited for the purpose of digestion and absorption. The mucosa is folded; the folds are covered with minute mucosal projections called villi. Each villus is a small tube of epithelium surrounding a small lymphatic vessel, or lacteal, and many capillaries. Tiny glandular pits, called the crypts of Lieberkühn, open at the bases of the villi; these pits secrete the enzymes necessary for intestinal digestion. Digested carbohydrates and proteins pass into the capillaries of the villi and then to the portal vein, which enters the liver; digested fats are absorbed into the lacteals in the villi, and they are transported through the lymphatic system into the general bloodstream. The lining of the small intestine also secretes a hormone called secretin, which stimulates the pancreas to produce digestive enzymes.

The large intestine is divided into the cecum, ascending colon, transverse colon, descending colon, sigmoid colon, and rectum. The cecum is a swollen sac located in the lower right-hand portion of the abdominal cavity; it is very large in herbivorous animals. The two important parts of the cecum in humans are the vestigal vermiform appendix (see Appendicitis), which often becomes diseased; and the ileocecal valve, a membranous structure between the cecum and the small intestine that regulates the passage of food material from the small intestine to the large intestine and also prevents the passage of toxic waste products from the large intestine back into the small intestine. The ascending colon rises along the right side of the abdominal cavity; the transverse colon runs across the body to the left side, where the descending colon travels downward. The sigmoid colon is the S-shaped portion of the large intestine as it enters the pelvic cavity. The rectum, about 15 cm (6 in) long, is the almost straight, terminal portion of the large intestine. At the exit of the rectum, called the anus, is a round muscle, the anal sphincter, that closes the anus. The large intestine has a smooth mucosal lining (only the rectum has folds) that secretes mucus to lubricate the waste materials.

Food and waste material are moved along the length of the intestine by rhythmic contractions of intestinal muscles; these contractions are called peristaltic movements. The entire intestine is held in place in the abdominal cavity by membranes called mesenteries.

Liver

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Liver, largest internal organ of the human body. The liver, which is part of the digestive system, performs more than 500 different functions, all of which are essential to life. Its essential functions include helping the body to digest fats, storing reserves of nutrients, filtering poisons and wastes from the blood, synthesizing a variety of proteins, and regulating the levels of many chemicals found in the bloodstream. The liver is unique among the body’s vital organs in that it can regenerate, or grow back, cells that have been destroyed by some short-term injury or disease. But if the liver is damaged repeatedly over a long period of time, it may undergo irreversible changes that permanently interfere with function.

The human liver is a dark red-brown organ with a soft, spongy texture. It is located at the top of the abdomen, on the right side of the body just below the diaphragm—a sheet of muscle tissue that separates the lungs from the abdominal organs. The lower part of the rib cage covers the liver, protecting it from injury. In a healthy adult, the liver weighs about 1.5 kg (3 lb) and is about 15 cm (6 in) thick.

Despite its many complex functions, the liver is relatively simple in structure. It consists of two main lobes, left and right, which overlap slightly. The right lobe has two smaller lobes attached to it, called the quadrate and caudate lobes.

Each lobe contains many thousands of units called lobules that are the building blocks of the liver. Lobules are six-sided structures each about 1 mm (0.04 in) across. A tiny vein runs through the center of each lobule and eventually drains into the hepatic vein, which carries blood out of the liver. Hundreds of cubed-shaped liver cells, called hepatocytes, are arranged around the lobule's central vein in a radiating pattern. On the outside surface of each lobule are small veins, ducts, and arteries that carry fluids to and from the lobules. As the liver does its work, nutrients are collected, wastes are removed, and chemical substances are released into the body through these vessels.

See also liver disease

Genetics

Genetics, study of the function and behavior of genes. Genes are bits of biochemical instructions found inside the cells of every organism from bacteria to humans. Offspring receive a mixture of genetic information from both parents. This process contributes to the great variation of traits that we see in nature, such as the color of a flower’s petals, the markings on a butterfly’s wings, or such human behavioral traits as personality or musical talent. Geneticists seek to understand how the information encoded in genes is used and controlled by cells and how it is transmitted from one generation to the next. Geneticists also study how tiny variations in genes can disrupt an organism’s development or cause disease. Increasingly, modern genetics involves genetic engineering, a technique used by scientists to manipulate genes. Genetic engineering has produced many advances in medicine and industry, but the potential for abuse of this technique has also presented society with many ethical and legal controversies.

Genetic information is encoded and transmitted from generation to generation in deoxyribonucleic acid (DNA). DNA is a coiled molecule organized into structures called chromosomes within cells. Segments along the length of a DNA molecule form genes. Genes direct the synthesis of proteins, the molecular laborers that carry out all life-supporting activities in the cell. Although all humans share the same set of genes, individuals can inherit different forms of a given gene, making each person genetically unique.

Sex

Sex is a physical and behavioral difference that distinguishes individual organisms according to their functions in the reproductive process. For information on issues of sexual health, sexual behavior, and sexual activity, see Human Sexuality.

Sex occurs at all levels of biological organization, with the exception of viruses. At the lowest level, bacteria conjugate and a length of the single chromosome is passed from the male, or donor cell, to the female, or recipient cell. At more advanced levels, multicellular individuals have specialized organs (gonads) that produce specialized sex cells (gametes). Upon fertilization, genetic information is transferred from the small, motile spermatozoa (male gametes) to the much larger ova (female gametes). Many organisms, including most plants, many protozoans and invertebrates, and some fishes, have both male and female gonads and are called hermaphroditic (see Hermaphroditism). Hermaphroditic organisms, however, are rarely self-fertilizing; the male and female reproductive organs ripen at different times—times that also coincide with those of other individuals, thereby ensuring cross-fertilization.