Circulatory System (Cambridge O Level Biology)

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

Circulatory System Diagram

An example of a double circulatory system

Circulatory Systems in Fish

• 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

Single Circulation Diagram

The single circulatory system in fish

Circulatory systems in Mammals

• 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)

Double Circulation Diagram

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