THE HEART/ CIRCULATORY SYSTEM

The heart and circulation have one primary purpose – to move substances around the body. In very small organisms, such as unicellular creatures, substances such as oxygen, carbon dioxide and digestive products are moved around the organism by diffusion.

Most complex multicellular organisms, however, are too large for diffusion to move substances around their bodies quickly enough. These animals usually have blood to carry vital substances around their bodies and a heart to pump it instead of relying on diffusion. In other words, they have a circulatory system. Some animals have more than one heart – the humble earthworm, for instance, has five.

In insects and some other animal groups, blood circulates in large open spaces. A simple heart pumps blood out into cavities surrounding the animal’s organs. Substances can diffuse between the blood and cells. When the heart muscle relaxes, blood is drawn from the cavity back into the heart, through small valved openings along its length. 

Many animals, including all vertebrates, have a closed circulatory system in which the blood is enclosed within tubes. This generates higher blood pressures as the blood is forced along fairly narrow channels instead of fl owing into large cavities. This means the blood travels faster and so the blood system is more effi cient at delivering substances around the body:

• The blood leaves the heart under pressure and fl ows along arteries and then arterioles (small arteries) to capillaries.

• There are extremely large numbers of capillaries. These come into close contact with most of the cells in the body, where substances are exchanged between blood and cells.

• After passing along the capillaries, the blood returns to the heart by means of venules (small veins) and then veins.

• Valves ensure that blood fl ows only in one direction.

Animals with closed circulatory systems are generally larger in size, and often more active than those with open systems.

Animals with a closed circulatory system have either single circulation or double circulation. Single circulation is found, for example, in fish.

• The heart pumps deoxygenated blood to the gills.

• Here gaseous exchange takes place; there is diffusion of carbon dioxide from the blood into the water that surrounds the gills, and diffusion of oxygen from this water into the blood.

• The blood leaving the gills then fl ows round the rest of the body before eventually returning to the heart.

Note that the blood fl ows through the heart once for each complete circuit of the body.

Birds and mammals, though, have double circulation:

• The right ventricle of the heart pumps deoxygenated blood to the lungs where it receives oxygen.

• The oxygenated blood then returns to the heart to be pumped a second time (by the left ventricle) out to the rest of the body.

This means that the blood flows through the heart twice for each complete circuit of the body. The heart gives the blood returning from the lungs an extra ‘boost’, which reduces the time it takes for the blood to circulate round the whole body.

This allows birds and mammals to have a high metabolic rate, because oxygen and food substances required for metabolic processes can be delivered more rapidly to cells.