How do capillaries exchange oxygen, nutrients, hormones, and wastes with body tissues?

Capillaries facilitate the exchange of oxygen, nutrients, hormones, and wastes between the blood and body tissues through their extremely thin walls, which consist of a single layer of endothelial cells surrounded by a basement membrane. This structure acts as a semipermeable membrane that allows for the efficient movement of substances.1

The exchange occurs primarily through three modes of transport:

• Diffusion: Substances soluble in the lipid membrane of capillary cells, such as oxygen and carbon dioxide, pass directly through the membranes. Diffusion is driven by differences in partial pressure, moving gases from areas of higher pressure to areas of lower pressure.12
• Ultrafiltration: Water-soluble substances that are insoluble in the lipid membrane, such as glucose and certain wastes, pass through minute, water-filled pores in the capillary walls.1
• Specific Receptors: Certain substances, such as cholesterol, are transported across the endothelium by specific receptors.1

Generally, oxygen and nutrients pass into tissues at the arteriolar end of the capillary, while carbon dioxide and metabolic waste products enter the vessel at the venous end. Hormones are also delivered to target tissues via this capillary network.134