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

The Gas Exchange System 🫁

In the lungs, the air you breathe (in the alveoli) meets the blood in tiny capillaries. Oxygen (O₂) moves from the alveoli into the blood, while carbon dioxide (CO₂) moves in the opposite direction. This swap is essential for life and happens through a simple process called diffusion.

Alveoli and Capillaries

Alveoli are like tiny balloons that fill with air during inhalation. Each balloon is surrounded by a network of microscopic blood vessels (capillaries). The walls of both alveoli and capillaries are extremely thin—just one cell thick—so gases can cross them easily.

The Diffusion Process

Diffusion is the movement of molecules from an area of high concentration to an area of low concentration. In the lungs:

1. Air enters the alveoli, raising the partial pressure of O₂ inside them.
2. O₂ diffuses across the alveolar wall into the capillary blood.
3. CO₂, which is higher in the blood, diffuses out into the alveoli.
4. Exhaled air carries the CO₂ away from the body.

Mathematically, diffusion can be described by Fick’s law:

where $J$ is the flux (amount of gas per unit area per unit time), $D$ is the diffusion coefficient, and $\frac{dC}{dx}$ is the concentration gradient.

Factors Affecting Diffusion

• Alveolar surface area – more surface means more gas can cross.
• Thickness of the alveolar–capillary membrane – thinner = faster diffusion.
• Partial pressure difference ($P_{O_2}$ and $P_{CO_2}$) between alveoli and blood.
• Temperature – higher temperatures increase diffusion rates.
• Blood flow (cardiac output) – keeps fresh blood at the capillary wall.

Partial Pressures in Alveoli and Capillaries

Exam Tips 📚

Remember:

• Use the partial pressure gradient to explain why O₂ moves into blood and CO₂ moves out.
• Show the diffusion equation and identify each variable when asked.
• Discuss how surface area and membrane thickness influence gas exchange efficiency.
• Explain the role of cardiac output in maintaining fresh blood at the capillary wall.
• Use the alveolar–capillary diagram (or table) to support your answers.