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Partial Pressures & Diffusion: What Pushes Gases Across

Gases do not need a pump — they slide down pressure gradients. Learn what a partial pressure is, how oxygen and carbon dioxide gradients are arranged, and the simple rules that govern how fast each gas diffuses.

What a partial pressure means

Air is a mixture of gases, mostly nitrogen and oxygen. Each gas contributes its own share of the total pressure, and that share is its partial pressure. Think of a crowded room: the total push on the walls comes from everyone, but you can ask how much push comes from one group alone. The partial pressure of oxygen is just the oxygen’s slice of the total.

Gases diffuse from higher partial pressure to lower partial pressure. So if we know the partial pressure of oxygen in the alveolus versus in the arriving blood, we know which way oxygen will move — and the bigger the difference, the faster it goes. The oxygen pressure already measured in arterial blood is written as PaO2, and the carbon dioxide pressure as PaCO2.

The gradients, laid out

Here is the flow of oxygen, in approximate pressures (in mmHg). Air enters rich in oxygen; blood returning to the lungs is oxygen-poor. The gradient between them drives oxygen into the blood. Meanwhile carbon dioxide runs the opposite way, from oxygen-poor blood out into the alveolus.

Oxygen (O2) — pushed INTO the blood:
  Inspired air (humidified)   PO2 ~150 mmHg
  Alveolar gas                PAO2 ~100 mmHg
  Blood arriving at alveolus  PO2  ~40 mmHg   <- oxygen-poor
  Gradient pushing O2 in:     ~100 - 40 = 60 mmHg
  Blood leaving alveolus      PaO2 ~100 mmHg  (now full)

Carbon dioxide (CO2) — pushed OUT of the blood:
  Blood arriving at alveolus  PCO2 ~46 mmHg
  Alveolar gas                PACO2 ~40 mmHg
  Gradient pushing CO2 out:   ~46 - 40 = 6 mmHg  (small but enough)

Why the small CO2 gradient still works:
  CO2 diffuses ~20x more easily than O2 (more soluble),
  so a tiny pressure difference moves plenty of gas.
Approximate partial-pressure gradients across the alveolar–capillary membrane. Numbers are typical resting values, not exact.

What makes diffusion fast or slow

  1. Surface area — more open, healthy alveoli mean more places for gas to cross. Emphysema destroys surface area and slows transfer.
  2. Membrane thickness — a thicker or scarred alveolar–capillary membrane (as in fibrosis) slows every molecule down.
  3. Pressure gradient — a bigger difference in partial pressure pushes harder. Breathing extra oxygen raises the alveolar value and steepens the gradient.
  4. Gas solubility & size — carbon dioxide crosses far more easily than oxygen, which is why oxygen is usually the first to fall when the membrane is damaged.

Doctors can actually measure how well the membrane transfers gas with a test called DLCO (the diffusing capacity). A low DLCO points to a damaged or destroyed exchange surface — for example, in fibrosis or emphysema — and it is one of the most useful clues in the whole field.