Gas Exchange in Humans
How your lungs exchange oxygen and carbon dioxide with your blood
Master breathing, gas exchange, and how exercise affects your breathing rate
The Big Picture
Your body is constantly respiring — breaking down glucose to release energy. This process needs oxygen
and produces carbon dioxide as waste. Your breathing system's job is to deliver oxygen from the air
to your blood and remove carbon dioxide from your blood back to the air.
The lungs are not muscles — they're passive. Instead, muscles like your diaphragm and
intercostal muscles do the work, contracting and relaxing to move air in and out. Inside the lungs,
millions of tiny air sacs called alveoli sit directly against blood capillaries, allowing gases to
swap places by diffusion. It's a beautifully simple system — once you see how pressure and volume work together,
the whole thing clicks into place.
When you exercise, your muscles demand more oxygen and produce more carbon dioxide. Your brain detects this change
and automatically speeds up your breathing — you don't have to think about it. Understanding why and how
this happens is what this chapter is about.
Features of Gas Exchange Surfaces
Every gas exchange surface in your body — whether it's an alveolus in your lungs, villi in your small intestine,
or root hair cells in a plant — shares the same design principles. They're optimised for one job: moving substances
across a barrier as fast as possible.
The Four Key Features
All gas exchange surfaces must have these four features to work well:
Large Surface Area
More space = more molecules can cross at once. Alveoli are arranged like bunches of grapes, giving the lungs about 70 m² of surface area — the size of a tennis court.
Thin Walls
Shorter diffusion distance = faster exchange. Alveoli and capillaries are only one cell thick.
Good Ventilation
Constant fresh air supply maintains a steep diffusion gradient. Breathing keeps the oxygen concentration high in the alveoli and CO₂ concentration low.
Good Blood Supply
Blood flowing past picks up oxygen and drops off CO₂. The pulmonary artery and pulmonary vein ensure the concentration gradient stays steep.
Exam Tip: One Principle, Many Surfaces
The alveolus, villus, and root hair cell all share these same four features. If you understand one gas exchange
surface deeply, you can apply that knowledge to any other exchange surface. Examiners love to test this transfer of understanding.
The Alveolus in Detail
The alveolus is a tiny air-filled sac where gas exchange actually happens. Here's what makes it work:
- Epithelial wall: One cell thick (squamous epithelium) — no wasted space
- Blood capillary: Wraps around the alveolus, carrying deoxygenated blood in and oxygenated blood out
- Moist lining: Gases dissolve in the thin film of moisture, then diffuse across
- Elastic tissue: Allows the alveolus to stretch and recoil during breathing