Skip to main content
SingaporeBiologySyllabus dot point

How does the human gas exchange system swap oxygen for carbon dioxide?

Describe the human gas exchange system and explain how the alveoli are adapted for gas exchange

A focused answer to the O-Level Biology outcome on human gas exchange. The path of air to the alveoli, how the alveoli are adapted for fast diffusion, and the differences between inhaled and exhaled air.

Generated by Claude Opus 4.88 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

Have a quick question? Jump to the Q&A page

Jump to a section
  1. What this dot point is asking
  2. The answer
  3. Examples in context
  4. Try this

What this dot point is asking

SEAB wants you to describe the human gas exchange system, the path air takes to reach the alveoli, and how the alveoli are adapted for efficient gas exchange by diffusion. You should be able to explain how oxygen enters the blood and carbon dioxide leaves it, and to compare inhaled and exhaled air.

The answer

The path of air

Air travels in through the nose or mouth, down the trachea (windpipe), which branches into two bronchi (one to each lung). Each bronchus divides into many smaller bronchioles, which end in tiny air sacs called alveoli. The trachea is held open by rings of cartilage, and its lining traps dust and microbes in mucus.

Gas exchange at the alveoli

Gas exchange happens in the alveoli, where the air is very close to the blood in the surrounding capillaries:

  • Oxygen diffuses from the air in the alveolus (high concentration) into the blood (lower concentration), where it binds to haemoglobin.
  • Carbon dioxide diffuses the other way, from the blood (high concentration) into the alveolus (lower concentration), to be breathed out.

Both gases move by diffusion, down their concentration gradients.

How the alveoli are adapted

The alveoli are adapted for fast diffusion:

  • A very large surface area (millions of alveoli), so a lot of gas crosses at once.
  • Walls one cell thick, giving a short diffusion distance.
  • A rich blood supply (a network of capillaries) that carries gases to and from the alveoli, keeping the gradients steep.
  • A moist surface, so gases dissolve before diffusing across.

These are the same features as any good exchange surface.

Inhaled versus exhaled air

Compared with inhaled air, exhaled air has:

  • Less oxygen (it has diffused into the blood for respiration).
  • More carbon dioxide (it has diffused out of the blood from respiration).
  • More water vapour (picked up from the moist alveolar lining).

Exhaled air is also warmer.

Examples in context

Example 1. Why smokers get breathless. Damage to the alveoli (as in emphysema) reduces the surface area for gas exchange, so less oxygen can diffuse into the blood. The person becomes breathless even with mild activity, showing how vital the alveolar surface area is.

Example 2. Athletes and lung capacity. Trained athletes can move air in and out efficiently and keep the alveolar gradients steep during exercise, so their muscles receive oxygen quickly. This supports more aerobic respiration during hard effort.

Try this

Q1. Name the tiny air sacs where gas exchange occurs. [1 mark]

  • Cue. The alveoli.

Q2. State two ways an alveolus is adapted for gas exchange. [2 marks]

  • Cue. Any two of: a large surface area, walls one cell thick (short diffusion distance), a rich blood supply, a moist surface.

Q3. Explain why exhaled air contains more carbon dioxide than inhaled air. [2 marks]

  • Cue. Carbon dioxide is made by respiration in the body and diffuses from the blood into the alveoli, so it is breathed out in greater amounts.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original5 marksThe alveoli are the site of gas exchange in the lungs. Describe four ways in which an alveolus is adapted for efficient gas exchange.
Show worked answer →

Four adaptations: there are millions of alveoli, giving a very large surface area for diffusion; each alveolus has a wall only one cell thick, giving a short diffusion distance; each alveolus is surrounded by a network of capillaries (a good blood supply) that carries gases to and from the alveolus, keeping the concentration gradients steep; the alveolar surface is moist, so gases dissolve before diffusing across.

Markers reward four correct adaptations, each linked to speeding diffusion (large surface area, thin wall, good blood supply, moist surface).

Original4 marksCompare inhaled (breathed in) air with exhaled (breathed out) air in terms of the amount of oxygen, carbon dioxide and water vapour, and explain the changes.
Show worked answer →

Exhaled air contains less oxygen than inhaled air, because oxygen has diffused from the alveoli into the blood for respiration. Exhaled air contains more carbon dioxide, because carbon dioxide (a waste product of respiration) has diffused from the blood into the alveoli to be breathed out. Exhaled air contains more water vapour, because it has picked up moisture from the wet surfaces of the alveoli.

Markers reward less oxygen, more carbon dioxide and more water vapour in exhaled air, each with the correct reason (diffusion linked to respiration, and evaporation from the moist lining).

Related dot points