http://www.ck12.org Chapter 1. Human Biology
FIGURE 1.41
The atria receive blood and the ventricles
pump blood out of the heart.Blood Flow Through the Heart
Blood flows through the heart in two separate loops. You can think of them as a “left side loop” and a “right side
loop.” The right side of the heart collects oxygen-poor blood from the body and pumps it into the lungs, where it
releases carbon dioxide and picks up oxygen. The left side carries the oxygen-rich blood back from the lungs into
the left side of the heart, which then pumps the oxygen-rich blood to the rest of the body. The blood delivers oxygen
to the cells of the body and returns to the heart oxygen-poor.
To move blood through the heart, the cardiac muscle needs to contract in an organized way. Blood first enters the
atria (Figure1.42). When the atria contract, blood is pushed into the ventricles. After the ventricles fill with blood,
they contract, and blood is pushed out of the heart.
So how is the blood kept from flowing back on itself?Valves(Figure1.42) in the heart keep the blood flowing in
one direction. The valves do this by opening and closing in one direction only. Blood only moves forward through
the heart. The valves stop the blood from flowing backward. There are four valves of the heart.
- The two atrioventricular (AV) valves stop blood from moving from the ventricles to the atria.
- The two semilunar (SL) valves are found in the arteries leaving the heart, and they prevent blood from flowing
back from the arteries into the ventricles.
Why does a heart beat? The “lub-dub” sound of the heartbeat is caused by the closing of the AV valves ("lub") and
SL valves ("dub") after blood has passed through them.
Vocabulary
- atrium: Small, thin-walled chamber at the top of the heart where the blood first enters.
- valve: Structure in the heart that prevents back-flow and keeps the blood flowing in one direction.
- ventricle: Muscular V-shaped chamber that pumps blood out of the heart.