579
CHAPTER FIFTEEN
Cardiovascular System
If the right ventricle of the heart is unable to pump blood out as rap-
idly as it enters, other parts of the body may develop edema because
the blood backs up into the veins, venules, and capillaries, increasing
blood pressure in these vessels. As a result of this increased
back pres-
sure,
osmotic pressure of the blood in the venular ends of the capillar-
ies is less eF
ective in attracting water from tissue ±
uid, and the tissues
swell. This is true particularly in the lower extremities if the person is
upright, or in the back if the person is supine. In the terminal stages of
heart failure, edema is widespread, and ±
uid accumulates in the peri-
toneal cavity of the abdomen. This condition is called
ascites.
Venules and Veins
Venules
(ven
u
ˉlz) are the microscopic vessels that continue
from the capillaries and merge to form
veins
(va
¯nz). The
veins, which carry blood back to the atria, follow pathways
that roughly parallel those of the arteries.
The walls of veins are similar to those of arteries in
that they are composed of three distinct layers. However,
the middle layer of the venous wall is poorly developed.
Consequently, veins have thinner walls that contain less
smooth muscle and less elastic tissue than those of compa-
rable arteries, but their lumens have a greater diameter (see
F
g. 15.25
b
).
Many veins, particularly those in the upper and lower
limbs, contain flaplike
valves
(called semilunar valves),
which project inward from their linings. Valves, shown in
f
gure 15.31
, are usually composed of two leafl
ets pushed
closed if the blood begins to back up in a vein. These valves
aid in returning blood to the heart because they are open as
long as the fl ow is toward the heart but close if it is in the
opposite direction.
pressure, which favors reabsorption. At the arteriolar end of
capillaries, the blood pressure is higher (35 mm Hg outward)
than the colloid osmotic pressure (24 mm Hg inward), so at
the arteriolar end of the capillary, F ltration predominates. At
the venular end, the colloid osmotic pressure is essentially
unchanged (24 mm Hg inward), but the blood pressure has
decreased due to resistance through the capillary (16 mm Hg
outward). Thus, at the venular end, reabsorption predominates
(f g. 15.30)
. (The interstitial fl uid also has hydrostatic pressure
and osmotic pressure, but the values are low and tend to can-
cel each other; as such, they are omitted from this discussion.)
Normally, more fl
uid leaves the capillaries than returns
to them because the net inward pressure at the venular ends
of the capillaries is less than the net outward pressure at the
arteriolar ends of the capillaries. Closed-ended vessels called
lymphatic capillaries collect the excess fl uid and return it
through lymphatic vessels to the venous circulation. Chapter 16
(pp. 617–618) discusses this mechanism.
Sometimes unusual events increase blood fl ow to cap-
illaries, and excess fl
uid enters spaces between tissue cells
(interstitial spaces). This may occur, for instance, in response
to certain chemicals such as
histamine
that vasodilate the
metarterioles and increase capillary permeability. Enough
fl uid may leak out of the capillaries to overwhelm lymphatic
drainage, and affected tissues become swollen (edematous)
and painful.
PRACTICE
36
What forces aF
ect the exchange of substances between blood
and the tissue ±
uid?
37
Why is the ±
uid movement out of a capillary greater at its
arteriolar end than at its venular end?
38
More ±
uid leaves the capillary than returns to it, so how is the
remainder returned to the vascular system?
FIGURE 15.30
Water and other substances leave capillaries because of a net outward pressure at the capillaries’ arteriolar ends. Water enters
at the capillaries’ venular ends because of a net inward pressure. Substances move in and out along the length of the capillaries according to their
respective concentration gradients.
Net force at arteriolar end
Outward force, including hydrostatic pressure
= 35 mm Hg
Inward force of osmotic pressure
= 24 mm Hg
Net outward pressure
= 11 mm Hg
Net force at venular end
Outward force, including hydrostatic pressure
= 16 mm Hg
Inward force of osmotic pressure
=
24
mm Hg
Net inward pressure
=
8 mm Hg
Blood
flow
from
arteriole
Capillary
Tissue
cells
Lymphatic
capillary
Blood
flow to
venule
Outward force,
including
hydrostatic
pressure
35 mm Hg
Inward force
of osmotic
pressure
24 mm Hg
Net outward
pressure
11 mm Hg
Outward force,
including
hydrostatic
pressure
16 mm Hg
Inward force of
osmotic pressure
24 mm Hg
Net inward
pressure
8 mm Hg
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