Why do capillaries permit the diffusion of materials whereas arteries and veins not?

Capillaries permit the diffusion of materials because they have thin walls and large surface areas. This allows substances to pass through the capillary walls by diffusion, which is the movement of molecules from an area of higher concentration to an area of lower concentration.

In contrast, arteries and veins have thicker walls and smaller surface areas, which makes it more difficult for substances to diffuse through them. Arteries also have higher blood pressure than capillaries, which can help to push substances through the capillary walls but can also damage the capillaries if the pressure is too high. Veins have lower blood pressure than capillaries, which can help to prevent substances from leaking out of the capillaries but can also make it more difficult for substances to diffuse into the capillaries.

The difference in the structure and function of capillaries, arteries, and veins allows the body to regulate the flow of blood and nutrients to different tissues. Capillaries allow for the exchange of materials between the blood and the tissues, while arteries and veins transport blood throughout the body.