Circulatory system

The circulatory system moves blood through the body. It consists of a heart, blood vessels and valves.

The heart acts as a pump. It pushes blood into the vessels, which provide pathways for blood to flow through the body and back to the heart. The heart can then pump the blood through the vessels again.

Valves help keep blood flowing in one direction. They allow blood to pass forwards but close if it starts to flow backwards.

Single circulation in fish

In a fish, blood travels from the heart to the gills, then to the rest of the body, before returning to the heart. This route is called a single circulation because blood passes through the heart once during each complete circuit.

The heart pumps oxygen-poor blood to the gills, where the blood takes up oxygen. From the gills, the blood flows directly to the rest of the body, without first returning to the heart. It delivers oxygen to body tissues, then returns to the heart to begin another circuit.

The route is heart → gills → body → heart.

Double circulation in mammals

In mammals, blood travels through two connected circuits. During one complete circuit, it passes through the heart twice: once on its way to the lungs and once on its way to the rest of the body. This arrangement is called double circulation.

1. In the pulmonary circuit, the heart pumps deoxygenated blood to the lungs. There, the blood picks up oxygen and returns to the heart.
2. In the systemic circuit, the heart pumps this oxygenated blood to the rest of the body. Body tissues take up oxygen, and the deoxygenated blood returns to the heart.

The route is therefore: heart → lungs → heart → body → heart.

Separation and pressure in double circulation

The right side of the heart pumps deoxygenated blood to the lungs, while the left side pumps oxygenated blood to the body. The two sides are separated, so oxygenated and deoxygenated blood do not mix in the heart.

Because the blood returns to the heart after passing through the lungs, it can be pumped again at a higher pressure for its journey around the body. This supplies body tissues rapidly. Blood is pumped to the lungs at a lower pressure, which helps protect their delicate blood vessels.