Breathing and gas exchange

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

Put these in the order air passes through them: alveoli, trachea, bronchioles, bronchi.

  1. Air goes down the windpipe: the trachea.
  2. The trachea splits into two bronchi, one to each lung.
  3. Each bronchus branches again and again into narrow bronchioles.
  4. They end in tiny air sacs, the alveoli, where gas exchange happens. trachea → bronchi → bronchioles → alveoli

Answer: trachea → bronchi → bronchioles → alveoli

Example 2

Explain how breathing in happens.

  1. The external intercostal muscles contract and pull the ribs up and out.
  2. The diaphragm contracts and flattens, moving down.
  3. So the volume of the chest increases and the pressure inside it falls below the air pressure outside.
  4. Air flows from high pressure to low pressure: into the lungs.

Answer: intercostal muscles and diaphragm contract; ribs move up and out, diaphragm flattens; chest volume increases, pressure falls, air flows in

Example 3

Give three ways in which alveoli are adapted for gas exchange.

  1. Millions of alveoli give a huge surface area: about the size of a tennis court.
  2. Their walls, and the capillary walls, are each only one cell thick, so the gases diffuse a very short distance.
  3. A rich blood supply carries oxygen away and brings carbon dioxide, keeping a steep concentration gradient. The moist lining lets the gases dissolve.

Answer: huge surface area; walls one cell thick (short distance); rich blood supply (and moist)

Example 4

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?

  1. Find how much the oxygen level falls. 21 − 16 = 5
  2. Divide by the starting amount and multiply by 100. 5 ÷ 21 × 100 = 23.8
  3. So about 24% of the oxygen breathed in is taken into the blood; the rest is breathed out again.

Answer: about 24%