HUMAN BIOLOGY

CirCulation: the heart and blood vessels 133

precapillary sphincter

A ring of smooth muscle

that regulates the flow of

blood into a capillary.

hoW do substances move betWeen blood

in capillaries and the surrounding

eXtracellular fluid?

• Substances enter and leave the blood in capillaries—and into
  and out of the extracellular fluid—by diffusion, through capillary
  pores, or by bulk flow.


• Movements of water and other substances into and out of
  capillaries help maintain blood pressure and the proper fluid
  balance between blood and tissues.

taKe-hoMe Message

The lymphatic system, which consists of lymph ves-

sels, lymph nodes, and some other organs, receives

fluid that leaves capillaries and returns it to the

blood. This system also plays a major role in body

defense, the subject of Chapter 9.

Overall, the movements of fluid and solutes

into and out of capillaries help maintain blood

pressure by adding water to, or subtracting it

from, blood plasma. The fluid traffic also helps

maintain the proper fluid balance between blood

and surrounding tissues.

blood in capillary beds flows

onward to venules

Capillary beds are the “turnaround points” for blood in the

cardiovascular system. They receive blood from arterioles,

and after the blood flows through the bed it enters channels

that converge into venules—the beginning of its return trip

to the heart.

At the point where a capillary branches into the capillary

bed, a wispy ring of smooth muscle wraps around it. This

structure, a precapillary sphincter, regulates the flow of

blood into the capillary. The smooth muscle is sensitive to

chemical changes in the capillary bed. It can contract and

prevent blood from entering the capillary, or it can relax

and let blood flow in (Figure 7.18).

For example, if you sit quietly and

listen to music, only about one-tenth

of the capillaries in your skeletal

muscles are open. But if you decide

to get up and dance, precapillary

sphincters will sense the demand for more blood flow to

your muscles to deliver oxygen and carry away carbon

dioxide. Many more of the sphincters will relax, allowing a

rush of blood into the muscle tissue. The same mechanism

brings blood to the surface of your skin when you blush or

become flushed with heat.

F i g u r e 7.17 Fluid may move by bulk flow out of a capillary bed.

blood

from

arteriole

outward

bulk flow

cells of

tissue

blood to

venule

inward osmotic

movement

Figure 7.18 This diagram shows the general direction of
blood flow through a capillary bed when vessel walls are fully
relaxed (A) and fully contracted (B). A precapillary sphincter
wraps around the base of each capillary. (© Cengage Learning)

arteriole venule

from heart to heart

precapillary

sphincters

arteriole venule

thoroughfare

channel

capillaries

from heart to heart

Fully relaxed arteriole and

sphincter muscles allow

maximum blood flow through the

arterioles and capillary networks.

Fully contracted arteriole and sphincter

muscles allow only a minimal amount of

blood to flow through a vessel called a

thoroughfare channel.

A Fully relaxed

B Fully contracted

© Cengage Learning

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