Structure and function of capillaries •
Figure 11.14
a.
Structure of capillaries
Endothelium
Lumen
Basement
membrane
The thin capillary walls allow gases and other
substances to pass through by diffusion.
Blood
Red blood
capillary
cell
900x
Capillaries are narrow (5-10p m in diameter). Blood
flows through them slowly and blood cells move
through them single file.
b.
Capillary
exchange
Blood flow
Tissue
cell
Lymph
capillary
drains the interstitial
fluid and returns fluid to blood plasma
(see Chapter 12).
Precapillary
sphincter
J
Interstitial
fluid
Capillary
Osmotic
pressure
Hydrostatic
pressure
I
V
,_________
Osmotic
pressure
Hydrostatic
pressure
V
.______
More fluid moves out
of the capillaries
at the arteriole end
More fluid moves
into the capillaries
at the venule end
Blood flow
Arteriole
Venule
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
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