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School Biology notes: Human circulatory system: Part 1. Heart and lungs

The human double circulatory system 1. Explaining the double circulatory system involving the heart and lungs and its advantages

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There are various sections to work through,  after 1 they can be read and studied in any order.

Index of notes on human circulatory system: heart, lungs & blood vessels

1. A double circulatory system - advantages

Oxygen is needed to produce energy from respiration chemistry to power the cells of any organism. Carbon dioxide is the waste product from respiration. So, a gas exchange system is required - this takes place in the alveoli of the lungs.

This is described on the Gas exchange in the lungs by diffusion page, so here we concentrate on the double circulatory system and how the heart works.

The circulatory system transports substances around the body

We humans, like other mammals, have a double circulatory system consisting of two closed circuits joined together with the heart acting as the central pump.

From the right ventricle of the heart, the first circuit pumps deoxygenated blood to the lungs to absorb fresh oxygen before being returned to the heart (right diagram).

Deoxygenated means the haemoglobin molecules in red blood cells are not carrying oxygen molecules, therefore the blood itself, is not carrying oxygen.

Diagram note: Deoxygenated blood is not blue, veins look blue due to an optical effect of the skin. Both oxygenated and deoxygenated blood are shades of red-pink, but using blue helps you follow the circulation diagrams!

Oxygenated means the haemoglobin molecules in red blood cells are carrying oxygen molecules, therefore the blood itself, is carrying oxygen.

In the second  circuit, the left ventricle of the heart pumps oxygenated blood from the lungs to all the organs and associated structures (the rest of the body except for the lungs) and returns it to the heart (left diagram).

After circulating round the body discharging its oxygen to all the cells of the body, the heart pumps the blood back to the lungs to be re-oxygenated.

Blood is pumped out of the heart under high pressure into the arteries.

The blood flows through the arteries to the capillaries and then the veins, in doing so , the pressure decreases.


The whole double circulation system is pretty complicated (right diagram).

The diagram is a simplified diagram of the double circulatory system of the human body as an example of the mammalian circulation structure.

Apart from the lungs, the blood vessel system must pass near all the rest of the exchange surfaces too e.g.

the villi in the gut (small intestine) to absorb food molecules and water,

the kidneys, where the blood is filtered to remove urea,

and the liver filters the blood from the digestion system e.g. one of its functions is the detoxification of various chemicals.

The advantages of the mammalian double circulatory system


Because the blood is returned to the heart after its absorbed oxygen, it can be pumped out around the body at a higher pressure AND at a faster rate.

This increases the rate of blood flow increasing the supply of oxygen to all the tissues and organs of the body.

This is vital for mammals to maintain their optimum body temperature - being warm blooded animals.

Also, the oxygenated blood is flows separately from the deoxygenated blood.

Note on fish:

Fish have a single circulatory system in which deoxygenated blood from the fish's body is pumped to the heart, which then pumps it through the gills to absorb oxygen from the water and round through the rest of the body in one continuous loop - just one circuit in operation (unlike the double circulatory system of mammals).

This single circulatory system is fine for cold blooded animals like fish, but not for warm blooded mammals.

For more on fish gills see

Other examples of exchange surfaces for substances in animals

Summary of learning objectives and key words or phrases

Be able to interpret diagrams explaining the double circulatory system of the heart and lungs in humans and other mammals and be able to explain the advantages of a  double circulatory system.



Index of biology notes on the human circulatory system - the heart, lungs and blood vessels

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