The circulatory system — Science, 11–13 years
The heart and blood vessels form a transport network that supplies cells and carries waste away.
What does the circulatory system do?
The circulatory system is a moving transport network. The heart pumps blood through arteries, tiny capillaries and veins; blood carries oxygen, nutrients and heat to cells, then carries carbon dioxide and other wastes away. Different parts of the network have different jobs, but they work as one continuous circuit.
Why do we need a circulation system?
In a large body, many cells are too far from the lungs and intestines to receive supplies by diffusion alone. The heart-and-vessel network solves this distance problem by bringing materials close to every cell. It also collects wastes quickly, so cells can keep releasing energy and carrying out their jobs.
Estimating blood pumped per minute
Suppose a resting heart beats 70 times each minute and pumps about 70 millilitres per beat. Multiply: 70 beats × 70 mL = 4,900 mL each minute. Since 1,000 mL equals 1 litre, that is 4.9 litres per minute. This is an estimate: exercise can make both heart rate and the amount per beat rise.
Arteries are not defined by oxygen
A reasonable shortcut is to say that arteries always carry oxygen-rich blood and veins always carry oxygen-poor blood. It works for most of the body, so the rule feels safe. The real distinction is direction: arteries carry blood away from the heart, while veins carry it towards the heart. Pulmonary vessels are the important exception to the oxygen shortcut.
The circulation in health
A pulse is the pressure wave made when the heart pushes blood into an artery, so counting it gives an estimate of heart rate. Doctors also measure blood pressure because unusually high or low pressure can signal a problem. During exercise, muscles need more oxygen, so the heart and breathing system work faster to support them.
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