The mammalian circulatory system

Mammals have small surface area to volume rations. A circulatory system is necessary to transport materials to and from the large volume of metabolically active tissue.

In mammals, during one complete pathway around the body, the blood flows through the heart twice. One circuit going from the heart to the lungs and back is pulmonary circulation. The other circuit from the heart to the rest of te body is the systematic circulation. Since there are two circuits, this is called a double circulatory system. Blood is pumped twice by the heart, from the right ventricle to the lungs and the left ventricle to the rest of the body.

These two chambers both have thick walls made of cardiac muscle, but the muscle of the left ventricle is thicker than the muscle of the right ventricle. Therefore, the blood pumped by the left ventricle is pushed harder and is under greater pressure than blood from the right ventricle. This two-pressure system has advantages:

• The low pressure in the pulmonary circulation pushes blood slowly to the lungs allowing more time for gas exchange. In addition, high pressure is not necessary to pump the blood over the shorter distances involved.
• The high pressure in the systematic circulation ensures blood is pumped to all the other body organs and allows tissue fluid to form in each organ.

The heart needs its own supply of blood - the coronary circulation - to provide cardiac muscle with oxygen and nutrients. The coronary arteries arise from the base of the aorta.

The heart

Structure of the heart

The heart is a highly specialised muscular organ with the function of pumping blood through the body. As mammals have a double circulation, the heart is really two pumps, with each side pumping blood through two separate circulatory systems. 

The two sides of the heart are separated by a thick muscular wall - the septum - that runs through the centre of the heart. Each 'pump' has an upper chamber, the atrium, and a lower chamber, the ventricle.

• Atria - relatively thin walled, as they receive blood from the lungs (left) or the body (right) and pump the blood into the ventricles.
• Ventricles - have much thicker walls as the pump blood to the lungs (right) or around the body (left). As the lungs are only a few centimetres from the heart, the right ventricle does not have to pump with as much force as the right ventricle which has to pump the blood all around the body.

The blood leaves the heart in 'pulses' that coincude with each heartbeat and it functions as a one-way pump. Valves prevent the backflow of blood and there are two types:

• The atrioventricular (tricuspid and bicuspid) valves - lie between the atria and ventricles, and prevent the backflow of blood back…