The alveolus

There are millions of alveoli in the lungs. They are where oxygen enters the blood and carbon dioxide leaves it, at the end of the airways.

Air reaches an alveolus through the trachea, a bronchus and then a bronchiole. Each bronchiole ends in a cluster of alveoli that looks like a bunch of grapes, and each alveolus is wrapped in a net of blood capillaries.

The alveolus is therefore the point where the respiratory system and the circulatory system meet, and questions on this structure usually test whether you can explain gas exchange as diffusion down a concentration gradient rather than as air being pushed into the blood.

Parts and functions

How structure suits function

The alveolus is adapted for fast diffusion: its wall is only one cell thick for a short distance, it has a very large total surface area, a moist lining so gases dissolve, and a dense capillary network to keep the concentration gradient steep.

Each adaptation links to one factor that affects the rate of diffusion, and an examiner awards the mark for the link, not for the feature alone.

• Wall one cell thick, the diffusion distance is short, so oxygen and carbon dioxide cross quickly.
• Millions of alveoli, the total surface area for gas exchange is very large, so more gas diffuses per second.
• Moist lining, oxygen dissolves in the film of water first, and only dissolved gases diffuse across the membrane.
• Dense capillary network, blood carrying oxygen away is replaced continuously, so the concentration of oxygen in the blood stays low and the gradient stays steep.
• Continuous ventilation, breathing replaces stale air with fresh air, keeping the oxygen concentration in the alveolus high and the carbon dioxide concentration low.
• Elastic fibres, the alveolus stretches to take in more air and recoils to expel it, so exchange continues with every breath.

Related processes

Gas exchange in the alveolus is pure diffusion. Oxygen concentration in the alveolar air is higher than in the deoxygenated blood arriving from the pulmonary artery, so oxygen diffuses into the blood, dissolves in the plasma and binds to haemoglobin in red blood cells to form oxyhaemoglobin.

Carbon dioxide concentration is higher in the blood than in the alveolar air, so it diffuses in the opposite direction and is exhaled.

Ventilation is the process that maintains the gradients. During inhalation the diaphragm contracts and flattens, the external intercostal muscles contract to lift the ribs, the volume of the thoracic cavity increases, the pressure inside drops below atmospheric pressure and air flows in.

During exhalation the reverse happens and elastic recoil of the alveoli helps push air out. Because the alveolus is the site of gas exchange, it also links to aerobic respiration: the oxygen it loads into the blood is used in mitochondria, and the carbon dioxide it removes is the waste product of that respiration.

Common labelling errors

Worked question

Question (in the style of Paper 2 Section A): Diagram X shows an alveolus and a blood capillary. (a) Name the process by which oxygen moves from the alveolus into the blood. [1 mark] (b) Explain two ways the alveolus is adapted for this process. [4 marks] (c) State how the blood entering the capillary differs from the blood leaving it. [1 mark]

Model answer: (a) Diffusion. (b) The alveolar wall is one cell thick, so the diffusion distance is short and gases cross rapidly.

The alveolus has a moist lining, so oxygen dissolves before diffusing across the membrane. (Alternative points: a large total surface area from millions of alveoli increases the rate of diffusion; a dense capillary network carries oxygenated blood away and maintains a steep concentration gradient.) (c) Blood entering is deoxygenated with a high carbon dioxide concentration; blood leaving is oxygenated with a lower carbon dioxide concentration.

Marking note: for part (b) each adaptation earns one mark for the feature and one for the explanation. Naming the feature alone scores half.

Source:SRC-DSKP-EN

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