describe gas exchange between air in the alveoli and blood in the capillaries

The Gas Exchange System

Objective

Describe the gas exchange that occurs between the air in the alveoli and the blood in the surrounding capillaries.

Key Concepts

• Alveoli are tiny air‑filled sacs at the end of the respiratory tree.
• Each alveolus is surrounded by a dense network of pulmonary capillaries.
• Gas exchange occurs by simple diffusion driven by partial pressure gradients.
• The respiratory membrane consists of the alveolar epithelium, capillary endothelium and their fused basement membranes.

Partial Pressure Gradients

Diffusion of a gas follows Fick’s law, which can be expressed as:

\$\text{Rate of diffusion} = \frac{D \, A}{T} (P1 - P2)\$

where:

• \$D\$ = diffusion coefficient of the gas
• \$A\$ = surface area of the respiratory membrane
• \$T\$ = thickness of the membrane
• \$P1 - P2\$ = difference in partial pressure of the gas on either side of the membrane

Typical Partial Pressures (mm Hg) at Sea Level

Steps in Gas Exchange

1. Inhaled air reaches the alveoli, raising the \$P{O2}\$ in alveolar air to about 100 mm Hg.
2. Blood arriving in pulmonary capillaries has a lower \$P{O2}\$ (\overline{40} mm Hg) and a higher \$P{CO2}\$ (\overline{46} mm Hg).
3. Oxygen diffuses from alveolar air into the blood because \$P{O2}\$ (alveolus) \$>\$ \$P{O2}\$ (blood).
4. Carbon dioxide diffuses from the blood into the alveolus because \$P{CO2}\$ (blood) \$>\$ \$P{CO2}\$ (alveolus).
5. The now oxygen‑rich blood is carried away by the pulmonary veins to the left side of the heart.
6. Exhaled air removes the CO₂ that entered the alveoli, maintaining the gradient for the next breath.

Factors Influencing the Rate of Diffusion

• Surface area (A): Large total alveolar surface (\overline{70} m²) maximises diffusion.
• Membrane thickness (T): Healthy lungs have a thin barrier (\overline{0}.5 µm). Conditions such as pulmonary fibrosis increase \$T\$ and reduce diffusion.
• Partial pressure difference: Altitude reduces atmospheric \$P{O2}\$, decreasing the gradient.
• Diffusion coefficient (D): Depends on gas solubility; O₂ diffuses more slowly than CO₂.

Clinical Relevance

Impaired gas exchange can arise from:

• Pulmonary edema – fluid fills alveolar spaces, increasing \$T\$.
• Emphysema – loss of alveolar walls reduces \$A\$.
• High altitude – reduced \$P{O2}\$ in inspired air.

Summary

Gas exchange between alveolar air and pulmonary capillary blood is driven by differences in partial pressures of oxygen and carbon dioxide. The efficiency of this process depends on the large surface area, minimal membrane thickness, and the magnitude of the pressure gradients, all of which are described by Fick’s law of diffusion.