Which of the following describes the properties that allow capillaries to exchange gas between blood cells and surrounding tissue?

a. capillaries are short vessels with thin walls that create strong pressures forcing gas-infused blood to diffuse to the surrounding tissue.
b. capillaries have thin walls and narrow diameters so that blood cells come close to the thin walls, across which gases can diffuse to surrounding tissue.
c. capillaries have valves along their walls that allow blood cells to move through them to surrounding tissue, where gas exchange can occur.
d. capillaries regulate the pressure of blood within them so that a steady stream of gas-infused blood cells passes into the surrounding tissue.

The answer is b. Capillaries have thin walls and narrow diameters so that blood cells come close to the thin walls, across which gases can diffuse to surrounding tissue.

Capillaries are the smallest of all blood vessels. They have very narrow diameters (5-10 micrometers) and thin walls. All of this allow the blood cells to come close to the walls and cross them so gases diffusion to surrounding tissue can occur.

Through the process of elimination:
- a. is not the right choice because thin walls could not persist strong pressure.
- c. is not the right choice because capillaries do not have valves like veins do.
- d. is not the right choice because capillaries are that small they cannot regulate the blood pressure within them.