Blood enters the heart through the atria. Blood from two vena cavae enters the right atrium. Blood from the pulmonary veins enters the left atrium.The atria fill, followed by the ventricles. Blood is prevented from flowing back into the atria by heart valves.
Humans have a double circulatory system. The heart pumps blood through two circuits:
The pulmonary circulation transports blood to the lungs. At the lungs:
The systemic circulation transports:
The systemic circulation is under high pressure - it has to deliver blood to the extremities of the body.
The pulmonary circulation is under lower pressure, because:
Blood passes through the heart twice in one complete circulation of the body.
Following the blood through one complete circulation of the body:
Note that although this is shown as a sequence, the atria contract at the same time. When blood has been delivered to the ventricles and they are full, they also contract at the same time. Part of the blood is being delivered to the body, while the remainder of the blood is being transported to the lungs. This means that oxygen can be picked up from the lungs, while at the same time, blood is also being delivered to the body.