Biology for AP Courses · The Respiratory System
Gas Exchange across Respiratory Surfaces
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In 30 seconds
Gas exchange is the passive diffusion of oxygen (O₂) and carbon dioxide (CO₂) across a respiratory surface, driven entirely by differences in Partial pressure The pressure contributed by one gas in a mixture, as if it alone filled the space Full entry → — no pumps or active transport are involved. In humans, the respiratory surface is the alveolar membrane in the lungs, where O₂ moves from inhaled air into the blood and CO₂ moves in the opposite direction. Three physical rules govern the whole process: Dalton's law Total pressure = sum of each gas's partial pressure Full entry → (each gas in a mixture exerts its own partial pressure), Henry's law Gas dissolving in liquid depends on its partial pressure and solubility Full entry → (gases dissolve in proportion to their partial pressure and solubility), and Fick's law Diffusion rate ∝ area × gradient × solubility / thickness Full entry → of diffusion (diffusion rate depends on surface area, pressure gradient, distance, and the gas's properties). Whatever the organism — a fish's gill, an insect's tracheae, or a mammal's lung — the same physics applies: gas moves from where its partial pressure is high to where it is low, across a membrane that is thin, large in area, and well supplied with blood.
Why this matters
Every cell in the body consumes O₂ for cellular respiration and produces CO₂ as a waste product, so the body's gas-exchange surfaces are the interface between the atmosphere and the bloodstream. When that interface fails — because of fluid in the lungs (pulmonary edema), destroyed alveoli (emphysema), or a thickened membrane (pulmonary fibrosis) — blood oxygen falls and CO₂ can accumulate, with life-threatening consequences. Understanding partial pressure gradients also explains everyday and high-stakes situations: why breathing at high altitude is harder, why carbon monoxide is dangerous even though it is invisible, and why divers must ascend slowly. On the AP® exam, the partial-pressure reasoning and the factors in Fick's law are classic free-response material, and the same concepts reappear in later chapters on circulation and gas transport.
The college version
Core Concepts
Partial pressure: the driving force
Dalton's law states that the total pressure of a gas mixture equals the sum of the pressures each gas would exert alone. At sea level, atmospheric pressure is about 760 mmHg, and since O₂ makes up roughly 21% of dry air, its partial pressure (PO₂) is about 160 mmHg. Each gas behaves independently: a gradient for O₂ exists even when other gases are present. Henry's law adds that the amount of gas that dissolves in a liquid is proportional to its partial pressure and its solubility — which is why fizzy drinks bubble when opened (CO₂ pressure drops) and why deep-sea divers breathing compressed air must account for nitrogen dissolving in their tissues. In the body, the relevant reference values are commonly taught as: alveolar PO₂ ≈ 100 mmHg, deoxygenated (venous) blood PO₂ ≈ 40 mmHg, and tissue PO₂ lower still. These gradients point O₂ inward and CO₂ outward.
The respiratory membrane: built for diffusion
The human respiratory surface is the Respiratory membrane The thin barrier between alveolar air and blood (type I cells + endothelium) Full entry →, where alveolar air and capillary blood are separated by only two thin cell layers: the type I pneumocytes lining the alveolus and the capillary endothelium, with a fused basement membrane between them. Commonly taught thickness is on the order of 0.2–0.5 µm — thin enough that O₂ and CO₂ diffuse across in well under a second. The lungs maximize the other half of Fick's law: roughly 300 million alveoli create a total surface area of about 70–100 m² (commonly taught), packed against a dense capillary network. Type II pneumocytes secrete Surfactant Phospholipid–protein mix that lowers surface tension Full entry →, a phospholipid–protein mixture that lowers surface tension and keeps tiny alveoli from collapsing — a detail that becomes critical when surfactant is missing (as in premature infants).
Fick's law: what controls the rate
Fick's law of diffusion summarizes the factors that determine how fast a gas crosses a membrane:
Rate of diffusion ∝ (Surface area × Partial-pressure gradient × Gas solubility) / (Membrane thickness × √molecular mass)
Each variable has a real consequence. Larger surface area (more alveoli) → faster exchange. A steeper partial-pressure gradient (bigger difference across the membrane) → faster exchange. A thinner membrane → faster exchange. CO₂ diffuses roughly 20–24 times faster than O₂ across biological membranes because it is far more soluble — which is why CO₂ exchange is rarely limited by diffusion, while O₂ often is. Anything that changes a variable changes gas exchange: emphysema shrinks surface area, pneumonia and edema thicken the membrane or fill the airspace with fluid, and altitude shrinks the gradient.
Two gradients, opposite directions
At the alveolus, the gradients run in opposite directions: O₂ moves from alveolar air (PO₂ ≈ 100 mmHg) into capillary blood (PO₂ ≈ 40 mmHg on arrival), while CO₂ moves from blood (PCO₂ ≈ 45 mmHg) into alveolar air (PCO₂ ≈ 40 mmHg). At the tissue capillaries, the same rules reverse: active cells consume O₂, keeping tissue PO₂ low, so O₂ leaves the blood; cells produce CO₂, keeping tissue PCO₂ high, so CO₂ enters the blood. This is why blood leaving the lungs is oxygen-rich and carbon-poor, and blood leaving tissues is the opposite.
Matching ventilation with perfusion
For exchange to work, the air reaching an alveolus (ventilation) must be matched by blood flow through its capillaries (perfusion). The ventilation/perfusion (V/Q) ratio describes this matching. In a healthy upright lung, gravity creates slight mismatches — more blood flow at the base, more ventilation at the apex — but the body fine-tunes the match by constricting blood vessels in poorly ventilated regions and airways in poorly perfused regions. When disease destroys this matching (e.g., a pulmonary embolism blocks perfusion, or a collapsed airway blocks ventilation), gas exchange suffers even though the lung tissue itself is normal.
Common Confusions
| Do not confuse | With | Difference |
|---|---|---|
| O₂ being "pumped" or actively transported into blood | Passive diffusion down a partial-pressure gradient | Gas moves because of concentration/pressure differences, not cellular work |
| Surfactant production | Type I cells make it | Type II pneumocytes make surfactant; type I cells are the thin exchange lining |
| Oxygen binding to hemoglobin | Oxygen crossing the respiratory membrane | Diffusion moves dissolved O₂ across the membrane; hemoglobin picks it up later in the blood — that's gas transport, the next topic |
| Breathing harder = always more O₂ uptake | Ventilation vs. diffusion | Faster breathing raises ventilation, but diffusion rate is set by area, gradient, and thickness (Fick's law) |
| CO₂ exchange is the limiting step | O₂ exchange is usually the bottleneck | CO₂ diffuses ~20–24× faster, so O₂ fails first in lung disease |
| PO₂ of inhaled air ≈ PO₂ in alveoli | Inhaled (≈160 mmHg) vs. alveolar (≈100 mmHg) | Alveolar air is humidified and mixed with stale CO₂-rich air, lowering its PO₂ |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Imagine a crowded playground: kids (oxygen molecules) squeeze through a gate into an empty playground, and other kids (carbon dioxide) squeeze out through the same gate in the other direction. They don't need anyone to push them — they just move from where there are lots of them to where there are few. Your lungs are a super-thin, super-big gate like that, so the swap happens almost instantly every time you breathe.
Worked example
Why does emphysema cause low blood oxygen even when the person breathes harder? A person with emphysema has lost alveolar walls, so the respiratory surface area is drastically reduced. Ventilation may even increase as the person breathes faster, but Fick's law shows the problem: rate of diffusion ∝ surface area. With less area, fewer O₂ molecules cross per second no matter how steep the gradient or how hard the person breathes. Blood leaving such lungs is under-saturated, producing chronic shortness of breath and low arterial O₂. Now contrast that with a healthy person hiking at high altitude: surface area and membrane thickness are normal, but the gradient has shrunk because inspired PO₂ is lower — the same Fick's-law framework explains both cases, and it predicts the treatment direction (increase inspired O₂, or in emphysema's case address the surface-area loss).
Key takeaways
- Gas exchange is passive diffusion down partial-pressure gradients — no energy, no pumps.
- Dalton's law: each gas exerts its own partial pressure; Henry's law: dissolved gas ∝ partial pressure × solubility.
- Fick's law: rate ∝ (area × gradient × solubility) / (thickness × √MW). Bigger area and gradient and thinner membrane = faster exchange.
- CO₂ diffuses ~20–24× faster than O₂ across respiratory membranes because of its high solubility.
- Respiratory membrane = type I pneumocytes + capillary endothelium, ~0.2–0.5 µm thin, ~70–100 m² total area (commonly taught values).
- Type II pneumocytes make surfactant; type I cells are the gas-exchange lining — a classic exam distinction.
- Alveolar PO₂ ≈ 100 mmHg, venous PO₂ ≈ 40 mmHg, tissue PO₂ lower still (commonly taught references).
- V/Q matching keeps ventilation and perfusion aligned; embolism and airway blockage break it.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
State Dalton's law and use it to explain why alveolar PO₂ (≈100 mmHg) is lower than atmospheric PO₂ (≈160 mmHg).
Show answer
Dalton's law: the total pressure of a gas mixture is the sum of the individual partial pressures. Alveolar air is humidified (water vapor adds pressure) and mixes with CO₂-rich air remaining in the lungs from previous breaths, so O₂ is diluted and its partial pressure falls to ≈100 mmHg.
Write Fick's law in words and predict what happens to O₂ diffusion when the respiratory membrane thickens (e.g., in fibrosis).
Show answer
Rate ∝ (surface area × gradient × solubility) / (membrane thickness × √MW). A thicker membrane divides the rate, so O₂ diffusion slows and blood O₂ falls.
Why does CO₂ diffuse across respiratory membranes much faster than O₂?
Show answer
Because CO₂ is roughly 20–24 times more soluble in biological membranes than O₂, its effective diffusion rate is much higher despite a similar molecular mass.
Which alveolar cell type makes surfactant, and why does its absence matter in premature infants?
Show answer
Type II pneumocytes. Without surfactant, surface tension collapses small alveoli, causing respiratory distress syndrome in premature infants whose surfactant system has not matured.
Describe the direction of O₂ and CO₂ movement at the alveolus and at the tissue capillaries.
Show answer
At the alveolus: O₂ leaves the airspace for the blood, CO₂ leaves the blood for the airspace. At tissue capillaries: O₂ leaves the blood for the cells, CO₂ leaves the cells for the blood.
What does V/Q mismatch mean, and give one example of a condition that causes it.
Show answer
V/Q mismatch means ventilation and blood flow are not aligned in a lung region — e.g., a pulmonary embolism blocks perfusion (high V/Q) or a mucus-plugged airway blocks ventilation (low V/Q), both reducing effective gas exchange.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Partial pressure
- The pressure contributed by one gas in a mixture, as if it alone filled the space
- Dalton's law
- Total pressure = sum of each gas's partial pressure
- Henry's law
- Gas dissolving in liquid depends on its partial pressure and solubility
- Respiratory membrane
- The thin barrier between alveolar air and blood (type I cells + endothelium)
- Type I pneumocyte
- Thin cell that lines the alveolus and allows gas diffusion
- Type II pneumocyte
- Cell that secretes surfactant
- Surfactant
- Phospholipid–protein mix that lowers surface tension
- Fick's law
- Diffusion rate ∝ area × gradient × solubility / thickness
- V/Q ratio
- Ventilation-to-perfusion match within the lungs
Sources & references
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