Circulatory System

Exam code: 5090

Written by: Ashika|Reviewed by: Caroline Carroll|Updated 2 July 2026

The circulatory system is a system of blood vessels with a pump and valves to ensure one-way flow of blood

An example of a double circulatory system

Fish have a two-chambered heart and a single circulation
This means that for each circuit of the body, the blood passes through the heart once

The single circulatory system in fish

Mammals have a four-chambered heart and a double circulation
This means that for each circuit of the body, the blood passes through the heart twice
The right side of the heart receives deoxygenated blood from the body and pumps it to the lungs (the pulmonary circulation)
The left side of the heart receives oxygenated blood from the lungs and pumps it to the body (the systemic circulation)

The double circulatory system in mammals

Blood travelling through the small capillaries in the lungs loses a lot of pressure that was given to it by the pumping of the heart, meaning it cannot travel as fast
By returning the blood to the heart after going through the lungs its pressure can be raised again before sending it to the body, meaning cells can be supplied with the oxygen and glucose they need for respiration faster and more frequently
Because the left side of the heart has to pump the blood much further and, in some cases, against gravity (e.g. to the human brain), it is advantageous to increase its pressure in order to reach those areas with sufficient flow and supply of oxygen and nutrients

Double Circulation