valve / valv/ • n. a device for controlling the passage of fluid through a pipe or duct, esp. an automatic device allowing movement in one direction only. ∎ (in full ther·mi·on·ic valve) Electr. British term for thermionic tube. ∎ Mus. a cylindrical mechanism in a brass instrument that, when depressed or turned, admits air into different sections of tubing and so extends the range of available notes. ∎ Anat. & Zool. a membranous fold in a hollow organ or tubular structure, such as a blood vessel or the digestive tract, that maintains the flow of the contents in one direction by closing in response to any pressure from reverse flow. ∎ Zool. each of the halves of the hinged shell of a bivalve mollusk or brachiopod, or of the parts of the compound shell of a barnacle. ∎ Bot. each of the halves or sections into which a dry fruit (esp. a pod or capsule) dehisces. DERIVATIVES: valved adj. [in comb.] a branchiopod has a two-valved outer covering valve·less adj.
valve, device for controlling the flow of fluids (liquids and gases). Valves vary in construction and size depending upon their function. Some are classified according to their method of operation or design, e.g., butterfly, gate, globe, lift, needle, piston, and slide valves. Valves are also named for the functions they perform, e.g., check valve (which permits flow in one direction only) and cutoff, bypass, exhaust, intake, safety (see safety valve), and throttle valves. Valves are operated automatically, by hand, or by special mechanism. Valves are employed in the carburetor, diesel engine, internal-combustion engine, pump, and steam engine. In Great Britain an electron tube may be referred to as a valve. In anatomy and physiology the term valve includes the flaps of tissue that help to control the direction of the flow of blood in the heart.
1. Any of various structures for restricting the flow of a fluid through an aperture or along a tube to one direction. Valves in the heart (see bicuspid valve; semilunar valve; tricuspid valve), veins, and lymphatic vessels consist of two or three flaps of tissue (cusps) fastened to the walls. The cusps are flattened to the walls to allow the normal passage of blood or lymph, but a reverse flow causes them to block the vessel or aperture, so preventing further backflow.
2. Any of the parts that make up a capsule or other dry fruit that sheds its seeds.
3. One of the two halves of the cell wall of a diatom.
4. Either of the two hinged portions of the shell of a bivalve mollusc.