A level Biology exam revision resources written by A level Examiners





 

There are three main components that make up mammalian circulatory systems<:

  • The heart
  • Blood vessels and
  • Blood

The circulation of mammals is referred to as a double circulation. This means that for each round trip of the body, the blood passes through the heart twice - the circuit of blood can be thought of as :

heartlungsheartbody

The blood therefore is pumped through a capillary bed, which slows it down and reduces it's pressure, then receives another pump, before it enters another capillary bed. Double circulatory systems are therefore high pressure systems.

Fish have a single circulatory system which is effectively a low pressure system - where the sequence is :

heartgillsbody,

which means the blood passes through two capillary beds before being pumped again. Below are figures showing the single circulatory system found in fish, and the double circulatory systems found in mammals.

Figure 1 : The single circulatory system found in fish
The single circulatory system found in fish
Figure 2 : The double circulatory systems found in mammals
The double circulatory systems found in mammals

The two circulations which make up the double circulatory system are:

  1. The Pulmonary Circulation

    This is the circulation of blood from the heart to the lungs, and back to the heart again.

    Deoxygenated blood returning to the heart from the body system is pumped by the right ventricle via the pulmonary artery. In the lungs the blood becomes oxygenated before returning to the left atrium via the pulmonary veins.

  2. The Systemic Circulation

    This is the circulation of blood to all parts of the body (except the lungs) and back again to the heart.

    The oxygenated blood from the left atrium is pumped from the left ventricle to all parts of the body via the aorta. The aorta is the body's main artery, and branches to supply all the body's organs, including the heart via the coronary arteries, with oxygenated blood.

As can be seen from figure 2, the right hand side of the heart supplies blood to the lungs. This branch of the circulatory system is known as the pulmonary circulation. The left hand side of the heart supplies blood to the rest of the body, and this branch of the circulatory system is known as the systemic circulation.



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