Gas exchange
Gas exchange
- Unit 1: Cells
- Unit 2: Organisms
- Unit 3: Microorganisms
- Unit 4: Digestion and Excretion systems
- Unit 5: Respiration system
- Unit 6: Circulatory system
- Unit 7: Nervous system
- Unit 8: Senses
- Unit 9: Musculoskeletal system
- Unit 10: Endocrine system
- Unit 11: Plants
- Unit 12: Reproductive system
- Unit 13: Genetics and Biotechnology
In this topic you will:
- find out how oxygen gets into your blood from the air, and how carbon dioxide goes in the other direction
- do an experiment to help you to think about why the air sacs in the lungs need to be very small
- do an experiment to compare how much carbon dioxide there is in the air you breathe in and the air you breathe out
Key words
- alveoli
- analogy
- capillaries
- diffusion
- expired air
- gas exchange
- haemoglobin
- inspired air
- limewater
Air sacs
The photograph shows a tiny part of the lungs, seen through a powerful microscope. You can see the lungs are mostly holes. These holes are called air sacs. Another name for them is alveoli.
There are also lots of very tiny blood vessels in the lungs, wrapped around the air sacs. You cannot see them in the photograph, but they are shown in the diagram below. These blood vessels are capillaries.
The structure of an air sac
This diagram shows one of the air sacs in the lungs. The air sac has a wall made of one layer of cells. These cells are very thin.
You can see that there is a blood capillary around the outside of the alveolus. The capillary is pressed tightly against the alveolus. The wall of the capillary is also made of a single layer of very thin cells.
Gas exchange in the air sacs
Inside the air sacs, oxygen from the air goes into the blood. Carbon dioxide from the blood goes into the air. This is called gas exchange.
Think about the blood capillary on the left of the diagram. The blood inside this capillary comes from the heart. Before reaching the heart, it came from the organs in the body. These organs contain cells that respire, using up oxygen and making carbon dioxide. So the blood in this capillary contains only a small amount of oxygen, and a lot of carbon dioxide.
Now think about the air inside the air sac. It came from outside the body, where the air contains a lot of oxygen and only a small amount of carbon dioxide.
Inside the alveolus, this air is very close to the blood. There are only two very thin cells between the air and the blood.
The oxygen particles in the air are a gas, so they are moving freely. They can easily move from the air, through the thin-walled cells and into the blood. This is called diffusion. You can find out more about diffusion in Topic 3.7. The oxygen particles move from where there are a lot of them (in the air) to where there are fewer of them (in the blood).
When the oxygen gets into the blood, it dissolves. (You can find out about dissolving in Topic 2.1.) The oxygen goes into the red blood cells where it combines with haemoglobin. You will find out what happens to it after that in Topic 1.6.
Now think about the carbon dioxide. There is a lot of it in the blood in the capillary, and only a small quantity in the air inside the air sac. So the carbon dioxide diffuses into the air in the air sac.
Quick fact: gas exchange and diffusion
Gas exchange happens by diffusion, where gases move from areas of higher concentration to lower concentration. Oxygen diffuses from the alveoli into the blood, while carbon dioxide diffuses in the opposite direction to be exhaled.
Common mistake
It’s a common misconception that oxygen moves into the blood because the body “wants” it. In reality, gases like oxygen and carbon dioxide move by diffusion—from areas of higher concentration to lower concentration—without any conscious direction.
Questions
Show Answer
Thin walls allow gases like oxygen and carbon dioxide to diffuse quickly between the air in the alveoli and the blood in the capillaries.
Show Answer
Oxygen moves by diffusion, from the air in the alveolus (where there is more oxygen) into the blood (where there is less oxygen).