Structure and function of capillaries •
Structure of capillaries
The thin capillary walls allow gases and other
substances to pass through by diffusion.
Capillaries are narrow (5-10p m in diameter). Blood
flows through them slowly and blood cells move
through them single file.
drains the interstitial
fluid and returns fluid to blood plasma
(see Chapter 12).
More fluid moves out
of the capillaries
at the arteriole end
More fluid moves
into the capillaries
at the venule end
In the capillaries, exchange of substances occurs as follows:
• Red blood cells exchange oxygen for carbon dioxide.
• Nutrients diffuse out of the blood into the interstitial fluid and into cells.
• Wastes diffuse from the cells into the interstitial fluid and into the capillaries
• Fluid moves out of the capillary at the arteriole end (net filtration) and into the capillary at the venule end (net absorption).
The direction of net fluid movement is a balance between hydrostatic pressure and osmotic pressure.
• Interstitial fluid drains into lymph capillaries, which ultimately return the fluid to the blood plasma.
Blood Vessels Are the Body's Plumbing 325