Cardiovascular Physiology
401
independent entities. However, any change in venous return
almost immediately causes equivalent changes in cardiac out-
put, largely through the Frank-Starling mechanism.
Venous
return and cardiac output therefore must be identical except for
very brief periods of time.
In summary (
Figure 12–46
), venous smooth muscle
contraction, the skeletal muscle pump, and the respiratory
pump all work to facilitate venous return and thereby enhance
cardiac output by the same amount.
The Lymphatic System
The
lymphatic system
is a network of small organs (lymph
nodes) and tubes (
lymphatic vessels
or simp
ly “
lymphat-
ics”) through which
lymph
—a fl uid derived from interstitial
uid—fl ows. The lymphatic system is not technically part of
the cardiovascular system, but it is described in this chapter
because its vessels provide a route for the movement of intersti-
tial fl uid to the cardiovascular system (
Figure 12–47a
).
Present in the interstitium of virtually all organs and tis-
sues are numerous
lymphatic capillaries
that are completely
distinct from blood vessel capillaries. Like the latter, they are
tubes made of only a single layer of endothelial cells resting on
a basement membrane, but they have large water-fi lled chan-
nels that are permeable to all interstitial fl uid constituents,
including protein. The lymphatic capillaries are the fi rst of the
lymphatic vessels, for unlike the blood vessel capillaries, no
tubes fl ow into them.
Small amounts of interstitial fl
uid continuously enter
the lymphatic capillaries by bulk fl ow (the precise mecha-
nisms that control this movement remain unclear). Now
known as lymph, the fl
uid fl ows from the lymphatic capil-
laries into the next set of lymphatic vessels, which con-
verge to form larger and larger lymphatic vessels. At various
points in the body—in particular the neck, armpits, groin,
and around the intestines—the lymph fl ows through lymph
nodes (
Figure 12–47b
), the function of which is described
in Chapter 18. Ultimately, the entire network ends in two
large lymphatic ducts that drain into the veins near the junc-
tion of the jugular and subclavian veins in the upper chest.
Valves at these junctions permit only one-way fl ow from lym-
phatic ducts into the veins. Thus, the lymphatic vessels carry
interstitial fl
uid to the cardiovascular system.
The movement of interstitial fl uid from the lymphat-
ics to the cardiovascular system is very important because, as
noted earlier, the amount of fl uid fi ltered out of all the blood
vessel capillaries (except those in the kidneys) exceeds that
absorbed by approximately 4 L each day. This 4 L is returned
to the blood via the lymphatic system. In the process, the
small amounts of protein that leak out of blood vessel capillar-
ies into the interstitial fl uid are also returned to the cardiovas-
cular system.
Failure of the lymphatic system due, for example, to occlu-
sion by infectious organisms (as in the disease
elephantiasis
)
allows the accumulation of excessive interstitial fl
uid. The result
can be massive swelling of the involved area (
Figure 12–48
).
Surgical removal of lymph nodes and vessels during the treat-
ment of breast cancer can similarly allow interstitial fl uid to
pool in affected tissues. The accumulation of large amounts of
interstitial fl uid from whatever cause is termed
edema
.
In addition to draining excess interstitial fl
uid, the lym-
phatic system provides the pathway by which fat absorbed
from the gastrointestinal tract reaches the blood (Chapter
15). The lymphatics also, unfortunately, are often the route
by which cancer cells spread from their area of origin to other
parts of the body (which is why cancer treatment sometimes
includes the removal of lymph nodes).
Mechanism of Lymph Flow
In large part, the lymphatic vessels beyond the lymphatic
capillaries propel the lymph within them by their own con-
tractions. The smooth muscle in the wall of the lymphatics
exerts a pumplike action by inherent rhythmical contractions.
Because the lymphatic vessels have valves similar to those in
veins, these contractions produce a one-way fl ow toward the
point at which the lymphatics enter the circulatory system.
The lymphatic vessel smooth muscle is responsive to stretch,
so when no interstitial fl uid accumulates, and therefore no
lymph enters the lymphatics, the smooth muscle is inactive.
Cardiac muscle
Stroke volume
End-diastolic
ventricular
volume
Atrial pressure
Venous return
Veins
Venous pressure
Activity of
sympathetic
nerves to veins
Skeletal
muscle pump
Inspiration
movements
Blood
volume
Figure 12–46
Major factors determining peripheral venous pressure, venous
return, and stroke volume. Reversing the arrows in the boxes would
indicate how these factors can decrease. The effects of increased
inspiration on end-diastolic ventricular volume are actually quite
complex, but for the sake of simplicity, they are shown here only as
increasing venous pressure.
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