What is the primary method of substance exchange in the capillaries?

The primary method of substance exchange in the capillaries is diffusion. This process allows for the movement of small molecules and gases, such as oxygen and carbon dioxide, across the capillary walls and between the blood and surrounding tissues. The concentration gradient created by the different concentrations of these substances on either side of the capillary wall drives diffusion; substances naturally move from areas of higher concentration to areas of lower concentration.

Diffusion is effective in capillaries because they have thin walls composed of a single layer of endothelial cells, facilitating the easy passage of small, nonpolar molecules. Additionally, the large surface area of capillaries and their close proximity to tissues enhance this process, ensuring that essential nutrients and gases are efficiently exchanged.

In contrast, active transport involves the movement of substances against their concentration gradient and requires energy, which is not the primary mechanism in capillary exchange. Osmosis specifically refers to the movement of water across a semipermeable membrane and is more relevant in contexts where there is a concentration gradient of solutes. Facilitated diffusion, although it relies on proteins to assist in the transport of substances across membranes, still operates along concentration gradients and is not the main mode for overall substance exchange in capillaries compared to simple diffusion.