right atrium to the left atrium to bypass the fetal
lungs.- The ductus arteriosus permits blood to flow from
the pulmonary artery to the aorta to bypass the
fetal lungs. - These fetal structures become nonfunctional after
birth, when the umbilical cord is cut and breathing
takes place.
Velocity of Blood Flow (see Fig. 13–9)
- Velocity is inversely related to the cross-sectional
area of a segment of the vascular system. - The total capillaries have the greatest cross-
sectional area and slowest blood flow. - Slow flow in the capillaries is important to permit
sufficient time for exchange of gases, nutrients, and
wastes.
Blood Pressure (BP)—the force exerted by
the blood against the walls of the blood ves-
sels (Fig. 13–9)
- BP is measured in mmHg: systolic/diastolic.
Systolic pressure occurs during ventricular con-
traction; diastolic pressure occurs during ventricu-
lar relaxation. - Normal range of systemic arterial BP: 90 to 120/60
to 80 mmHg. - BP in capillaries is 30 to 35 mmHg at the arterial
end and 12 to 15 mmHg at the venous end—high
enough to permit filtration but low enough to pre-
vent rupture of the capillaries. - BP decreases in the veins and approaches zero in
the caval veins. - Pulmonary BP is always low (the right ventricle
pumps with less force): 20 to 25/8 to 10 mmHg.
This low BP prevents filtration and accumulation
of tissue fluid in the alveoli.
Maintenance of Systemic BP
- Venous return—the amount of blood that returns
to the heart. If venous return decreases, the heart
contracts less forcefully (Starling’s law) and BP
decreases. The mechanisms that maintain venous
return when the body is vertical are:
- Constriction of veins with the valves preventing
backflow of blood - Skeletal muscle pump—contraction of skeletal
muscles, especially in the legs, squeezes the deep
veins - Respiratory pump—the pressure changes of
inhalation and exhalation expand and compress
the veins in the chest cavity
- Heart rate and force—if heart rate and force
increase, BP increases. - Peripheral resistance—the resistance of the arteries
and arterioles to the flow of blood. These vessels
are usually slightly constricted to maintain normal
diastolic BP. Greater vasoconstriction will increase
BP; vasodilation will decrease BP. In the body,
vasodilation in one area requires vasoconstriction
in another area to maintain normal BP. - Elasticity of the large arteries—ventricular systole
stretches the walls of large arteries, which recoil
during ventricular diastole. Normal elasticity low-
ers systolic BP, raises diastolic BP, and maintains
normal pulse pressure. - Viscosity of blood—depends on RBCs and plasma
proteins, especially albumin. Severe anemia tends
to decrease BP. Deficiency of albumin as in liver or
kidney disease tends to decrease BP. In these cases,
compensation such as greater vasoconstriction will
keep BP close to normal. - Loss of blood—a small loss will be rapidly com-
pensated for by faster heart rate and greater vaso-
constriction. After severe hemorrhage, these
mechanisms may not be sufficient to maintain nor-
mal BP. - Hormones—(see Fig. 13–10) (a) Norepinephrine
stimulates vasoconstriction, which raises BP; (b)
epinephrine increases cardiac output and raises BP;
(c) ADH increases water reabsorption by the kid-
neys, which increases blood volume and BP; (d)
aldosterone increases reabsorption of Naions by
the kidneys; water follows Naand increases blood
volume and BP; (e) ANP increases excretion of
Na ions and water by the kidneys, which de-
creases blood volume and BP.
Distribution of Blood Flow- Metabolically active tissues require more oxygen,
and receive a greater proportion of the blood vol-
ume as it circulates (see Fig. 13–11). - Blood flow is increased by the dilation of arterioles
and precapillary sphincters. - In less active tissues, arterioles and precapillary
sphincters constrict. - Organs receive sufficient oxygen, and BP for the
body is maintained within the normal range.
316 The Vascular System