Why exchange gases
Every cell respires, taking in oxygen and giving out carbon dioxide. Gaseous exchange is how an organism supplies that oxygen and removes that waste gas across a specialised surface, always by diffusion down concentration gradients.
A good exchange surface shares the same features everywhere in biology: it is large (big surface area), thin (short diffusion distance), moist, and well supplied with a transport system to keep the gradient steep.
The human breathing system
Air travels down the trachea, which branches into two bronchi, then into finer bronchioles ending in millions of tiny air sacs — the alveoli. It is here that exchange happens: oxygen diffuses from the alveolar air into the blood, and carbon dioxide diffuses the other way.
The alveoli are perfectly built for it — a huge combined surface area, walls one cell thick, moist linings, and a dense network of capillaries carrying blood right past them.
| Alveolus feature | Why it helps |
|---|---|
| Millions of sacs | huge surface area |
| Walls one cell thick | short diffusion distance |
| Moist lining | gases dissolve before diffusing |
| Dense capillaries | keeps the concentration gradient steep |
Breathing in and out
Breathing (ventilation) moves air in and out. To breathe in, the diaphragm and intercostal muscles contract, the chest volume increases and the pressure inside falls, so air is pushed in. To breathe out, these muscles relax, the volume falls and the pressure rises, forcing air out. Do not confuse breathing (the movement of air) with respiration (the chemical release of energy in cells).
Breathing is not respiration
Breathing is the physical movement of air in and out of the lungs. Respiration is the chemical process, in every cell, that releases energy from glucose. Breathing simply supplies the oxygen respiration needs.
Exchange in plants
Plants exchange gases too, through the stomata on their leaves. In daylight, photosynthesis uses up carbon dioxide and releases oxygen faster than respiration uses it, so the net movement is CO₂ in and O₂ out. In the dark, only respiration occurs, so the net movement reverses.
A worked example
Explain how the structure of an alveolus makes it efficient at gaseous exchange.
- There are millions of alveoli, giving a very large total surface area.More area means more diffusion at once.
- Their walls are only one cell thick.A short distance lets gases diffuse quickly.
- They are surrounded by many capillaries carrying blood.This keeps a steep concentration gradient for oxygen and carbon dioxide.
- Their linings are moist.Gases dissolve in the moisture before diffusing across.
Large surface area, thin walls, a rich blood supply and a moist lining together make diffusion of gases fast and efficient.
Breathing rate and exercise
During exercise the muscles respire faster, needing more oxygen and producing more carbon dioxide. The body responds by making breathing faster and deeper, so more air is moved through the lungs each minute, and the heart beats faster to carry the gases quickly. If oxygen still cannot be supplied fast enough, the muscles respire anaerobically, producing lactic acid — the reason you keep breathing hard for a while after stopping, to repay the oxygen debt.
Smoking and the lungs
Tobacco smoke damages the breathing system in several ways. Tar collects in the lungs, contains carcinogens that cause lung cancer, and paralyses the tiny cilia that sweep mucus and dirt out — so infections build up (smoker's cough, bronchitis). Smoke also breaks down the alveoli walls, reducing the surface area for exchange (emphysema), and the carbon monoxide in it cuts the blood's oxygen-carrying capacity.
Why smokers get breathless
Smoking destroys alveoli (less surface area), clogs the airways with mucus (cilia paralysed), and its carbon monoxide steals oxygen-carrying capacity — so less oxygen reaches the blood.