9.1 Human gas exchange
The gas exchange system
Air passes from the nose or mouth through the larynx (voice box), down the trachea (windpipe), which divides into two bronchi (one for each lung). The bronchi divide into smaller bronchioles, which end in tiny air sacs called alveoli. The lungs are protected inside the rib cage and are separated from the abdomen by the diaphragm.
Features of a gas exchange surface
The alveoli
| Feature | How it helps gas exchange |
|---|---|
| millions of alveoli | a very large surface area |
| walls one cell thick, and capillary walls one cell thick | a very short diffusion distance, so fast diffusion |
| a dense network of capillaries | a good blood supply that carries oxygen away and brings carbon dioxide, keeping a steep concentration gradient |
| moist lining | gases dissolve in it before diffusing across |
| breathing movements | refresh the air in the alveoli, so the oxygen concentration stays high and the carbon dioxide concentration stays low |
Oxygen diffuses from the air in the alveoli into the blood (where it combines with haemoglobin in red blood cells). Carbon dioxide diffuses from the blood into the air in the alveoli and is breathed out.
The composition of air
| Gas | Inspired (atmospheric) air | Expired air |
|---|---|---|
| nitrogen | about 78% | about 78% (unchanged) |
| oxygen | about 21% | about 16% |
| carbon dioxide | about 0.04% | about 4% |
| water vapour | variable (low) | high (saturated) |
| temperature | air temperature | warmer (body temperature) |
Expired air has less oxygen (used in respiration), more carbon dioxide (made in respiration) and more water vapour (evaporates from the moist lining of the lungs) than inspired air.
Investigation. Breathe through limewater: expired air turns limewater milky much faster than inspired air, because it contains more carbon dioxide. A hydrogencarbonate indicator also changes colour faster with expired air. Breathing out onto a cold mirror shows the water vapour (it mists).
Breathing
| Breathing in (inhaling) | Breathing out (exhaling) | |
|---|---|---|
| external intercostal muscles | contract | relax |
| internal intercostal muscles | relax | contract (in forced breathing) |
| ribs | move up and out | move down and in |
| diaphragm | contracts and flattens | relaxes and returns to a dome shape |
| volume of the thorax | increases | decreases |
| pressure in the thorax | decreases (below atmospheric) | increases (above atmospheric) |
| air movement | air moves in | air is forced out |
Air always moves from a region of higher pressure to a region of lower pressure.
The diaphragm contracts and flattens, and the external intercostal muscles contract and lift the ribs. The volume of the thorax increases, so the pressure falls below atmospheric pressure and air is forced into the lungs.
The effect of exercise
During exercise the muscles respire faster, so they need more oxygen and make more carbon dioxide. The breathing rate and depth (volume of each breath) increase, which brings in more oxygen and removes the extra carbon dioxide. The increased carbon dioxide in the blood is detected by the brain, which sends signals to the breathing muscles.
Investigation. Count the number of breaths in one minute at rest, then after exercise (for example, running on the spot for 2 minutes). The rate is higher after exercise and takes time to return to normal. Depth can be measured with a spirometer.
Protection of the gas exchange system
Air contains dust, bacteria and other particles. The airways are lined with:
- goblet cells, which make mucus;
- mucus, which traps particles and pathogens;
- ciliated cells, with tiny hair-like cilia that beat and sweep the mucus with the trapped particles up to the throat, where it is swallowed.
This stops particles and pathogens from reaching the alveoli. Smoking damages the cilia, so mucus builds up and causes coughing and infection.
Practise this chapter
Every past-paper question on these topics, with the mark scheme.