a.
Sphincters relaxed: blood flowing
through capillaries
How arterioles control blood flow into capillaries
Arterioles are smaller, thick-walled vessels that vary
in size from 15 ^m to 300 ^m in diameter. They have an
endothelial layer covered by one or two layers of smooth
muscle. At the distal end, the smooth muscle forms a
pre-
capillary sphincter
, which controls blood flow into the
capillaries (
Figure 11.13
). Like arteries, arteriolar va-
soconstriction and vasodilation can help maintain blood
pressure and control blood flow to organs.
Next, let’s take a closer look at capillaries.
Capillaries Are Thin Exchangers
Between Blood and Tissues
A
capillary
is a thin-walled, microscopic vessel consisting
of only an endothelial layer and a basement membrane
(
Figure 11.14
). Blood flows slowly through these narrow
vessels, where exchange of gases, nutrients, wastes, and
fluid occurs. The exchange of fluid is governed by a balance
between
hydrostatic pressure,
which
drives fluid out of the capillary, and
osmotic pressure
, which draws it
back in. Nutrient-rich fluid moves
out of the capillary via filtration,
while waste-laden fluid is returned
to the capillary, using absorption. In the arteriolar end,
there is net filtration because blood hydrostatic pressure is
b. Sphincters contracted: blood flowing
through thoroughfare channel
• Figure 11.13
the dominant force. However, as blood moves through the
capillary, hydrostatic pressure diminishes and osmotic pres-
sure increases due to the decrease in fluid volume within
the capillary. So, in the venule end, there is net absorption
because osmotic pressure is the dominant force. About
85-90% of the filtered fluid gets absorbed. The remaining
filtered fluid drains into lymph capillaries, eventually re-
turning to the blood plasma. The overall effect is that blood
volume does not change.
The interstitial fluid also remains relatively consistent
during this exchange. In the condition known as
edema
,
however, the interstitial fluid level increases. Edema can
be caused by many factors, but they generally fall into
the following categories: increased hydrostatic pressure
(causing too much filtration), decreased osmotic pressure
(causing too little absorption), or a lymphatic system is-
sue (leading to inability to return the excess filtrate to the
system). Because excess fluid is left in the tissues, visible
swelling is a common sign of edema.
At the capillary level, local conditions regulate vasodi-
lation and vasoconstriction. Chemicals released from cells
can vasodilate nearby arterioles and increase blood flow to
the cells. Vasoconstrictors released by cells can have the
opposite effects. The ability of tissues to adjust blood flow
based on local conditions is called
autoregulation
.
Now let’s take a closer look at veins and venules.
osmotic pressure
The pressure of a fluid
due to its solute con-
centration, mostly the
protein content.
324 CHAPTER 11
The Cardiovascular System: Heart, Blood Vessels, and Circulation
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