Explain why capillaries have a very small lumen and walls that are only one cell thick.

Capillaries are the smallest blood vessels in the body, and their structure is specifically designed for their function, which is to facilitate the exchange of materials between the blood and the body's cells. Here's why they have a very small lumen and walls that are only one cell thick:

1. Small Lumen: The small lumen of capillaries slows down the flow of blood. This is important because it gives more time for the exchange of materials (like oxygen, nutrients, and waste products) between the blood and the body's cells.

2. Thin Walls: The walls of capillaries are only one cell thick to minimize the distance over which this exchange of materials has to occur. The shorter the distance, the faster the rate of diffusion. This allows for efficient exchange of materials.

In summary, the small lumen and thin walls of capillaries are crucial for their role in material exchange between blood and body cells.