Blood flows continuously through the body’s blood vessels. The heart is the pump that makes this possible. The right and left sides of the heart work together. The steps described in the following table are repeated over and over again, causing blood to flow continuously to the heart, lungs, and body.
| Right side of the heart | Left side of the heart |
|---|---|
| 1. Blood enters the heart through the superior and inferior vena cava, emptying oxygen-poor blood from the body into the right atrium. | 1. Oxygen-rich blood is returned to the heart through the pulmonary veins into the left atrium. |
| 2. The right atrium contracts and pumps blood through the open tricuspid valve into the right ventricle. | 2. The left atrium contracts and pumps blood through the open mitral valve into the left ventricle. |
| 3. When the right ventricle is full, the tricuspid valve closes. This prevents blood from flowing backward into the atria while the ventricle contracts. | 3. When the left ventricle is full, the mitral valve shuts. This prevents blood from flowing backward into the atrium while the ventricle contracts. |
| 4. As the right ventricle contracts, blood leaves the heart through the pulmonary valve via the pulmonary trunk to the right and left pulmonary arteries and into the lungs, where it is oxygenated. | 4. As the left ventricle contracts, blood leaves the heart through the aortic valve into the ascending aorta, where it is distributed to the rest of the body. |