Gaseous Exchange in Humans and Plants
Senior 2 · Biology
Gaseous Exchange in Humans and Plants
Lesson 3 of 5
20 min
Let's begin
Each of your lungs contains around 300 million alveoli. Spread out flat, their surface would cover roughly 75 m², about the floor of a large classroom, all packed inside your chest. This huge surface is where oxygen gets into your blood with every breath.
Diffusion is the net movement of particles from a region where they are more concentrated to a region where they are less concentrated. The difference in concentration is the concentration gradient. The steeper the gradient, the faster the diffusion.
Worked example
Blood arriving at the lungs in the pulmonary artery has come from the body's tissues, which used up its oxygen and added carbon dioxide. Air in an alveolus has just been refreshed by breathing. Which way does each gas move?
The route for oxygen: air in the alveolus → dissolves in the moisture lining it → through the alveolus wall → through the capillary wall → into the plasma → into a red blood cell, where it combines with haemoglobin to form oxyhaemoglobin. The blood, now oxygenated, returns to the heart in the pulmonary vein.
| Feature | Why it speeds up diffusion |
|---|---|
| Enormous number of alveoli | very large total surface area |
| Wall only one cell thick, pressed against a capillary wall that is also one cell thick | very short diffusion distance |
| Moist inner lining | gases dissolve before diffusing through the cells |
| Dense network of capillaries with blood always flowing | oxygen is carried away and carbon dioxide brought in, keeping the gradient steep |
| Ventilation by breathing | brings in fresh air, keeping oxygen high and carbon dioxide low in the alveolus |
Special cells in the lining also release a detergent-like liquid (surfactant) that stops the tiny, wet alveoli sticking together and collapsing when you breathe out.
Worked example
A lung disease reduces the alveolar surface area of a patient from 75 m² to 45 m². By what percentage has the surface for diffusion fallen, and what will the patient notice? Solution: (75 − 45) ÷ 75 × 100 = 40%. With 40% less surface, less oxygen enters the blood per breath, so the patient becomes breathless, especially when active.
Open the Diffusion simulation (PhET, version 1.1.5). Put many particles on one side of the divider and few on the other, then remove the divider. Watch the net movement from the crowded side to the emptier side until the two sides even out. Then raise the temperature and see how the speed of diffusion changes.
Tip
Four features for every exchange surface: large area, thin, moist, good blood supply. For each one, give the reason: "thin, so the diffusion distance is short".
Direction answers must name both the gas and the gradient: "oxygen diffuses from the alveolus, where its concentration is high, into the blood, where it is low."
Why does carbon dioxide diffuse out of the blood in the lungs?
Solution: Its concentration is higher in the blood arriving from the tissues than in the freshly breathed air in the alveolus, so it diffuses down its concentration gradient into the alveolus.
Explain how continuous blood flow helps gas exchange.
Solution: Flowing blood carries oxygenated blood away and brings in blood that is low in oxygen, so the oxygen concentration in the capillary stays lower than in the alveolus. The gradient stays steep.
If breathing stopped for a short time, what would happen to the concentration gradients in the alveoli?
Solution: Oxygen in the alveolar air would fall and carbon dioxide would rise, so both gradients would become less steep and gas exchange would slow down.
A patient's alveolar surface area falls from 80 m² to 60 m². Calculate the percentage decrease.
Solution: (80 − 60) ÷ 80 × 100 = 25%.
Remember
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
