A double circulation
Mammals have a double circulatory system: blood passes through the heart twice for each complete circuit of the body. The pulmonary circulation carries deoxygenated blood from the heart to the lungs and returns oxygenated blood; the systemic circulation carries oxygenated blood from the heart to all other organs and returns deoxygenated blood. Two circuits keep pressure high in the body while protecting the delicate lung capillaries.
Key idea
The right side of the heart pumps deoxygenated blood to the lungs; the left side pumps oxygenated blood to the body. The two sides are separated by the septum, so the bloods never mix.
Structure of the heart
The heart has four chambers: two thin-walled atria on top and two thick-walled ventricles below. Blood returns to the atria, which pump it into the ventricles; the ventricles then pump blood out of the heart. The left ventricle has a thicker muscular wall than the right because it must generate a higher pressure to force blood all around the body, whereas the right ventricle only pumps to the nearby lungs.
Valves and the cardiac cycle
Valves ensure blood flows in one direction only. The atrioventricular valves (bicuspid on the left, tricuspid on the right) prevent backflow from ventricles to atria; the semilunar valves prevent backflow from the arteries into the ventricles. Valves open and close because of pressure differences on either side.
Remember
- Ventricles have thicker walls than atria.
- The left ventricle wall is thicker than the right.
- The coronary arteries supply the heart muscle itself with oxygen.
- A faster heart rate during exercise delivers more oxygen and glucose to muscles.