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 correct answer is option b.

Explanation:

Capillaries refer to the fine bifurcated blood vessels and are the minutest blood vessels exhibiting a diameter of about 5 to 10 micrometers. They are about one-celled thick, that is, they are very thin-walled. They are regarded as the location of exchange of substances between the blood and the cells. The smaller molecules like gases, water, and ions are interchanged via simple diffusion, while the bigger molecules like amino acids, glucose, albumin, and others are passed via active or passive transport process.

when you breathe in, or inhale, your diaphragm contracts (tightens) and moves downward. this increases the space in your chest cavity, into which your lungs expand. the intercostal muscles between your ribs also enlarge the chest cavity. they contract to pull your rib cage both upward and outward when you inhale.