output is approximately 5000 mL per minute. Exercise
cardiac output is three times that, about 15,000 mL
per minute. Keep in mind that the volume of blood is
the same in both cases, but that during exercise the
blood is being circulated more rapidly.
Compare the amounts of blood flowing to various
organs and tissues during exercise and at rest. During
exercise, the heart receives about three times as much
blood as it does when the body is at rest. The very
active skeletal muscles receive about ten times as much
blood. The skin, as an organ of heat loss, receives
about four times as much blood. Other organs, how-
ever, can function adequately with less blood. Blood
flow is reduced to the digestive tract, to the kidneys,
and to other parts of the body such as bones.
When the exercise ceases, cardiac output will
gradually return to the resting level, as will blood flow
to the various organs. These changes in the distribu-
tion of blood ensure sufficient oxygen for active tissues
and an appropriate blood pressure for the body as a
whole.REGULATION OF BLOOD PRESSURE
The mechanisms that regulate systemic blood pressure
may be divided into two types: intrinsic mechanisms
and nervous mechanisms. The nervous mechanisms
involve the nervous system, and the intrinsic mecha-
nisms do not require nerve impulses.INTRINSIC MECHANISMS
The term intrinsicmeans “within.” Intrinsic mecha-
nisms work because of the internal characteristics of
certain organs. The first such organ is the heart. When310 The Vascular System
Adrenal glandPituitaryHeartNorepinephrineEpinephrineAldosteroneADHANPIncreases
excretion of
Na+,
water followsKidneyIncreases
reabsorption
of H 2 OIncreases
reabsorption of
Na+,
water followsIncreases rate
and force of
contractionVasoconstrictionRaises
B.P.Lowers
B.P.Figure 13–10. Hormones that affect blood pressure. See text for further description.
QUESTION:Which two hormones have opposite functions, and what are these functions?