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The respiratory system

Gas exchange happens inside the lungs. Roll over the image to see the different parts of the respiratory system and a description of the jobs they do.

Breathing action
	Your lungs are inside your chest cavity (along with your heart). The chest cavity is sealed so no air can get in or out of it. When you breathe in, the diaphragm, which is a sheet of muscle, contracts and moves downwards. At the same time, your ribs are pulled up and outwards by the contraction of the intercostal muscles. These two changes make the chest cavity expand. Therefore the pressure around the lungs is reduced. However, the lungs are open to the air which is now at a higher pressure. So air is forced into the lungs. It feels like you are sucking air in.

Picture 6. Alveoli give a tremendous surface area for gas exchange. They have moist, thin walls and a good blood supply.

Picture 5. The main parts of the breathing system. Roll your cursor over the red blobs to see what they are and what they do.

Picture 7. Roll over the play button to see the animation.
Open the animation in a new window.

Gas exchange at the alveoli

There are about 300 million alveoli in each lung. If they were spread out, these hollow sacs could cover the surface of a tennis court. This large surface area makes the lungs very efficient at gas exchange.

Alveoli are found at the end of the branching bronchioles and so they have a good air supply. Their walls are very thin and have a moist surface. They are covered by a network of capillaries which transport the gases. This structure is specialised to make alveoli excellent at gas exchange.

Picture 8. The lungs have an extensive network of blood vessels. This excellent blood supply is needed to transport oxygen away from the lungs efficiently.
Wellcome Library, London.

Diffusion drives gas exchange

The definition of diffusion can sound a little tricky but it is worth remembering. It is:

Diffusion is the movement of gas molecules from a region of their high concentration into a region where they have a lower concentration.

Take a look at the page about dialysis to find out more about diffusion.

Question 2

Look at the paragraphs below. They describe the exchange of gases in the lungs. Choose the missing words from the drop down lists.

Diffusion is the movement of a gas from an area of concentration to one of concentration.

The concentration of carbon dioxide in the blood coming from the heart is . This means that diffuses from the capillary into the alveolus and is then breathed .

Blood leaving the alveolus is rich in . This is because there is a low concentration of in the blood coming from the heart and a high concentration of oxygen in the . Diffusion drives the movement of oxygen from the into the .