What is the primary substance that undergoes diffusion through capillaries?

The process of diffusion through capillaries involves the movement of substances from areas of higher concentration to areas of lower concentration. In the context of capillaries, various substances—such as carbon dioxide, nutrients, and hormones—can diffuse through the thin walls of these blood vessels.

Capillaries have a unique structure that allows for the exchange of gases, nutrients, and waste products between the blood and surrounding tissues. The diffusion process is facilitated by the concentration gradients of these substances. For example, carbon dioxide, which is a waste product of cellular metabolism, moves from the tissues (where its concentration is higher) into the blood (where it is lower) to be transported to the lungs for exhalation. Similarly, nutrients present in higher concentrations in the blood diffuse into the surrounding tissues where they are needed for metabolic processes. Hormones, which are also released into circulation, travel from their sites of secretion to distant organs or tissues through a similar diffusion process once they reach the capillary level.

Therefore, the correct answer encapsulates the idea that capillaries are involved in the diffusion of all these substances, making the choice encompassing all of the listed components the most comprehensive and accurate response.