Human Physiology, 14th edition (2016)

Cardiac Output, Blood Flow, and Blood Pressure 471

metabolism of the brain regions responsible for motor control

and somatosensory information. By contrast, during heavier

exercise (at greater than 60% of the maximal oxygen uptake)

the cerebral blood flow decreases somewhat. This is because

the person hyperventilates, which lowers the blood CO 2 and

thereby produces cerebral vasoconstriction. This cerebral

vasoconstriction during heavy exercise may contribute to cen-

tral fatigue (chapter 12, section 12.4).

blood flow (cardiac output); (2) metabolic vasodilation in the
exercising muscles; and (3) the diversion of blood away from
the viscera and skin. For similar reasons, coronary blood flow
to cardiac muscle also increases significantly during exercise.
The total blood flow to the brain is relatively constant
( fig. 14.20 ), but recent studies suggest that cerebral blood flow
increases somewhat during light to moderate exercise. This
is believed to result from vasodilation induced by increased

Figure 14.20 The distribution of blood flow (cardiac output) during rest and heavy exercise. At rest, the cardiac
output is 5 L per minute ( bottom of figure ); during heavy exercise the cardiac output increases to 25 L per minute ( top of figure ). At rest,
for example, the brain receives 15% of 5 L per minute ( 5  750 ml/min), whereas during exercise it receives 3% to 4% of 25 L per minute
(0.03  3  25 L/min  5  750 ml/min; at 4%, the brain receives 1000 ml/min). Flow to the skeletal muscles increases more than twentyfold
because the total cardiac output increases (from 5 L/min to 25 L/min) and because the percentage of the total received by the muscles
increases from 15% to 80%. Note that the percentage of blood flow to the skin during heavy exercise was immeasurably low, so
no percentage is indicated. Also, the percentages for the blood flow to different organs at rest only add up to 95% because these
percentages are estimated averages.

100%

5 L/min

20%–25% 4%–5% 20%

Cardiac output

= 5 L/min

Cardiac output

= 25 L/min

Heavy

exercise

Heavy

exercise

Rest

Rest

3%–5% 15% 4%–5% 15%–20%

~0.75 L/min

~20 L/min

100% 3%–5% 4%–5% 2%–4% 3%–4% 80%–85%

25 L/min

0.5%–1%