Breathing and gas exchange · Year 7
Every cell in your body needs oxygen for respiration and makes carbon dioxide that must be removed. Your lungs swap these two gases with the air you breathe, about 12 to 20 times a minute at rest. Breathing moves air in and out; gas exchange happens in millions of tiny air sacs called alveoli.
Key facts
- Air goes: nose or mouth → trachea (windpipe) → bronchi → bronchioles → alveoli. Rings of cartilage keep the trachea open.
- Breathing in: the intercostal muscles contract and lift the ribs up and out, and the diaphragm contracts and flattens. The chest gets bigger, the pressure inside falls, and air rushes in.
- Breathing out: the intercostal muscles and diaphragm relax, the ribs move down and in, the diaphragm domes up. The chest gets smaller, the pressure rises, and air is pushed out.
- In the alveoli, oxygen diffuses from the air into the blood, and carbon dioxide diffuses from the blood into the air.
- Alveoli are good at gas exchange: a huge surface area, walls only one cell thick, a moist lining and a rich blood supply.
- Inhaled air is about 21% oxygen and 0.04% carbon dioxide; exhaled air is about 16% oxygen and 4% carbon dioxide. Both are about 78% nitrogen.
- Smoking damages the lungs: tar coats the airways and stops the cilia sweeping out mucus, and it can break down alveoli and cause cancer.
Worked examples
Example 1
Inhaled air has 21% oxygen and exhaled air has 16%. How much of the oxygen in each breath is taken into the blood, as a percentage of what was breathed in?
Answer: about 24%