capillaries

• The walls of capillaries are just one cell thick
•  allow the exchange of molecules between the blood and the body's cells
• molecules can diffuse across their walls
• Oxygen diffuses through the capillary wall, into the tissue fluid, and the cells.
• carbon dioxide diffuses from the cells into the tissue fluid, then across the capillary walls into the blood plasma.
• Glucose diffuses from the blood plasma, across the capillary walls to the tissue fluid, and then to the cells.
• The waste product urea diffuses from the cells of the liver, to the tissue fluid, and then across the capillary walls into the blood plasma.