valve

The structure of a vein, showing how the valves prevent blood from flowing backwards.

In animals, a structure for controlling the direction of the blood flow. In humans and other vertebrates, the contractions of the beating heart cause the correct blood flow into the arteries because a series of valves prevents back flow. Diseased valves, detected as ‘heart murmurs’, have decreased efficiency. The tendency for low-pressure venous blood to collect at the base of limbs under the influence of gravity is counteracted by a series of small valves within the veins. It was the existence of these valves that prompted the 17th-century physician William Harvey to suggest that the blood circulated around the body.

valve

In technology, a device that controls the flow of a fluid. Inside a valve, a plug moves to widen or close the opening through which the fluid passes.

Common valves include the cone or needle valve, globe valve, and butterfly valve, all named after the shape of the plug. Specialized valves include the one-way valve, which permits fluid flow in one direction only, and the safety valve, which cuts off flow under certain conditions.

valve

In brass wind instruments, a mechanism for diverting the air flow through an extension loop, to vary the length and thus the pitch of the instrument. Most valve instruments are of the piston type, but older French horns have rotary valves operated by levers.