How it Works Your Heart

The normal heart is a strong, hard-working pump made of muscle tissue. It's about the size of a person's fist. The heart has four chambers. The upper two chambers are the right artium and left atrium, and the lower two are the right ventricle and left ventricle (see Figure A in diagram). Blood is pumped through the chambers, aided by four heart valves. The valves open and close to let the blood flow in only one direction. The four heart valves are:
Each valve has a set of "flaps" (also called leaflets or cusps). The mitral valve normally has two flaps; the others have three flaps.

Dark bluish blood, low in oxygen, flows back to the heart after circulating through the body. It returns to the heart through veins and enters the right atrium. This chamber empties blood through the tricuspid valve (B) into the right ventricle.
The right ventricle pumps the blood under low pressure through the pulmonary valve into the pulmonary artery. From there the blood goes to the lungs where it gets fresh oxygen (C). After the blood is refreshed with oxygen, it's bright red. Then it returns by the pulmonary veins to the left atrium. From there it passes through the mitral valve (D) and enters the left ventricle.
The left ventricle pumps the red oxygen-rich blood out through the aortic valve into the aorta (E). The aorta takes blood to the body's general circulation. The blood pressure in the left ventricle is the same as the pressure measured in the arm.