Gas exchange in humans

Cambridge IGCSE Biology 0610 · Core and Extended

Syllabus points

• Describe the features of gas exchange surfaces.
• Identify the parts of the breathing system and state their functions.
• Compare the composition of inspired and expired air, and describe the test for carbon dioxide.
• Explain the role of the ribs, intercostal muscles and diaphragm in ventilation.
• Explain the effects of physical activity on the rate and depth of breathing.
• Explain how the lungs are protected from pathogens and particles.

Features of a gas exchange surface

Every gas exchange surface in biology, from an alveolus to a gill to a leaf, has the same four features, and each has the same reason:

• Large surface area — more gas can diffuse at once.
• Thin, usually one cell thick — a short diffusion distance.
• Good blood supply — carries gases away, keeping the concentration gradient steep.
• Well ventilated — brings fresh air in, keeping the gradient steep.

The last two are the ones commonly left out. Both work by maintaining the concentration gradient, and saying so is what earns the mark.

The alveoli deliver all four: there are about 300 million of them giving a surface area of roughly 70 m², their walls are one cell thick, each is wrapped in capillaries, and breathing refreshes the air inside.

The breathing system

Air travels: nose or mouth → trachea → bronchi → bronchioles → alveoli.

• Trachea — held open by rings of cartilage, so it cannot collapse when pressure inside drops during inhalation. The rings are C-shaped so the oesophagus behind can expand when swallowing.
• Bronchi — one to each lung, also supported by cartilage.
• Bronchioles — finer tubes, no cartilage, with muscle in their walls that can narrow them.
• Alveoli — tiny air sacs where gas exchange happens.

Gas exchange at the alveolus

Air in the alveolus is high in oxygen and low in carbon dioxide. Blood arriving in the capillary is the reverse. So:

• Oxygen diffuses from alveolus into blood, and combines with haemoglobin in red blood cells.
• Carbon dioxide diffuses from blood into the alveolus and is breathed out.

Both are diffusion, down concentration gradients, and neither needs energy.

Inspired and expired air

Two points are often got wrong. Expired air still contains plenty of oxygen, about 16%, which is why mouth-to-mouth resuscitation works. And nitrogen does not change, because it takes no part in respiration.

The test for carbon dioxide is limewater, which turns from clear to milky or cloudy. In the standard experiment, breathing in through one tube and out through another shows expired air turns limewater milky much faster than inspired air does.

Hydrogencarbonate indicator is the other reagent worth knowing. It is orange in normal air, turns yellow when carbon dioxide increases, and purple or magenta when carbon dioxide falls. It is used to show respiration and photosynthesis in the same experiment.

Ventilation

Air moves because of pressure differences, and pressure changes because volume changes. Getting the causal order right is what these questions test.

Breathing in:

1. External intercostal muscles contract, pulling the ribcage up and out.
2. The diaphragm contracts and flattens, moving down.
3. Volume of the thorax increases.
4. Pressure inside decreases, falling below atmospheric pressure.
5. Air flows in.

Breathing out:

1. The intercostal muscles relax, so the ribcage moves down and in.
2. The diaphragm relaxes and returns to its domed shape.
3. Volume decreases.
4. Pressure increases above atmospheric.
5. Air flows out.

Note that the diaphragm contracts to flatten and relaxes to dome. Answers frequently reverse this. And air is never pulled or pushed by the lungs themselves: they have no muscle, and simply follow the movement of the chest wall.

Physical activity

During exercise, muscles respire faster, using more oxygen and producing more carbon dioxide.

Increased carbon dioxide in the blood is detected by the brain, which increases the rate and depth of breathing. Deeper breaths bring in more air per breath, and faster breathing brings in more breaths, so more oxygen is delivered and carbon dioxide removed more quickly.

Breathing stays raised after exercise stops, to repay the oxygen debt from anaerobic respiration and to remove the lactic acid that built up.

Protecting the lungs

• Goblet cells produce mucus, which traps dust, bacteria and other particles.
• Ciliated cells have cilia that beat in waves, sweeping the mucus up the airways to the throat, where it is swallowed.

Smoking damages this system. Tar paralyses and eventually destroys the cilia, so mucus is no longer swept away. It collects in the airways, and the only way to shift it is coughing, which is smoker's cough. The trapped bacteria in the stationary mucus make chest infections and bronchitis much more likely.