Atria: Collect blood from the veins.
Ventricles: Pump blood from the heart at high pressure.
Valves: Only allow blood flower in one direction.
a) Atrio-ventricular valves: Valves between the atria and their corresponding ventricle.
b) Semilunar valves: Valves between the ventricles and the main artery that leaves them.
1. Blood flows into the atria from the veins.
2. The atria contract, forcing the atrio-ventricular valves to open.
3. Blood is forced through the open valves into the ventricles.
4. The semilunar valves are closed, so the ventricles fill with blood.
5. The walls of the ventricles contract powerfully, increasing the pressure within the ventricles.
6. This pressure forces the atrio-ventricular valves to close, and the semilunar valves to open.
7. Blood, under pressure, is forced into the arteries.
8. The atria begin filling again.
9. As pressure in the ventricles drop, the semilunar valves close to prevent backflow of blood into the ventricle. When pressure is further reduced, the atrioventricular valve reopens and blood begins to pass from the atria to the ventricles.
10. The next heartbeat begins as the walls of the atria contract again, repeating the cycle.