Gas exchange vs breathing vs respiration: three different things
Examiners deliberately test whether you can separate these:
- Breathing (ventilation): the muscular movements that move air in and out of the lungs.
- Gas exchange: the diffusion of oxygen into the blood and carbon dioxide out, across the alveoli.
- Respiration: the chemical reactions in cells that release energy from glucose.
Using 'breathing' for 'respiration' is one of the most reliably penalised slips in the whole subject. Read the command word and answer the process actually named.
Alveoli: adaptations linked to function
This is the bankable long-answer. State each feature WITH its consequence:
- Large total surface area (millions of alveoli). So more gas diffuses at once.
- Thin walls, one cell thick: so the diffusion distance is short and diffusion is fast.
- Good (rich) blood supply from surrounding capillaries. So a steep concentration gradient is maintained, keeping diffusion going.
- Moist lining: so gases dissolve before diffusing across.
The single most common cap on marks is listing the features without the 'so that' clause. 'Alveoli have a large surface area' = 1 mark; 'a large surface area so more oxygen can diffuse at once' = 2. Every adaptation must end in a diffusion consequence.
Inspired vs expired air, and the limewater/CO2 evidence
| Component | Inspired air | Expired air |
|---|---|---|
| Oxygen | Higher (~21%) | Lower (~16%) |
| Carbon dioxide | Lower (~0.04%) | Higher (~4%) |
| Water vapour | Less | More (saturated) |
| Temperature | Cooler | Warmer |
The reason for the differences is the paying point: oxygen is used and carbon dioxide produced by respiration in cells. Limewater turning from clear to cloudy/milky is the standard test for the higher CO2 in expired air; a question often asks you to design the comparison using two tubes (breathing in vs out through limewater).
The breathing mechanism: pressure logic students reverse
Breathing in (inspiration): the diaphragm contracts and flattens, the external intercostal muscles contract pulling the ribs up and out, so the volume of the thorax increases, the pressure inside decreases below atmospheric pressure, and air flows in. Breathing out reverses every step: muscles relax, volume decreases, pressure increases, air flows out. The trap is the pressure–volume relationship: larger volume means lower pressure (so air rushes in). Students routinely write it backwards. Anchor it: chest gets bigger → pressure drops → air comes in.