SECTION A SUMMARY
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:
II. Resistance is directly proportional to the viscosity of a ﬂ
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
The Cardiovascular System
Chambers through which blood ﬂ ows from veins to ventricles. Atrial contraction adds to ventricular ﬁ lling but is not
essential for it.
Chambers whose contractions produce the pressures that drive blood through the pulmonary and systemic vascular
systems and back to the heart.
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.
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.
Major sites of nutrient, metabolic end product, and ﬂ uid exchange between blood and tissues.
Sites of nutrient, metabolic end product, and ﬂ uid exchange between blood and tissues.
Low-resistance conduits for blood ﬂ ow back to the heart. Their capacity for blood is adjusted to facilitate this ﬂ ow.
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.
Includes erythrocytes that function mainly in gas transport, leukocytes that function in immune defenses, and
platelets (cell fragments) for blood clotting.
bulk ﬂ ow
inferior vena cava
superior vena cava
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?