Lungs and alveoli
When you breathe in, air travels down the trachea, splits into two bronchi, then into finer and finer bronchioles, and finally arrives at tiny air sacs called alveoli. This is where the actual swap happens: oxygen moves out of the air and into the blood, and carbon dioxide moves the other way. That swap is gas exchange, and it works entirely by diffusion, gases moving from where there are lots of them to where there are fewer.
- Gas exchange
- The movement of oxygen from the alveoli into the blood, and carbon dioxide from the blood into the alveoli, by diffusion down concentration gradients.
The alveoli are not just bags. Every feature of them is an adaptation to make diffusion as fast as possible:
| Adaptation | Why it helps diffusion |
|---|---|
| Millions of alveoli | Give a very large total surface area (around 70 square metres) for gases to cross. |
| Walls one cell thick | Short diffusion distance, so gases cross quickly. |
| Dense capillary network | Blood constantly removes oxygen and delivers carbon dioxide, keeping a steep concentration gradient. |
| Moist lining | Gases dissolve before they diffuse across the membrane. |
| Ventilated by breathing | Fresh air keeps alveolar oxygen high and carbon dioxide low, maintaining the gradient. |
Describe the path of an oxygen molecule from the air in an alveolus to a red blood cell.
The mechanics of ventilation
Ventilation just means breathing: moving air in and out of the lungs. The lungs have no muscle of their own, so two sets of muscles do the work, the diaphragm (a sheet of muscle below the lungs) and the intercostal muscles (between the ribs). The trick to understanding it is pressure: air always moves from high pressure to low pressure, so changing the volume of the chest changes the pressure and makes air flow.
| Step | Breathing in (inhalation) | Breathing out (exhalation) |
|---|---|---|
| Diaphragm | Contracts and flattens, moving down | Relaxes and domes upwards |
| Intercostal muscles | Contract, pulling ribs up and out | Relax, so ribs move down and in |
| Chest volume | Increases | Decreases |
| Chest pressure | Falls below atmospheric pressure | Rises above atmospheric pressure |
| Air | Rushes in to the lungs | Is forced out of the lungs |
Volume up, pressure down
The whole thing hangs on one idea: bigger space, lower pressure. If you can say "volume increases, so pressure drops, so air moves in", you can rebuild the rest of the answer from memory.
Aerobic respiration
Once oxygen reaches the cells, respiration uses it to release energy from glucose. Respiration is not the same as breathing: breathing moves air, while respiration is the chemical reaction inside every cell that transfers energy for life processes. When oxygen is used, it is aerobic respiration, and it happens in the mitochondria.
Word equation:
Balanced symbol equation:
Aerobic respiration releases a lot of energy per glucose molecule because the glucose is broken down completely. That energy is used for muscle contraction, keeping warm, active transport, and building large molecules.
Anaerobic respiration and oxygen debt
During hard exercise your muscles can need energy faster than your lungs and blood can deliver oxygen. When that happens the cells switch to anaerobic respiration, releasing energy from glucose without oxygen. In humans this produces lactic acid.
In yeast and plant cells anaerobic respiration is called fermentation and gives different products, which is why it is used to make bread and alcohol:
| Feature | Aerobic | Anaerobic (in humans) |
|---|---|---|
| Oxygen needed? | Yes | No |
| Products | Carbon dioxide and water | Lactic acid |
| Energy released | Large amount | Small amount |
| Glucose broken down | Completely | Only partly |
Lactic acid is a problem: it builds up in the muscles, lowers the pH and causes cramp and fatigue. To clear it, the body needs extra oxygen after exercise stops. This extra oxygen is the oxygen debt.
- Oxygen debt
- The extra oxygen the body must take in after exercise to break down the lactic acid that built up during anaerobic respiration. It is why you keep breathing hard for a while after you stop running.
Where this is assessed
Explaining how alveoli are adapted and linking ventilation to pressure is Criterion A (knowing and understanding). Interpreting a graph of breathing rate before, during and after exercise is Criterion C (processing and evaluating), so practise reading those recovery curves.
Check yourself
1. Give three ways an alveolus is adapted for gas exchange, and explain each. +
Any three of: a large surface area (many alveoli) so more gas crosses at once; thin walls one cell thick so the diffusion distance is short; a good blood supply from capillaries so a steep concentration gradient is maintained; a moist lining so gases dissolve before diffusing. Each one speeds up diffusion.
2. Describe what the diaphragm and ribs do when you breathe in, and why air enters. +
The diaphragm contracts and flattens and the intercostal muscles pull the ribs up and out. This increases the volume of the chest, which lowers the pressure inside below atmospheric pressure, so air moves in from high pressure to low pressure.
3. Write the balanced symbol equation for aerobic respiration, and say why anaerobic respiration in muscles is a problem. +
Aerobic respiration: C6H12O6 + 6O2 → 6CO2 + 6H2O. Anaerobic respiration in muscles produces lactic acid, which builds up, causes fatigue and cramp, and creates an oxygen debt that must be repaid after exercise.
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