The normal heart is a strong, hardworking pump made of muscle tissue. It's about the size of a person's fist. The heart has four chambers.
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. 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. The right ventricle pumps the blood under low pressure through the pulmonary valve into the pulmonary artery. After the blood is refreshed with oxygen, it's bright red.