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U N I T 5
Circulatory Function
The Systemic Circulation
and Control of Blood Flow
The systemic vascular system is a closed system of ves-
sels that distributes blood from the heart to the tis-
sues and returns blood to the heart. Its three divisions
include the arterial system, which delivers blood to the
tissues; the venous system, which returns blood to the
heart; and the capillaries of the microcirculation, which
separate the arterial and venous systems and is the site
where nutrients and gas exchange take place.
Blood Vessels
All blood vessels, except the capillaries, have walls
composed of three layers, or coats, called
tunicae
(Fig. 17-17). The outermost layer of a vessel, called the
tunica externa
or
tunica adventitia
, is composed pri-
marily of loosely woven collagen fibers that protect the
blood vessel and anchor it to the surrounding structures.
The middle layer, the
tunica media
, is largely a smooth
muscle layer that constricts to regulate and control the
diameter of the vessel. Larger arteries have an external
elastic lamina that separates the tunica media from the
tunica externa. The innermost layer, the
tunica intima
,
consists of a single layer of flattened endothelial cells
with minimal underlying subendothelial connective
tissue. The endothelial layer provides a smooth and
slippery inner surface for the vessel that prevents plate-
let adherence and blood clotting.
The layers of the different types of blood vessels vary
with vessel function. The walls of the arterioles, which
control blood pressure, have large amounts of smooth
muscle. Veins are thin-walled, distensible, and collapsible
vessels. Capillaries are single-cell–thick vessels designed
for the exchange of gases, nutrients, and waste materials.
Vascular smooth muscle cells, which form the pre-
dominant cellular layer in the tunica media, produce
vasoconstriction or dilation of blood vessels. Smooth
muscle contracts slowly and generates high forces for
long periods with low energy requirements; using only
1/10 to 1/300 the energy of skeletal muscle. These char-
acteristics are important in blood vessels that must
maintain their tone day in and day out.
Compared with skeletal and cardiac muscle, smooth
muscle has a less–well-developed sarcoplasmic reticu-
lum for storing intracellular calcium, and it has very
few fast sodium channels. Therefore, depolarization of
smooth muscle relies largely on extracellular calcium,
which enters through calcium channels in the muscle
membrane. Sympathetic nervous system control of vas-
cular smooth muscle tone occurs by way of receptor-
activated opening and closing of the calcium channels.
In general,
α
-adrenergic receptors are excitatory, in that
they causing the channels to open and produce vaso-
constriction, and
β
-adrenergic receptors are inhibitory,
in that they causing the channels to close and produce
vasodilation. Calcium channel–blocking drugs cause
vasodilation by blocking calcium entry through the cal-
cium channels.
Smooth muscle contraction and relaxation also
occur in response to local tissue factors such as lack of
oxygen, increased hydrogen ion concentrations, and
■■
The cardiac cycle, which describes the pumping
action of the heart, is divided into two parts:
systole, during which the ventricles contract and
blood is ejected from the heart; and diastole,
during which the ventricles relax and allow for
filling to occur.The cardiac output, or amount
of blood that the heart pumps each minute,
represents the stroke volume, or amount of
blood pumped with each beat; and the heart
rate, the number of times the heart beats each
minute. Cardiac reserve refers to the maximum
percentage of increase in cardiac output that can
be achieved above the normal resting level.
■■
The heart’s ability to increase its output
according to body needs depends on: (1) the
preload, or filling of the ventricles (i.e., end-
diastolic volume); (2) the afterload, or resistance
to ejection of blood from the heart; (3) cardiac
contractility, which is determined by the
interaction of the actin and myosin filaments
of cardiac muscle fibers; and (4) the heart rate,
which determines the frequency with which
blood is ejected from the heart.
SUMMARY CONCEPTS
(continued)
Artery
Vein
Tunica intima
Tunica
media
Tunica
externa
Tunica
media
Tunica
externa
FIGURE 17-17.
Medium-sized artery and vein, showing the
relative thickness of the three layers.
