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Human Physiology II · Systems Physiology

Gas Exchange

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On this page 7 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Worked example
  6. Key takeaway
  7. Study tools

In 30 seconds

Gas exchange moves oxygen from alveolar air into pulmonary capillary blood and carbon dioxide in the reverse direction, driven entirely by partial-pressure gradients. says each gas contributes a proportional to its fraction, and says the amount that dissolves is proportional to its partial pressure times its . Oxygen diffuses inward because alveolar PO₂ exceeds capillary PO₂, while CO₂ diffuses out down the reverse gradient. The rate is set by , and efficiency depends on matching ventilation to perfusion (V/Q); mismatches waste either air or blood flow.

Why this matters

Gas-exchange physiology underpins pulse oximetry, arterial blood-gas analysis, and the alveolar-arterial (A-a) oxygen gradient. A widened A-a gradient signals that the lung is not fully equilibrating blood with alveolar gas — as in membrane thickening, shunt, or V/Q mismatch. Reference ranges and diagnostic thresholds vary by institution and jurisdiction; these notes support education only and do not replace clinical instruction or supervision.

The college version

1. Partial pressures: Dalton's law

Dalton's law states that each gas in a mixture exerts a partial pressure equal to the total pressure times its fractional concentration, and the partial pressures add to the total: Pgas = Fgas × Ptotal. In dry atmospheric air at sea level (Ptotal ≈ 760 mmHg), oxygen is ~21%, so PO2 ≈ 0.21 × 760 ≈ 160 mmHg.

2. Solubility: Henry's law

Henry's law states that the amount of gas that dissolves in a liquid is proportional to its partial pressure times its solubility: concentration = solubility × partial pressure. CO₂ is far more soluble than O₂, so at equal partial pressures much more CO₂ dissolves.

3. The alveolar gas equation and the respiratory membrane

The estimates alveolar PO2: PAO2 = FIO2 × (PB - PH2O) - PACO2R, where FIO2 = inspired oxygen fraction, PB = barometric pressure, PH2O = water-vapor pressure (~47 mmHg at body temperature), PACO2 = alveolar CO₂, and R = respiratory exchange ratio (~0.8). The — alveolar epithelium, fused basement membranes, and capillary endothelium — is ~0.5 µm thick with ~70 m² of area, nearly ideal for diffusion.

How it works

  1. Inspired air is humidified and mixed with alveolar gas, setting alveolar PO₂ ≈ 100 and PCO₂ ≈ 40 mmHg.
  2. Mixed venous blood (PO₂ ≈ 40, PCO₂ ≈ 45 mmHg) reaches the pulmonary capillaries.
  3. Oxygen diffuses down its gradient from alveolus to blood; CO₂ diffuses down the reverse gradient.
  4. Fick's law sets the rate: large area, thin membrane, steep gradient, and high solubility all speed transfer.
  5. Blood equilibrates within ~0.25 s, leaving with arterial PO₂ ≈ 100 and PCO₂ ≈ 40 mmHg.
  6. Local control mechanisms match ventilation to perfusion in each region; persistent V/Q mismatch leaves some blood poorly oxygenated or some air unused.

Common confusions

Do not confuseWithDifference
Partial pressureConcentrationPartial pressure is the driving force; concentration also depends on solubility (Henry)
Dalton's lawHenry's lawDalton describes partial pressures in gas mixtures; Henry describes how much dissolves in liquid
DiffusionPerfusionDiffusion = molecular movement across the membrane; perfusion = blood flow delivering/picking up gas
Low V/QHigh V/QLow = blood without air (shunt-like); high = air without blood (dead-space-like)
Alveolar PO₂Arterial PO₂Alveolar is the air-side value; arterial is after equilibration (normally nearly equal)

Memory aids

"Gases go their own way (Dalton), in proportion to how well they dissolve (Henry), as fast as the door allows (Fick), if air and blood show up together (V/Q)." For numbers: 100/40 out, 40/45 in — alveolar air is 100 (O₂)/40 (CO₂); venous blood is the reverse.

Quick review

Topic Recap

Gas exchange is diffusion down partial-pressure gradients. Dalton's law defines each gas's partial pressure, Henry's law adds solubility, the alveolar gas equation predicts alveolar PO₂, and Fick's law sets the diffusion rate across the thin, vast respiratory membrane. The key clinical variable is V/Q matching, because even normal diffusion cannot compensate for air and blood delivered to different places.

Knowledge Check

  1. What is the partial pressure of oxygen in dry atmospheric air at sea level?
  2. Why does CO₂ move so readily across the membrane despite a tiny partial-pressure gradient?
  3. State Fick's law and name one change that would slow diffusion.
  4. A lung region is perfused but not ventilated. Is its high or low, and what results?
  5. What partial pressures characterize mixed venous blood?

Answers and Rationales

  1. About 160 mmHg — 0.21 × 760 mmHg by Dalton's law.
  2. Because CO₂ is much more soluble than O₂, so even a small gradient (45 → 40 mmHg) moves large amounts (Henry's law).
  3. Rate = (Area × diffusion coefficient × gradient) ÷ thickness. Thickening the membrane (edema, fibrosis) or shrinking the area slows diffusion.
  4. Low (near zero, "shunt-like"). Blood passes through without picking up oxygen and returns poorly oxygenated.
  5. PO₂ ≈ 40 mmHg and PCO₂ ≈ 45 mmHg.
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Imagine a crowded room (the blood) and an empty hallway (the alveolus) joined by a doorway. People (oxygen molecules) drift from crowded to empty spots on their own until both sides are equally crowded — no pump needed. Carbon dioxide is the same crowd moving the other way, because for CO₂ the blood is the crowded side.

The key trick is that each gas moves on its own. Oxygen ignores how much CO₂ is around; it only "looks" at its own crowding — its partial pressure. How many molecules get through the door depends on how wide it is (area and thinness) and how fast they walk (solubility).

Where it stops being exact: the room and hallway never fully equalize, because fresh air keeps arriving and blood keeps leaving, and the door isn't always the same width. Making sure air and blood show up at the same doorway at the same time is V/Q matching, and it's where real lungs most often fall short.

Simple Example

Inflate a balloon and open its neck: CO₂-rich air flows out because room air has almost no CO₂.

Worked example

  1. Setting the gradients. Alveolar air has PO2 ≈ 100 mmHg and PCO2 ≈ 40 mmHg; mixed venous blood arriving at the capillaries has PO2 ≈ 40 and PCO2 ≈ 45 mmHg. Oxygen diffuses alveolus → blood, CO₂ blood → alveolus, each down its own gradient.
  2. Fick's law: V̇gas = A × D × (P1 - P2)T (V̇gas = transfer rate, A = membrane area, D = diffusion coefficient ∝ solubility ÷ √molecular weight, (P1 - P2) = partial-pressure gradient, T = thickness). Larger area and gradient speed diffusion; thicker membranes and low solubility slow it.
  3. Equilibration. Blood spends only ~0.75 s in the capillary yet normally equilibrates its PO₂ with alveolar air well within that time, leaving with PO2 ≈ 100 and PCO2 ≈ 40 mmHg. Membrane thickening (fibrosis, edema) or area loss (emphysema) slows this.
  4. V/Q matching. Efficiency requires ventilation (V̇) and perfusion (Q̇) in the same alveoli; the ideal ratio is ~0.8–1.0. Low V/Q (shunt-like): perfused but not ventilated, so blood passes without picking up O₂. High V/Q (dead-space-like): ventilated but not perfused, so air is wasted. Both lower arterial PO₂ and raise arterial PCO₂.

Key takeaways

  • High yield: Each gas diffuses down its own partial-pressure gradient (Dalton), not down total-pressure differences.
  • High yield: CO₂ is far more soluble than O₂, so its small gradients still move large amounts (Henry).
  • High yield: Alveolar PO₂ ≈ 100 and PCO₂ ≈ 40 mmHg; mixed venous blood is ~40/45.
  • High yield: Fick's law: rate ↑ with area and gradient, ↓ with thickness.
  • High yield: Ideal V/Q ≈ 0.8–1.0; low V/Q = shunt-like (blood without air), high V/Q = dead-space-like (air without blood).

Keep learning

Ready to build on this? Continue to the next lesson.

Study tools & related lessonsYou’ll learn to · Key vocabulary · Related

You’ll learn to

  • Apply Dalton's law of partial pressures to calculate the partial pressure of each gas in air and in the alveoli.
  • Explain Henry's law and why gas solubility determines how much gas dissolves in blood.
  • Use the alveolar gas equation to predict alveolar PO₂ and explain what changes it.
  • Describe diffusion across the respiratory membrane with Fick's law and explain how ventilation-perfusion (V/Q) matching determines gas-exchange efficiency.

Key vocabulary

Dalton's law
Each gas's partial pressure = fraction × total pressure
Partial pressure
The pressure one gas contributes
Henry's law
Dissolved gas = solubility × partial pressure
Solubility
How readily a gas dissolves in liquid
Alveolar gas equation
Formula estimating alveolar PO₂
Respiratory membrane
Alveolar epithelium + fused basement membranes + endothelium
Fick's law
Diffusion rate ∝ area × gradient ÷ thickness
Ventilation (V)
Airflow reaching the alveoli
Perfusion (Q)
Blood flow through pulmonary capillaries
V/Q ratio
Ventilation ÷ perfusion (~0.8 ideal)

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