Lymph

Lymph, common name for the fluid carried in the lymphatic system. Lymph is diluted blood plasma containing large numbers of white blood cells, especially lymphocytes, and occasionally a few red blood cells. Because of the number of living cells it contains, lymph is classified as a fluid tissue.

Lymph diffuses into and is absorbed by the lymphatic capillaries from the spaces between the various cells constituting the tissues. In these spaces lymph is known as tissue fluid, plasma that has permeated the blood capillary walls and surrounded the cells to bring them nutriment and to remove their waste substances. The lymph contained in the lacteals of the small intestine is known as chyle.

The synovial fluid that lubricates joints is almost identical with lymph, as is the serous fluid found in the body and pleural cavities. The fluid contained within the semicircular canals of the ear, although known as endolymph, is not true lymph.

Corticoid

Corticoid, also corticosteroid, any steroid hormone that derives from the outer layer, or cortex, of the adrenal gland. Among the corticoids are the glucocorticoids, such as cortisone and cortisol (see Hydrocortisone), essential to metabolism and the body's reaction to stress; and the mineralocorticoids, which have other important regulatory functions, such as maintaining ion balance. The adrenal cortex also secretes sex hormones, although these are produced in much greater quantities by the testes and ovaries. See Endocrine System; Steroids.

Heredity

Heredity, process of transmitting biological traits from parent to offspring through genes, the basic units of heredity. Heredity also refers to the inherited characteristics of an individual, including traits such as height, eye color, and blood type.

Heredity accounts for why offspring look like their parents: when two dogs mate, for example, they have puppies, not kittens. If the parents are both Chihuahuas, the puppies will also be Chihuahuas, not great Danes or Labrador retrievers. The puppies may be a little taller or shorter, a little lighter or a lot heavier than their parents are. Their faces may look a little different, or they may have different talents and temperaments. In all the important characteristics, however—the number of limbs, arrangement of organs, general size, fur type—they will share the traits of their parents. The principles of heredity hold true not only for a puppy but also for a virus, a roundworm, a pansy, or a human.

Adenoids

Adenoids, lymphoid tissue at the back of the throat, which usually shrinks and disappears by adolescence. Enlargement of this tissue, however, is fairly common in children and may interfere with breathing. Symptoms of enlarged adenoids include a nasal voice, persistent breathing through the mouth, snoring, and restless sleep. Formerly these tissues were routinely removed in children, because it was thought that inflamed adenoids led to recurrent infections and colds. More recently, medical science has recognized this condition as usually benign, and the number of adenoidectomies has consequently declined. See also Tonsil.

Sex Hormone

Sex Hormone, any of several chemical substances that affect the development and functioning of the reproductive system in vertebrates, or animals with a backbone.

The sex hormones are divided into three major groups: gonadotropins, gonadal hormones, and lactogens. Gonadotropins stimulate the gonads, which are sperm- or egg-producing organs. The male gonads are the testes, which produce sperm, and the female gonads are the ovaries, which produce eggs. Gonadotropins are secreted by the pituitary gland, which is located in the center of the brain and is controlled by an area of the brain known as the hypothalamus. Gonadotropins, such as the leutinizing hormone (LH) and the follicle-stimulating hormone (FSH) in females and the interstitial cell-stimulating hormone (ICSH) in males, control the development and functions of the ovaries and testes, including menstruation and sperm production. Gonadal hormones such as estrogen, progesterone, and testosterone are secreted primarily by the testes and ovaries, placenta (the sac of nutritive tissue that supports and protects the fetus), and adrenal glands. Their chief function is to regulate the development of the secondary sex characteristics such as deepening of the voice in males and distribution of body hair. The third group, the lactogens, are secreted by the pituitary gland and are necessary for the secretion of milk in the mammary glands of mammals. They are also believed to affect maternal behavior patterns.

Epinephrine

Epinephrine, hormone secreted by the adrenal gland. The pure compound, first isolated by the Japanese chemist Jokichi Takamine, is also known as adrenaline. It was formerly prepared from adrenal glands, but it is now made synthetically.

Epinephrine normally is present in the bloodstream in minute quantities. In times of excitement or emotional stress, however, large additional quantities are secreted, exerting a marked effect on body structures in preparation for physical exertion. For example, epinephrine stimulates the heart, constricts the small blood vessels, raises the blood pressure, liberates sugar stored in the liver, and relaxes certain involuntary muscles while contracting others. It is widely used as a drug to stimulate the heart in cases of shock, to prevent bleeding, and to expand lung bronchioles in acute asthma attacks.

The adrenal gland also secretes a substance chemically related to epinephrine, called norepinephrine, noradrenaline, or levoarterenol. In general, the function of norepinephrine seems to be the maintenance of normal blood circulation. It is also the chemical agent responsible for transmission of nerve impulses in the autonomic nervous system. Large amounts of epinephrine and norepinephrine are produced by some tumors of the adrenal glands, resulting in a great increase in blood pressure.

Bile

Bile, bitter, neutral, or slightly alkaline fluid secreted by the liver and passed through a duct into the gallbladder, where it is stored and, as necessary, released into the duodenum. As formed in the liver, bile is a thin, watery fluid to which the gallbladder adds a mucous secretion, forming a complex thickened and stringy substance consisting of salts and bile salts, proteins, cholesterol, hormones, and enzymes. The gallbladder returns water containing salts and other materials to the circulation and concentrates the complex further by a tenfold reduction of the bile salts, which the liver synthesizes from cholesterol. Such foods as fats, egg yolk, and foods rich in cholesterol cause concentrated bile, together with secretions from the pancreas, to be discharged into the duodenum to promote digestion, to stimulate peristalsis and absorption, and to carry off excess cholesterol and the disintegration products of overage red blood cells. The hemoglobin of such disintegrating cells degrades rapidly into reddish-yellow bilirubin, predominant in the bile of carnivorous and omnivorous animals, and biliverdin, a green pigment that appears in the bile of herbivores. Under normal conditions, the liver efficiently clears these pigments.

Certain conditions create an inability to excrete bile, and this may create serious disabilities, such as jaundice. In obese and immobilized persons, in pregnant women, and in cases of obstruction of flow of bile, gallstones may be formed by precipitation of bilirubin in combination with calcium and cholesterol. Stasis frequently coexists with inflammation and infection of the gallbladder; this may alter the concentration of bile constituents and create debris around which bile and its components may precipitate to form gallstones, which may then block the common bile duct to reduce or stop the flow of bile. Inflammation and infection, together with the consequent regurgitation of bile into the liver, may damage that organ, sometimes causing cirrhosis.