Anatomy PhysiologyDown the Gradient: How Your Alveoli Trade Gases with the Blood
Atlas the explorer-guide stands inside a glowing, balloon-like alveolus, holds a pressure gauge, and traces oxygen molecules drifting across a paper-thin moist wall into a bright red capillary below.
- Describe how oxygen and carbon dioxide move across the alveolar membrane by diffusion.
- Explain why each gas moves from higher partial pressure to lower partial pressure.
- Identify the structural features of alveoli — including thin membrane, moisture, and vast collective surface area — that make gas exchange fast and efficient.
- Relate the act of breathing to maintaining the partial-pressure gradients that deliver oxygen to blood.
Key terms
- Alveolus
- A tiny air sac in the lung where gas exchange occurs across a thin, moist wall wrapped in capillaries.
- Partial pressure
- The share of total pressure contributed by one gas, which determines that gas's diffusion direction.
- Diffusion
- The passive net movement of gas molecules from a region of higher to lower partial pressure.
- Concentration gradient
- A difference in partial pressure across a membrane that drives gas movement without any energy input.
- Respiratory membrane
- The thin, moist alveolar-capillary barrier that gases cross quickly during exchange.
Diffusion Down Partial-Pressure Gradients
Gas exchange is entirely passive: oxygen and carbon dioxide diffuse independently, each from where its own partial pressure is higher to where it is lower. In the alveolus, oxygen sits near 100 mmHg while arriving venous blood is near 40 mmHg, so oxygen diffuses into the blood. Carbon dioxide runs the opposite way, higher in the returning blood than in the alveolus, so it diffuses out to be exhaled. No pump moves these gases — the gradients alone do the work, which is why partial pressure, not bulk air flow, is the quantity that governs direction.
Structural Features That Speed Exchange
Three structural features make alveolar gas exchange remarkably fast. First, an enormous collective surface area — roughly 50 to 70 square metres from 300 to 500 million alveoli — lets huge volumes of gas exchange at once. Second, an extremely thin, moist respiratory membrane shortens the diffusion distance so gases cross in a fraction of a second. Third, breathing continuously refreshes alveolar air, keeping oxygen high and carbon dioxide low so the gradients stay steep. Reduce any of these — collapse alveoli, thicken the membrane, or stop ventilation — and exchange slows dramatically.
Worked examples
Determine the direction oxygen moves across the alveolar membrane and explain why.
- Compare partial pressures: alveolar PO2 ≈ 100 mmHg, arriving blood PO2 ≈ 40 mmHg.
- Apply the rule that a gas diffuses from higher to lower partial pressure.
- Since alveolar oxygen is higher, oxygen diffuses from the alveolus into the blood.
Answer: Oxygen moves from alveolar air into the blood, down its partial-pressure gradient.
Explain why holding your breath slows gas exchange.
- Without fresh breaths, alveolar oxygen falls as it diffuses into blood and CO2 rises as it diffuses out.
- Shrinking the alveolar-blood difference reduces both partial-pressure gradients.
- Because diffusion rate depends on the gradient, gas exchange slows as the gradients flatten.
Answer: Breath-holding lets gradients shrink, so diffusion and gas exchange slow down.
Activity
Match each labeled card to the role it plays in getting oxygen from a breath into the blood.
Practice
Explain how each breath maintains the steep partial-pressure gradients that gas exchange depends on.
Predict how a thickened alveolar membrane, as in some lung diseases, would affect the rate of oxygen diffusion.
Common mistakes to avoid
- The body pumps oxygen across the alveolar wall.Oxygen crosses passively by diffusion down its partial-pressure gradient; no pump or energy is used for this step.
- Venous blood arriving at the lungs is rich in oxygen.Venous blood arriving at the lungs is oxygen-poor, having already delivered oxygen to body tissues.
Check your understanding
In a healthy lung, which way does oxygen move across the alveolar membrane, and why?
Why does breathing in and out keep gas exchange going efficiently?
A common misconception is that the body actively pumps oxygen across the alveolar wall. Why is that wrong?
Which two structural features of alveoli work together to make gas exchange rapid?
Recap
Gas exchange is passive diffusion: oxygen moves from high alveolar partial pressure into the blood, and carbon dioxide moves the opposite way to be exhaled. A vast thin moist surface speeds it, and continuous breathing keeps the partial-pressure gradients steep so exchange stays fast.
Reflect
Why is relying on diffusion instead of an active pump an efficient design for the lungs?