Structure and Functions The body’s parts can be categorized either regionally or functionally. Regionally, the body consists of a trunk to which are attached two upper extremities, two lower extremities, and a head that is attached by means of a neck. Functionally, the body consists of a digestive system, a circulatory system, an excretory system, a respiratory system, a reproductive system, a nervous system, an endocrine system, an integument (skin), a skeleton, and a series of muscles.
Regionally, the trunk can be divided into an upper portion called the chest or thorax, containing ribs, and a lower, rib-free portion called the abdomen. Internally, the thorax and abdomen are separated by a muscular sheet called the diaphragm. The upper extremities of the trunk include the arms, forearms, and hands; the lower extremities include the thighs, lower legs, and feet. The head includes the brain and the major sense organs, such as the eyes and ears; the neck is the narrower, flexible part that connects the head to the trunk. The ventral (front) surface of the abdomen is often divided around the umbilicus into upper-left, upper-right, lower-left, and lower-right quadrants.
Functionally, the body consists of a number of organ systems. The digestive system breaks down foods into simpler substances and absorbs them. The circulatory system transports oxygen and other materials around the body. The excretory system rids the body of many waste products, while the respiratory system rids the body of carbon dioxide and adds oxygen to the blood. The reproductive system produces sex cells and, in females, provides an environment for the development of an embryo. The nervous system sends signals in the form of nerve impulses from one part of the body to another, and the endocrine organs send chemical messengers (hormones) through the bloodstream. The integument, or skin, protects the outer surface of the body from infection, injury, and desiccation (drying out); it also maintains the body’s internal temperature by providing insulation and preventing the body from overheating during exercise through sweating. The skeleton serves as the body’s framework and consists of 206 separate bones; these bones support the body’s other organs and also protect the heart, the lungs, and especially the central nervous system, including the brain and the spinal cord. The muscles contract and produce movements.
Each major organ system is constructed of several major organs. The major organs contained within the thorax, for example, are the heart, lungs, and thymus. The major organs contained within the abdomen include the stomach, spleen, liver, pancreas, small intestine (consisting of the duodenum, ileum, and jejunum), large intestine (consisting of the cecum, colon, and rectum, with the colon further divided into an ascending colon, transverse colon, descending colon, and sigmoid colon), and bladder. The kidneys and urinary ducts lie along the dorsal body wall of the abdomen. Also contained within the lower abdomen are the uterus, ovaries, and fallopian tubes in the female and the vas deferens and prostate gland in the male. In males, two downward extensions of the abdominal cavity form the scrotal sacs that surround the testes.
The thoracic and abdominal cavities, and the scrotal cavities in males, are all considered part of the general body cavity, or coelom. Each part of the coelom is lined on all sides with a thin, single layer of flat (squamous) cells known as the peritoneum. The peritoneum forming the outer wall of these cavities is called the parietal peritoneum; the peritoneum on the outer surface of the internal organs, or viscera, is called the visceral peritoneum.
Each type of organ is made of a number of different tissues. The four major types of tissues are epithelial tissues, connective tissue, muscle tissue, and nervous tissue. Epithelial tissues, or epithelia, include those tissues that originate in broad, flat surfaces; their functions include protection, absorption, and secretion. Epithelia can be single layered (simple) or many layered (stratified). Their cells can be flat (squamous), tall and skinny (columnar), or equal in height and width (cuboidal). Some simple epithelia have nuclei at two different levels, giving the false appearance of different layers; these tissues are called pseudostratified. Some simple squamous epithelia have special names: the inner lining of most blood vessels is called the endothelium, while the lining of the body cavities (including all parts of the coelom) is called the mesothelium. Kidney tubules and most small ducts are also lined with simple squamous epithelia. The pigmented layer of the eye's retina and the front surface of the lens are examples of simple cuboidal epithelia. Simple columnar epithelia form the inner lining of most digestive organs and the linings of the small bronchi and the gallbladder. The epithelia lining the fallopian tubes, nasal cavities, and bronchi are ciliated, meaning that the cells have small, hairlike extensions called cilia.
The outer layer of skin
is a stratified squamous epithelium; other stratified squamous epithelia line the inside of the mouth, esophagus, and vagina. Sweat glands and other glands in the skin are lined with stratified cuboidal epithelia. Most of the urinary tract is lined with a special kind of stratified cuboidal epithelium, called a transitional epithelium, that allows a large amount of stretching. Parts of the pharynx, the larynx, the urethra, and the ducts of the mammary glands are lined with stratified columnar epithelium.
Glands are composed of epithelial tissues that are highly modified for secretion. They may be either exocrine glands, whose secretions exit by ducts to targets nearby, or endocrine glands, whose secretions are carried by the bloodstream to targets some distance away. The salivary glands in the mouth, the glandular lining of the stomach, and the sebaceous glands of the skin are examples of exocrine glands. The thyroid gland, the adrenal gland, and the pituitary gland are examples of endocrine glands. The pancreas has both exocrine and endocrine portions; the exocrine parts secrete digestive enzymes, while the endocrine parts, called the islets of Langerhans, secrete the hormones insulin and glucagon.
Connective tissues are tissues containing large amounts of material called extracellular matrix, located outside the cells. The matrix may be a liquid such as blood plasma, a solid containing fibers of collagen and related proteins, or an inorganic solid containing calcium salts, as in bone. Blood and lymph are connective tissues with a liquid matrix (plasma) that can solidify when the blood clots. In addition to plasma, blood contains red cells (erythrocytes), white cells (leukocytes), and the tiny platelets that help form clots. The many kinds of leukocytes include granular types (basophils, neutrophils, and eosinophils, all named according to the staining properties of their granules), monocytes, and several types of lymphocytes. Lymph contains lymphocytes and plasma only.
Most connective tissues have a solid matrix, which includes fibrous proteins such as collagen and elastic fibers in some cases. If all the fibers are arranged in the same direction, as in ligaments and tendons, the tissue is called regular connective tissue. The dermis of the skin is an example of an irregular connective tissue in which the fibers are arranged in all directions. Loose connective tissue and adipose (fat) tissue both have very few fibers. The simplest type of loose connective tissue with the fewest fibers is sometimes called areolar connective tissue. Adipose tissue is a connective tissue in which the cells are filled with fat deposits.
Hemopoietic (blood-forming) tissue occurs in bone marrow and the thymus, and it contains the immature cell types that develop into most connective tissue cells, including blood cells. Cartilage tissue matrix contains a shock-resistant complex of protein and sugar-like (polysaccharide) molecules. Cartilage cells usually become trapped in this matrix and eventually die, except for those closest to the surface. Bone tissue gains its supporting ability and strength from a matrix containing calcium salts. Its typical cells, called osteocytes, contain many long strands by which these cells exchange nutrients and waste products with other osteocytes and ultimately with the bloodstream. Bone also contains osteoclasts, large cells responsible for bone resorption and the release of calcium into the bloodstream.
Mesenchyme is an embryonic connective tissue made of wandering, amoeba-like cells. During embryological development, the mesenchyme cells develop into many different cell types, including hemocytoblasts, which give rise to most blood cells, and fibroblasts, which secrete protein fibers and then usually differentiate into other cell types.
Muscle tissues are specially modified for contraction. When a nerve impulse is received, overlapping fibers of the proteins actin and myosin slide against one another to produce the contraction. The three types of muscle tissue are smooth muscle, cardiac muscle, and skeletal muscle. The term striated muscle is sometimes used to refer to cardiac and skeletal muscle, both of which have cylindrical fibers marked by cross-bands, which are also called cross-striations. The striations are caused by the lining up of the contractile proteins actin and myosin. Smooth muscle contains cells with tapering ends and centrally located nuclei. Muscular contractions are smooth, rhythmic, and involuntary, usually not subject to fatigue. The cells are not cross-banded. Smooth muscle occurs in many digestive organs, reproductive organs, and skin, as well as in many other organs. Cardiac muscle occurs only in the heart. Its cross-striated fibers branch and come together repeatedly. Contractions of these fibers are involuntary, rhythmic, and without fatigue. Nuclei are located in the center of each cell; cell boundaries are marked by dark-staining structures called intercalated disks. Skeletal muscle occurs in the voluntary muscles of the body. Their cylindrical, cross-striated fibers contain many nuclei but no internal cell boundaries; a multinucleated fiber of this type is called a syncytium. Skeletal muscle is capable of rapid, forceful contractions, but it fatigues easily. Skeletal muscle tissue always attaches to connective tissue structures.
Nervous tissues contain specialized nerve cells called neurons that respond rapidly to stimulation by conducting nerve impulses. All neurons contain RNA-rich granules, called Nissl granules, in the cytoplasm. Neurons with a single long extension of the cell body are called unipolar, those with two long extensions are called bipolar, and those with more than two long extensions are called multipolar. There are two types of extensions: dendrites conduct impulses toward the cell body, while axons generally conduct impulses away from the cell body. Many axons are surrounded by a multilayered fatty substance called the myelin sheath, which is composed of many layers of cell membrane wrapped around the axon.
Nervous tissues also contain several types of neuroglia, cells that hold nervous tissue together. Many neuroglia cells have projections that wrap around the neurons and help nourish them. The many types of neuroglia include the tiny microglia and the larger protoplasmic astrocytes, fibrous astrocytes, and oligodendroglia.
Two major tissue types make up most of the brain and spinal cord, or central nervous system. The first type, gray matter, contains the cell bodies of many neurons, along with smaller amounts of axons, dendrites, and neuroglia cells. The second type, white matter, contains mostly the axons (and sometimes also the dendrites) of neurons whose cell bodies lie elsewhere along with the myelin sheaths that surround many of the axons. Clumps of cell bodies are called nuclei when they are found within the brain and ganglia when they occur elsewhere. Bundles of axons are called tracts within the central nervous system and nerves when they appear peripherally.
The body can be described by the use of directional terms, which are defined in a relative manner according to the location of a given body part or segment. Some important directional terms are superior, inferior, cranial, caudal, dorsal, ventral, medial, lateral, radial, ulnar, anterior, and posterior.
Disorders and Diseases Diseases or disorders that affect the entire body are called systemic or multisystem diseases. For example, fevers or febrile diseases raise the body’s temperature. Many fevers are caused by infectious diseases such as influenza (actually a series of different viral infections). Influenzas cause fever, sore throat, muscle aches, coughs, headache, fatigue, and a general feeling of malaise.
Edema, or tissue swelling, is marked by an increase in the amount of extracellular fluid in several parts of the body at once. In the case of pulmonary edema, the fluid stains pink and fills the usually empty lung spaces (alveoli).
Most cancers are recognized by abnormalities of the cells in which they occur. The most dangerous cancers are marked by large tumors with ill-defined, irregular margins. If the cancer tumor is well defined, is small, and has a smooth, circular margin, then it is much less of a threat. Cancers are especially dangerous when they undergo metastasis, a process by which they produce wandering cells that spread throughout the body.
Juvenile diabetes mellitus (also called diabetes mellitus, type 1, and insulin-dependent diabetes mellitus, or IDDM), like most endocrine disorders, has systemic consequences throughout the body, including damage to nearly all the blood vessels. The primary defect in this disorder is a lack of insulin, which impairs the body’s ability to use glucose. Another endocrine disorder with systemic consequences is Addison’s disease, caused by a deficiency of adrenocorticotropic hormone (ACTH), which normally stimulates the cortex of the adrenal gland. Symptoms include weakness, loss of appetite, fatigue, weight loss, and reduced tolerance to cold. These symptoms result from imbalances in the levels of glucose and mineral salts throughout the body.
Systemic lupus erythematosus, a connective tissue disease, often produces red skin lesions marked by degeneration and flattening of the lower layers of the epidermis, drying and flaking of the outermost layer, dilation of the blood vessels under the skin, and the leakage of red blood cells out of these vessels, adding to the red color. (The word “erythematosus” means “red.”)
Muscular dystrophy has several forms; the most common is marked in its advanced stages by enlarged muscles in which the muscle tissue is replaced by a fatty substance. Another muscular disease, myasthenia gravis, is often marked by overall enlargement of the thymus and an increase in the number of thymus cells. Myocardial infarction, a form of heart disease marked by damage to the heart muscle, is noticed in histological section by dead, fibrous scar tissue replacing the muscle tissue in the heart wall. In patients with arteriosclerosis, the usually elastic walls of the arteries become thicker and more fibrous and rigid; many of the same patients also suffer from atherosclerosis, a buildup of deposits on the inside of the blood vessel, partially or completely blocking blood flow.
In nervous tissue, damage to peripheral nerves often results in a process called chromatolysis in the cell bodies of the neurons from which these axons arise. The nuclei of these cells enlarge and become displaced to one side, while the Nissl granules disperse and the cell body as a whole undergoes swelling. Increased deposits of fibrous tissue characterize multiple sclerosis and certain other disorders of the nervous system. Some of these diseases are also marked by a degeneration of the myelin sheath around nerve fibers. In the case of a cerebrovascular stroke, impaired blood supply to the brain causes degeneration of the neuroglia and is followed by general tissue death and the replacement of the neuroglia by fibrous tissue. Cranial hematoma (abnormal bleeding) results in the presence of blood clots, complete with blood cells and connective tissue fibers, in abnormal locations. Alzheimer’s disease is marked by granules of a protein-like substance called amyloid, often containing aluminum, surrounded by additional concentric layers of similar composition.
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