What controls what enter a capillary?

The entry of substances into a capillary is primarily controlled by the structure of the capillary walls, which are composed of a single layer of endothelial cells. These cells have small pores and junctions that allow selective permeability, permitting certain molecules, such as oxygen and nutrients, to pass while restricting larger molecules and cells. Additionally, the concentration gradients of substances and the presence of specific transport mechanisms, such as diffusion and active transport, influence what enters the capillary. Overall, these factors work together to regulate the exchange of materials between blood and surrounding tissues.