vitamin An organic substance that naturally occurs in plants or animal tissue that is essential for normal metabolism of the body and to life. Vitamins play a part in dozens of crucial activities in the body: Some are antioxidants, preventing oxidation of cells and potentially preventing cancer; others permit or deny chemical reactions involved in…
vitamin therapy The use of vitamins to prevent or cure disease, often as a complementary therapy to accompany medication or other treatments. One variant on this theme, megavitamin therapy, is still rather controversial. It is important to consult a physician before adding vitamin supplements to a health regimen. See also Appendix C, “Vitamins.”
vitelline duct See yolk stalk.
vitiligo A condition in which the skin turns white due to the loss of pigment from the melanocytes, cells that produce the pigment melanin that gives the skin color. In vitiligo, the melanocytes are destroyed, leaving depigmented patches of skin. The hair that grows in areas affected by vitiligo may also turn white. The skin…
vitreous humor A clear, jelly-like substance that fills the middle of the eye.
vocal cord One of the two small bands of muscle that form a V-shape within the larynx. When a person breathes, the vocal cords relax, and air moves through the space between them without making a sound. When a person talks or sings, the vocal cords tighten up and move closer together. Air from the…
void To urinate. The term void is also sometimes used to indicate the elimination of solid waste (defecation).
volar Pertaining to the palm or the sole. For example, the volar surface of the forearm is the portion of the forearm that is on the same side as the palm of the hand.
volume, stroke See stroke volume.
voluntary Done in accordance with the conscious will of the individual. The opposite of voluntary is involuntary. The terms voluntary and involuntary apply to the human nervous system and its control over muscles. The nervous system is divided into two parts: somatic and autonomic. The somatic nervous system operates the skeletal muscles, which are under…
volvulus Abnormal twisting of a portion of the gastrointestinal tract, usually the intestine, which can impair blood flow. Volvulus can lead to gangrene and death of the involved segment of the gastrointestinal tract, intestinal obstruction, perforation of the intestine, and peritonitis. The stomach, small intestine, cecum, and sigmoid colon are all subject to volvulus. Malrotation…
vomit 1 Matter from the stomach that is ejected in tandem with symptoms of nausea. When vomit is reddish or coffee-ground colored, it indicates serious internal bleeding. Also known as vomitus. 2 To expel vomit. Also known as emesis.