Function: Breathing and Respiration

Air constantly enters and leaves the lungs, enabling the tissues of the body to receive an adequate supply of oxygen and to dispose of their waste product, carbon dioxide. Breathing is controlled by the respiratory centre in a part of the brain known as the medulla. The respiratory centre stimulates the intercostal muscles around the chest cavity to contract and relax so that we breathe in and out.

How breathing works

During breathing, air moves from areas of high pressure to areas of low pressure. When the pressure in the lungs is lower than the pressure in the atmosphere, air enters the airways. If the pressure in the lungs increases, air moves out of the lungs and is then exhaled.


The diaphragm and intercostal muscles between the ribs contract. As a result, the ribcage expands, creating a decrease in pressure, and air is drawn into the lungs.


After breathing in, the diaphragm and intercostal muscles relax and the ribcage contracts. Pressure inside the lungs increases and air moves out of the lungs in order to be exhaled.

Chest X-rays

These normal chest X-rays show the volume of air in the lungs during inhalation and exhalation, which is achieved by changes in the position of the ribs and diaphragm.

How breathing is regulated

Even while we sleep, our basic breathing rhythm is controlled by a collection of nerve cells in the brain called the respiratory centre. From there, messages travel down nerves to the diaphragm and rib muscles and stimulate them so that we continue breathing. As activity changes, so does the amount of carbon dioxide in the blood. Receptors in some of the large arteries detect the changes and send instructions to the brain.

How carbon dioxide influences breathing

Nerve cells in arteries and in the brain’s respiratory centre are sensitive to levels of carbon dioxide in the blood. A slight rise in the carbon dioxide level increases the breathing rate to return the level to normal.

Gas exchange in the body

The exchange of oxygen and carbon dioxide gases occurs constantly throughout the body. In the lungs, oxygen crosses the delicate walls of the alveoli (air sacs) and enters tiny blood vessels (capillaries), where it binds to the molecule haemoglobin in the red blood cells. At the same time, carbon dioxide is released from the blood into the alveoli and exhaled through the mouth or nose. In tissue cells, blood from the lungs exchanges its supply of oxygen for carbon dioxide.

Exchange of gases

Body cells require a constant supply of oxygen in order to obtain energy to survive. In addition, waste products from the body cells, mainly carbon dioxide, must be transported away from the cells.

Cross section of alveoli

This magnified view of lung tissue shows the alveolar air spaces and their thin walls. The tiny holes seen in the walls are blood vessels.

From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.

The subjects, conditions and treatments covered in this encyclopaedia are for information only and may not be covered by your insurance product should you make a claim.

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