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Oxygenated blood

Blood which is carrying oxygen in the form of oxyhaemoglobin

Double circulation

A circulation made up of two separate systems, one flowing from the heart to the lungs and back and the other flowing around the rest of the body

Deoxygenated blood

Blood which has given up the oxygen it was carrying to the tissues


A large organ in the upper abdomen which manufactures, stores and breaks down substances as required by the body


A common term for the digestive system.

The circulatory system

The body transport system

A large multicellular organism such as a person needs a transport system to move substances around to the cells. In the human body the transport system consists of:

  • The blood (the liquid which carries substances around your body)
  • The blood vessels (the pipes which carry the blood)
  • The heart (the pump which moves the blood around the body)

Together they make up the circulatory or cardiovascular system.

Functions of the cardiovascular system

The circulatory system has a number of different functions in the body. These include:

  • Carrying food from the gut to the cells
  • Carrying oxygen from the lungs to the cells
  • Carrying carbon dioxide from the cells to the lungs
  • Carrying urea and other toxins from the liver to the kidneys
  • Carrying chemical messages (hormones) around the body from the glands where they are made to their target organs
  • Distributing heat around the body

The double circulation

We are very active animals so our cells need lots of oxygen. We have a very efficient system which makes sure our cells get the oxygen they need.

Your heart is not a single pump - it is in effect two pumps joined and working together. One part of the heart pumps deoxygenated blood to the lungs to get rid of waste carbon dioxide and pick up oxygen. The other part of the heart pumps this oxygenated blood all around the body. This is a double circulation.

A simple model of a double circulation

A simple model of a double circulation

However your circulatory system is not a simple system of boxes. There are different types of blood vessels (arteries, veins and capillaries) and the blood flows through lots of different organs.

Every time the heart pumps the blood is pushed around the circulatory system. It flows in a constant stream (see the animation below).

The human circulatory system

The human circulatory system

Blood flow in the circulatory system