Cardiovascular Physiology
385
Arterial Blood Pressure
What are the factors determining the pressure within an elas-
tic container, such as a balloon fi lled with water? The pressure
inside the balloon depends on (1) the volume of water, and (2)
how easily the balloon walls can stretch. If the walls are very
stretchable, large quantities of water can be added with only
a small rise in pressure. Conversely, the addition of a small
quantity of water causes a large pressure rise in a balloon that
is diffi
cult to stretch. The term used to denote how easily a
structure stretches is
compliance:
Compliance =
volume/
pressure
The higher the compliance of a structure, the more easily it
can be stretched.
These principles apply to an analysis of arterial blood
pressure. The contraction of the ventricles ejects blood into
the pulmonary and systemic arteries during systole. If a pre-
cisely equal quantity of blood were to fl ow simultaneously out
of the arteries, the total volume of blood in the arteries would
remain constant, and arterial pressure would not change.
Such is not the case, however. As shown in
Figure 12–30
,
a volume of blood equal to only about one-third the stroke
volume leaves the arteries during systole. The rest of the
stroke volume remains in the arteries during systole, distend-
ing them and raising the arterial pressure. When ventricular
contraction ends, the stretched arterial walls recoil passively,
like a stretched rubber band being released, and blood contin-
ues to be driven into the arterioles during diastole. As blood
leaves the arteries, the arterial volume and, therefore, the
arterial pressure slowly fall, but the next ventricular contrac-
tion occurs while there is still adequate blood in the arteries
to stretch them partially. Therefore, the arterial pressure does
not fall to zero.
The aortic pressure pattern shown in
Figure 12–31a
is
typical of the pressure changes that occur in all the large sys-
temic arteries. The maximum arterial pressure reached during
Table 12–4
Functions of Endothelial Cells
1. Serve as a physical lining that blood cells do not normally
adhere to in heart and blood vessels.
2. Serve as a permeability barrier for the exchange of
nutrients, metabolic end products, and fl
uid between
plasma and interstitial fl
uid; regulate transport of
macromolecules and other substances.
3. Secrete paracrine agents that act on adjacent vascular
smooth muscle cells; including vasodilators—prostacyclin
and nitric oxide (endothelium-derived relaxing factor,
EDRF)—and vasoconstrictors—notably endothelin-1.
4. Mediate angiogenesis (new capillary growth).
5. Play a central role in vascular remodeling by detecting
signals and releasing paracrine agents that act on adjacent
cells in the blood vessel wall.
6. Contribute to the formation and maintenance of
extracellular matrix.
7. Produce growth factors in response to damage.
8. Secrete substances that regulate platelet clumping, clotting,
and anticlotting.
9. Synthesize active hormones from inactive precursors
(Chapter 14).
10. Extract or degrade hormones and other mediators
(Chapters 11, 13).
11. Secrete cytokines during immune responses (Chapter 18).
12. Infl uence vascular smooth-muscle proliferation in the
disease atherosclerosis.
120
Diastolic
Systolic
Diastolic
Systolic
Pressure (mmHg)
Pulmonary
circulation
Systemic
circulation
Arteries
Arterioles
Capillaries
Venules
Veins
80
0
40
Figure 12–29
Pressures in the systemic and pulmonary
vessels.
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