C h a p t e r 1 7
Control of Cardiovascular Function
397
Interstitial
fluid
Tissue cell
Venule
Arteriole
Lymphatic
capillary
Blood
capillary
Interstitial fluid
Opening
Anchoring filament
Endothelium of
lymphatic capillary
Lymph
Tissue cell
A
B
FIGURE 17-21.
(A)
Location of the lymphatic capillary. Blood
from the arterial side of the capillary moves into the interstitial
spaces and is reabsorbed in the venous side of the capillary
bed.
(B)
Details of the lymphatic capillary with its anchoring
filaments and overlapping edges that serve as valves and can
be pushed open, allowing the inflow of interstitial fluids and
suspended particles.
back to the heart. Importantly, the lymphatics can carry
proteins and large particulate matter away from the tis-
sue spaces, neither of which can be removed by absorp-
tion into the venous system. The lymphatic system is
also the main route for absorption of fats and fat soluble
vitamins from the gastrointestinal tract.
The lymphatic system is made up of vessels similar
to those of the blood vessels in the circulatory system.
These vessels commonly travel along with an arteriole
or venule or with a companion artery and vein. The ter-
minal lymphatic vessels are made up of a single layer of
connective tissue with an endothelial lining and resemble
blood capillaries. The lymphatic vessels lack tight junc-
tions and are loosely anchored to the surrounding tis-
sues by fine filaments (Fig. 17-21). The loose junctions
permit the entry of large particles, and the filaments
hold the vessels open under conditions of edema, when
the pressure of the surrounding tissues would otherwise
cause them to collapse. The lymph capillaries drain into
larger lymph vessels that ultimately empty into the right
and left thoracic ducts (Fig. 17-22). The thoracic ducts
empty into the circulation at the junctions of the subcla-
vian and internal jugular veins.
Although the divisions are not as distinct as in the cir-
culatory system, the larger lymph vessels show evidence
of having intimal, medial, and adventitial layers simi-
lar to those of blood vessels. Contraction of the smooth
muscle in the medial layer of the larger collecting lymph
channels assists in propelling lymph fluid toward the
thorax. External compression of the lymph channels
by active and passive movements of body parts also
aids in forward propulsion of lymph fluid. The rate of
flow through the lymphatic system by way of all of the
various lymph channels, approximately 120 mL/hour,
is determined by the interstitial fluid pressure and the
activity of lymph pumps.
The Microcirculation
The most important function of the circulatory sys-
tem occurs in the microcirculation, which consists of
the arterioles, capillaries, and venules. It is here that
the transport of nutrients to the tissues and removal of
metabolites takes place. Blood enters the microcircula-
tion through an arteriole, passes through the capillar-
ies, and leaves by way of a small venule (Fig. 17-23).
Small cuffs of smooth muscle, the precapillary sphinc-
ters, are positioned at the arterial end of the capillary.
The smooth muscle tone of the arterioles, precapillary
sphincters, and venules controls blood flow through
the capillary bed. Depending on venous pressure, blood
flows through the capillary channels when the precapil-
lary sphincters are open.
An important aspect of the circulatory system, which
occurs at the level of the microcirculation, is the abil-
ity of organs and tissues to regulate their blood flow
based on metabolic needs. Local control is particularly
important in tissues such as skeletal muscle and in the
heart, organs in which the metabolic activity and need
for blood flow vary extensively; and in the brain where
metabolic activity and need for blood flow remain rela-
tively constant.
Autoregulation of Blood Flow
Autoregulation
is a local control mechanism that auto-
matically adjusts tissue blood flow independent of sys-
temic factors. For example, blood flow to organs such
as the heart, brain, and kidneys remains relatively con-
stant, although blood pressure may vary over a range
of 60 to 180 mm Hg. In contrast to the mean arterial
pressure, which is controlled by systemic mechanisms
that adjust the cardiac output to maintain that pressure,
changes in blood flow to the individual body tissues are
controlled intrinsically by modifying the diameter of
local arterioles feeding the capillaries.
There are two mechanisms that control autoregula-
tion: metabolic and myogenic. In most tissues, declining
levels of nutrients, particularly oxygen, are the stron-
gest stimuli for autoregulation. Substances released
by metabolically active tissues (such as potassium and
hydrogen ions, lactic acid, and adenosine, which is a
breakdown product of ATP) serve as autoregulation
stimuli. Whatever the precise stimuli, the net result is an
immediate vasodilation of the arterioles serving the cap-
illaries of the metabolically deprived tissues. Inadequate
blood perfusion to an organ is quickly followed by a
decline in its metabolic rate and, if prolonged, death
of its cells. Likewise, excessively high arterial pressure
and tissue perfusion can be dangerous because it may
damage the more fragile blood vessels. The
myogenic
(myo = muscle; gen = organ) control mechanisms rely on
stretch of the vascular smooth muscle in the vessel wall.
