Gas Exchange - MCAT Biology | Premier MCAT Prep

Gas Exchange

7 min read Updated Mar 26, 2026

Gas exchange is the entire reason the respiratory system exists. Every tube, every muscle, every membrane in the previous sections is designed to get oxygen from the atmosphere into your blood and carbon dioxide from your blood into the atmosphere. The mechanism is simple: diffusion down partial pressure gradients. No pumps, no active transport, no ATP. Just molecules moving from where there are more of them to where there are fewer.

Dalton’s Law of Partial Pressures

The total pressure of a gas mixture equals the sum of the individual partial pressures of each gas. This is the same Dalton’s law from general chemistry - here we apply it to respiration.

The composition of dry atmospheric air:

Gas	Percentage	Partial Pressure (at 760 mmHg)
Nitrogen (N2)	78%	593 mmHg
Oxygen (O2)	21%	160 mmHg
Argon (Ar)	0.9%	7 mmHg
Carbon dioxide (CO2)	0.04%	0.3 mmHg

Alveolar Gas Composition

The air inside alveoli is not the same as atmospheric air. It differs because:

Water vapor is added (airways humidify incoming air to 100% humidity), which dilutes all other gas partial pressures
Oxygen is continuously being absorbed into the blood
Carbon dioxide is continuously being added from the blood

Gas	Atmospheric Air	Alveolar Air
$P_{\text{O}_{2}}$	160 mmHg	~104 mmHg
$P_{\text{CO}_{2}}$	0.3 mmHg	~40 mmHg
$P_{\text{H}_{2}\text{O}}$	Variable	47 mmHg (at 37°C)

The water vapor pressure at body temperature (37°C) is always 47 mmHg. This must be subtracted when calculating alveolar gas pressures.

Henry’s Law

Henry’s Law governs how much gas dissolves in a liquid (like blood):

Fick’s Law of Diffusion

Fick’s Law describes the rate at which a gas diffuses across a membrane like the respiratory membrane:

How Fick’s Law Applies to Disease

Every factor in Fick’s Law can be disrupted by disease:

Factor	Normal	Disease State	Effect
Surface area (A)	~70 m²	Emphysema (alveolar walls destroyed)	Decreased diffusion
Membrane thickness (T)	~0.5 µm	Pulmonary fibrosis, edema (thickened barrier)	Decreased diffusion
Pressure gradient (ΔP)	60 mmHg for O2	High altitude (low atmospheric $P_{\text{O}_{2}}$ )	Decreased diffusion
Diffusion coefficient (D)	Normal	Not usually altered by disease	N/A

Diagram showing gas exchange at the alveolar capillary interface, with CO2 entering the red blood cell and being converted to bicarbonate by carbonic anhydrase, while O2 diffuses into the blood — Gas exchange at the capillary level. CO2 enters the RBC and is converted to bicarbonate (HCO3-) by carbonic anhydrase, while O2 dissolves in plasma and binds hemoglobin. *Credit: OpenStax Anatomy & Physiology 2e, CC BY 4.0*

The Partial Pressure Gradients Driving Gas Exchange

Gas exchange happens at two locations: the lungs and the tissues.

At the lungs (external respiration):

Gas	Alveolar Air	Pulmonary Capillary Blood (arriving)	Direction
O2	104 mmHg	40 mmHg	Into blood
CO2	40 mmHg	46 mmHg	Into alveolus

Oxygen diffuses from the alveolus ( $P_{\text{O}_{2}}$ = 104) into the blood ( $P_{\text{O}_{2}}$ = 40) because of the 64 mmHg gradient. CO2 diffuses from the blood ( $P_{\text{CO}_{2}}$ = 46) into the alveolus ( $P_{\text{CO}_{2}}$ = 40) because of the 6 mmHg gradient. Blood leaving the lungs has $P_{\text{O}_{2}}$ of ~100 mmHg and $P_{\text{CO}_{2}}$ of ~40 mmHg.

At the tissues (internal respiration):

Gas	Systemic Capillary Blood	Tissue Cells	Direction
O2	100 mmHg	~40 mmHg	Into tissues
CO2	40 mmHg	~46 mmHg	Into blood

The process reverses: oxygen diffuses from blood into tissues, and CO2 diffuses from tissues into blood.

Normal vs. Impaired Gas Exchange

Under normal conditions, blood flowing through the pulmonary capillary has enough time to fully equilibrate with alveolar air - oxygen transfer is very efficient. In diseases that thicken the membrane (fibrosis, edema) or reduce the surface area (emphysema), diffusion slows and blood may not fully equilibrate before leaving the capillary, resulting in hypoxemia.

Why does CO2 diffuse faster than O2 across the respiratory membrane despite having a much smaller partial pressure gradient?

Click to reveal answer

CO2 is approximately 20 times more soluble than O2 in the aqueous respiratory membrane. According to Fick's Law, diffusion rate depends on the diffusion coefficient (which includes solubility) multiplied by the pressure gradient. CO2's much higher solubility more than compensates for its smaller pressure gradient (6 mmHg vs. 60 mmHg for O2).

According to Fick's Law, which two disease processes would impair gas exchange, and how?

Click to reveal answer

Emphysema reduces surface area (A) and pulmonary fibrosis/edema increases membrane thickness (T). Fick's Law: Rate = (A x D x ΔP) / T. Decreasing A (emphysema destroys alveolar walls) or increasing T (fibrosis deposits scar tissue, edema adds fluid to the membrane) both reduce diffusion rate. Both cause hypoxemia before hypercapnia because CO2 diffuses 20x faster than O2.