Double Circulatory System (Pulmonary and Systemic)

The human circulatory system is a double circulatory system. This means it consists of two separate circuits: the pulmonary circuit and the systemic circuit.
In the pulmonary circuit, deoxygenated blood from the body is pumped into the lungs by the right side of the heart. Here, the blood unloads carbon dioxide (a waste product) and picks up oxygen. This process is known as pulmonary ventilation.
After oxygenation in the lungs, the blood returns to the left side of the heart. This oxygen-rich blood is then ready to be pumped around the systemic circuit.
The systemic circuit carries the oxygenated blood from the left side of the heart to all other parts of the body. Here, the oxygen and nutrients are unloaded to the body cells, and waste products like carbon dioxide are picked up.
The deoxygenated blood from the systemic circuit returns to the right side of the heart, and the cycle begins again.
The oxygenated blood (from the systemic circuit) and the deoxygenated blood (from the pulmonary circuit) never mix, minimising the risk of oxygen-poor blood being transported around the body.
This double circulation system allows for efficient blood flow, as the blood is quickly oxygenated and then promptly delivered to the tissue cells that need it.
It’s important to understand the two different types of blood vessels involved in these circuits. Arteries carry blood away from the heart. In general, arteries involved in the systemic circuit carry oxygenated blood while those involved in the pulmonary circuit carry deoxygenated blood. Veins, on the other hand, carry blood towards the heart.
The one exception to this rule is the pulmonary artery and pulmonary vein. The pulmonary artery carries deoxygenated blood from the heart to the lungs, while the pulmonary vein carries oxygenated blood from the lungs back to the heart.
Valves in the heart ensure blood flows in the correct direction, preventing back-flow.