independent entities. However, any change in venous return
almost immediately causes equivalent changes in cardiac out-
put, largely through the Frank-Starling mechanism.
return and cardiac output therefore must be identical except for
very brief periods of time.
In summary (
), 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
is a network of small organs (lymph
nodes) and tubes (
ics”) through which
—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 (
Present in the interstitium of virtually all organs and tis-
sues are numerous
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
), 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
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-
Failure of the lymphatic system due, for example, to occlu-
sion by infectious organisms (as in the disease
allows the accumulation of excessive interstitial ﬂ
uid. The result
can be massive swelling of the involved area (
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
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.
nerves to veins
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.