What does the walls of alveoli and capillaries are permeable mean?

The phrase "the walls of alveoli and capillaries are permeable" refers to the characteristic feature of the respiratory system that enables efficient gas exchange between the lungs and the bloodstream. Here's what it means:

1. Alveoli:

Alveoli are tiny, sac-like structures in the lungs where gas exchange occurs. Their walls are composed of a thin layer of epithelial cells and an extensive network of capillaries.

2. Capillaries:

Capillaries are the smallest blood vessels in the body. They form a dense network surrounding the alveoli, creating a large surface area for gas exchange.

3. Permeability:

The term "permeable" means that the walls of the alveoli and capillaries allow certain substances to pass through them easily. In this case, the substances of interest are oxygen and carbon dioxide.

4. Gas Exchange:

The permeability of the alveolar and capillary walls is crucial for efficient gas exchange. Oxygen from the inhaled air diffuses across the thin walls of the alveoli and capillaries, moving into the bloodstream. Simultaneously, carbon dioxide, a waste product of cellular respiration, moves from the bloodstream into the alveoli to be exhaled.

5. Diffusion:

Gas exchange occurs through a process called diffusion. Oxygen and carbon dioxide move down their concentration gradients, from areas of higher concentration to areas of lower concentration. The thinness of the alveolar and capillary walls minimizes the distance for diffusion, ensuring rapid and efficient gas exchange.

In summary, the permeability of the alveolar and capillary walls allows oxygen and carbon dioxide to diffuse freely between the lungs and the bloodstream, facilitating respiration and providing the body with a continuous supply of oxygen while removing waste carbon dioxide.