 |
 |
 |
 |
 |
 |
 |
 |
 |
  |
The heart is a muscle. Heart muscle is usually called cardiac muscle and like all muscles it contracts and relaxes to produce movement. As the heart contracts it squeezes blood out into arteries which carry the blood off to all parts of the body. When the heart muscle relaxes it becomes like an empty bag which can then fill up with blood that has returned from its journey around the body and arrives at the heart through the veins.
|
|
Deoxygenated blood from the body at low pressure enters the right atrium while oxygenated blood, also at low pressure, from the lungs enters the left atrium. Both atria expand as they fill up. The bicuspid and tricuspid valves are both closed while the atria fill up. As they fill the pressure rises and this forces the valves to open allowing some blood to pass into the ventricles.
The muscles of the atria walls then contract and force the rest of the blood into the ventricles. The muscles of the ventricle walls then contract, increasing the pressure on the blood and forcing the bicuspid and tricuspid valves to close and the aortic and pulmonary valves to open. The blood therefore, leaves the heart to begin its journey round the body.
Having left the heart the blood is at a high pressure and the pressure is now reduced in the ventricles as the muscles contract. This difference in pressure makes the aortic and pulmonary valves close to stop the blood flowing back into the heart.
This is one cardiac cycle and is then repeated continuously.