Why a pump needs valves
A pump that pushes equally in both directions moves nothing. To make blood go forward only, the heart has four valves — flaps of thin, strong tissue that swing open when blood pushes the right way and slam shut the instant it tries to flow back. They are passive: no muscle pulls them open or closed; pressure difference alone moves them.
The four valves split into two groups. Two sit between an atrium and a ventricle (the atrioventricular valves), and two guard the exits where the ventricles empty into the great arteries (the semilunar valves). Knowing which is which makes the whole layout click.
Where each valve sits
- Tricuspid valve — between the right atrium and right ventricle (it has three flaps, hence “tri-”).
- Pulmonary valve — at the exit of the right ventricle into the lung artery.
- Mitral valve — between the left atrium and left ventricle (it has two flaps).
- Aortic valve — at the exit of the left ventricle into the aorta.
Tethers, and the sounds valves make
The two atrioventricular valves face a problem: when the ventricle squeezes hard, what stops the flaps from flipping backwards into the atrium like an umbrella in the wind? The answer is a set of fine cords, the chordae tendineae, anchored to small muscular bumps on the ventricle wall called papillary muscles. They act like the guy-lines of a parachute, holding the valve flat against the back-pressure.
Valves closing are what you actually hear through a stethoscope. The familiar “lub-dub” is two events: the first heart sound (“lub”) is the two atrioventricular valves snapping shut as the ventricles begin to squeeze; the second heart sound (“dub”) is the aortic and pulmonary valves closing as the ventricles relax. When a valve doesn't open or close cleanly, blood flows turbulently and you may hear a heart murmur.