Cardiac Output34 35 45 24 1 8005

FIGURE 4 Distribution of cardiac output during rest and maximal exercise, when cardiac output increases from 5 to 25 L/min. (Adapted from Powers, SK, Howley, ET., Exercise physiol., 1997.)

FIGURE 4 Distribution of cardiac output during rest and maximal exercise, when cardiac output increases from 5 to 25 L/min. (Adapted from Powers, SK, Howley, ET., Exercise physiol., 1997.)

vasoconstrict arterioles. Sympathetic activation of vasoconstrictor pathways occurs in response to a central command (i.e., feedforward signals) from the motor cortex to medullary cardiovascular centers and from a muscle chemoreflex. The muscle chemoreflex is a sympathetic response to stimulation of small-diameter, nonmyelinated sensory nerves in exercising muscles by lactic acid and K+ ion. Arterial chemoreceptors may be stimulated by H+ above the anaerobic threshold which would be expected to produce a bradycardia and vasoconstriction, as observed in the diving reflex. However, the diving reflex is inhibited by increased ventilation, so arterial chemoreceptors have no significant effects on the cardiovascular system during exercise.

Changes in the distribution of blood flow during dynamic exercise are shown in Fig. 4. Most of the cardiac output is directed toward exercising muscle, including the heart. The brain receives almost the same absolute blood flow at rest and at exercise, so it represents a much smaller percentage of the cardiac output during exercise. However, other organs, such as the kidney and splanchnic bed, actually experience decreased absolute blood flow. Both renal mechanisms for correcting arterial acidosis and gastrointestinal absorption of fuels are too slow to make a significant difference in the acid-base balance or metabolism during exercise. Sympathetic vasoconstriction in these vascular beds explains the decreased flow;

hence, the redistribution of blood flow among organs during exercise depends on an integrated response of local and reflex mechanisms.

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