364
Chapter 12
SECTION A SUMMARY
System Overview
I. The key components of the circulatory system are the heart,
blood vessels, and blood.
II. The cardiovascular system consists of two circuits: the
pulmonary circulation—from the right ventricle to the lungs
and then to the left atrium, and the systemic circulation—from
the left ventricle to all peripheral organs and tissues and then
to the right atrium.
III. Arteries carry blood away from the heart, and veins carry
blood toward the heart.
a. In the systemic circuit, the large artery leaving the left side of
the heart is the aorta, and the large veins emptying into the
right side of the heart are the superior vena cava and inferior
vena cava. The analogous vessels in the pulmonary circulation
are the pulmonary trunk and the four pulmonary veins.
b. The microcirculation consists of the vessels between arteries
and veins: the arterioles, capillaries, and venules.
Pressure, Flow, and Resistance
I. Flow between two points in the cardiovascular system is
directly proportional to the pressure difference between
those points and inversely proportional to the resistance:
F
=
P
/
R
.
II. Resistance is directly proportional to the viscosity of a fl
uid
and to the length of the tube. It is inversely proportional to the
fourth power of the tube’s radius, which is the major variable
controlling changes in resistance.
SECTION A KEY TERMS
Table 12–1
The Cardiovascular System
Component
Function
Heart
Atria
Chambers through which blood fl ows from veins to ventricles. Atrial contraction adds to ventricular fi lling but is not
essential for it.
Ventricles
Chambers whose contractions produce the pressures that drive blood through the pulmonary and systemic vascular
systems and back to the heart.
Vascular system
Arteries
Low-resistance tubes conducting blood to the various organs with little loss in pressure. They also act as pressure
reservoirs for maintaining blood fl ow during ventricular relaxation.
Arterioles
Major sites of resistance to fl ow; responsible for the pattern of blood fl ow distribution to the various organs;
participate in the regulation of arterial blood pressure.
Capillaries
Major sites of nutrient, metabolic end product, and fl uid exchange between blood and tissues.
Venules
Sites of nutrient, metabolic end product, and fl uid exchange between blood and tissues.
Veins
Low-resistance conduits for blood fl ow back to the heart. Their capacity for blood is adjusted to facilitate this fl ow.
Blood
Plasma
Liquid portion of blood that contains dissolved nutrients, ions, wastes, gases, and other substances. Its composition
equilibrates with that of the interstitial fl
uid at the capillaries.
Cells
Includes erythrocytes that function mainly in gas transport, leukocytes that function in immune defenses, and
platelets (cell fragments) for blood clotting.
aorta
361
arteriole
361
artery
361
atrium
361
blood
360
blood vessels
360
bulk fl ow
360
capillary
361
cardiovascular system
360
circulatory system
360
erythrocytes
360
formed elements
360
heart
360
hematocrit
360
hemodynamics
362
hydrostatic pressure
362
inferior vena cava
361
leukocytes
360
microcirculation
361
plasma
360
platelet
360
portal system
362
pulmonary artery
361
pulmonary circulation
361
pulmonary trunk
361
pulmonary vein
361
resistance (
R
)
362
superior vena cava
361
systemic circulation
361
vascular system
360
vein
361
ventricle
361
venule
361
viscosity
363
SECTION A REVIEW QUESTIONS
1. What is the oxygen status of arterial and venous blood in the
systemic versus the pulmonary circulation?
2. State the formula relating fl ow, pressure difference, and resistance.
3. What are the three determinants of resistance?
4. Which determinant of resistance is varied physiologically to
alter blood fl ow?
5. How does variation in hematocrit infl uence the hemodynamics
of blood fl ow?
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