Key Biological Principle: Why Have a Heart and Circulation?:

• Heart and circulation have one primary purpose - to move substances around the body
• In very small organisms, substances such as oxygen, carbon dioxide etc. move around the organism via diffusion
• Diffusion is the movement of molecules or ions from a region of high concentration to low concentration
• More complex organisms are too large for diffusion to move substances quickly enough
• These animals have blood to carry vital substances around their bodies and a heart to pump it - a circulatory system

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 animal's organs
• When the heart muscle relaxes, blood is drawn from the cavity back into the heart, through small valved openings along its length

Closed Circulatory Systems:

• Vertebrates have a circulatory system where the blood is enclosed within tubes
• This generates higher blood pressures as the blood is forced along narro channels instead of flowing into large cavities
• The blood leaves the heart under pressure and flows along arteries and arterioles (smaller arteries) to capillaries
• There are many capillaries - these come into close contact with most body cells, where substance exchange takes place 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 the blood flows only in one direction

Single Circulatory Systems:
This type of circulation is found in (e.g.) fish:

• The heart pumps deoxygenated blood to the gills
• Here gaseous exchange takes place; there's diffusion of carbon dioxide from the blood into the water surrounding the gills, and diffusion of oxygen from this water to the blood
• The blood leaving the gills then flows round the rest of the body before eventually returning to the heart
• Note that the blood flows through the heart once for each complete circuit of the body

Double Circulatory Systems:
Birds and mammals 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 left ventricle) out to the rest of the body
• This means the blood flows through the heart twice for each complete circuit of the body
• The heart gives the blood returning from the lungs and extra 'boost', which reduces the time it takes for the blood to circulate round the 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