1.1 how does the circulation work?

1.1 biology as level

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what is cardiovascular disease?

Cardiovascular diseases (CVDs) are diseases of the heart and circulation.

The main forms of cardiovascular diseases are coronary heart disease and strokes.

coronary heart disease is the most common cause of death in the UK.

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why have a heart and circulation?

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 and carbon dioxide are moved around the body by diffusion.

Diffusion is the movement of molecules or ions from a region of their high concentration to a region of their low concentration, by relatively slow random movement.

most complex multicellular organisms are too large for diffusion to work efficiently. These animals usually have blood to carry substances around the body and a heart to pump it instead of relying on diffusion. In other words, they have a circulatory system.

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Open circulatory systems

In insects and some other animals blood circulates in large open spaces, A simple heart pumps blood out into cavities surrounding the animals organs. This is an open circulatory system.

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 opening along its length.

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closed circulatory systems

Many animals including all vertebrates, have a closed circulatory system in which the blood is enclosed within tubes.

This creates higher blood pressure as the blood flows along narrow channels. This means the blood travels faster and the blood system is more effiecient at moving substances around the body.

- The blood leaves the heart under pressure and flows along arteries and then arteioles (small arteries) to capillaries.

- There are a large amount of capillaries which come into close contact with most of the cells in the body, where substances are exchanges 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 flows in only one direction.

Animals with closed systems and generally larger in size and more active.

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single circulatory systems

Animals with a closed circulatory system either have single or double circulatory systems.

single circulation is found, for example, in fish.

- The heart pumps deoxygenated bloos to the gills.

- Here gaseus exchange takes place, carbon dioxide diffuses into the water and oxygen from the water diffuses into the blood.

- The blood leaving the gills then flows round the restof the body before returning to the heart.

Note that the blood flows through the heart only once for each complete circuit of the body.

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double circulatory systems

Birds and mammals have double circulatory systems.

- the right ventricle of the heart pumps deoxiginated blood  to the lungs where  it receives oxygen.

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

Note 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 blody.

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.

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