ORGAN BLOOD FLOW
Understanding the concepts presented in this chapter will enable the student to:
Describe the distribution of cardiac output among major organs when a person
is at rest.
Describe how various tissue and endothelial factors influence tissue blood flow.
Explain how extravascular compression alters blood flow in the heart and con-
tracting skeletal muscle.
Define autoregulation of blood flow, reactive hyperemia, and active (functional)
hyperemia and describe their mechanisms in different organs.
Compare and contrast autonomic control of blood flow in major vascular beds
of the body.
Describe the specialized vascular anatomy and function in the following organs:
brain, heart, intestines and liver, skin, kidneys, and lungs.
This chapter describes the blood flow to dif-
ferent organs of the body. The first part of the
chapter emphasizes local regulatory mecha-
nisms by which organs regulate their own
blood flow to meet the metabolic and func-
tional requirements of the organ. The second
part of the chapter examines blood flow in spe-
cific organs of the body.
DISTRIBUTION OF CARDIAC
We have previously seen that arterial pressure
is generated as the heart pumps blood into the
systemic circulation. This arterial pressure
serves as the driving force for blood flow to
all the organ systems. The relative distribu-
tion of blood flow to the organs is regulated
by the vascular resistance of the individual
organs, which is influenced by extrinsic (neu-
rohumoral) and intrinsic (local regulatory)
mechanisms as summarized in Chapter 5,
Table 7-1 summarizes the distribution of
cardiac output when a person is at rest. Most
of the cardiac output (-80%) goes to the gas-
trointestinal tract, kidneys, skeletal muscle,
heart, and brain, although these organs make
up <50% of the body mass. This relative dis-
tribution of cardiac output, however, changes
greatly depending on environmental condi-
tions and the state of physical activity. For
example, in a hot, humid environment, the rel-
ative blood flow to the skin increases substan-
tially as the body attempts to maintain its core
temperature by losing heat to the environment.
When a person exercises, the increased cardiac
output primarily goes to the active skeletal
muscles, heart, and skin (see Chapter 9); at
the same time, blood flow decreases to the gas-
trointestinal and renal circulations. Another
example of change in cardiac output distribu-
tion occurs following a meal, when blood flow
to the gastrointestinal circulation increases.
Instead of one “normal” blood flow for an
organ, there is a range of blood flows. Basal
flow refers to the flow that is measured under
basal conditions (i.e., when a person is in a