Circulatory Changes During Exercise

While the vascular resistance in skeletal muscles decreases during exercise, the resistance to flow through the visceral organs and skin increases. This increased resistance occurs because of vasoconstriction stimulated by adrenergic sympathetic fibers, and it results in decreased rates of blood flow through these organs. During exercise, therefore, the blood flow to skeletal muscles increases because of three simultaneous changes: (1) increased total blood flow (cardiac output); (2) metabolic va-sodilation in the exercising muscles; and (3) the diversion of blood away from the viscera and skin. Blood flow to the heart also increases during exercise, whereas blood flow to the brain does not appear to change significantly (fig. 14.19).

During exercise, the cardiac output can increase fivefold— from about 5 L per minute to about 25 L per minute. This is primarily due to an increase in cardiac rate. The cardiac rate,

Cardiac Output, Blood Flow, and Blood Pressure 425

Table 14.5

Changes in Skeletal Muscle Blood Flow Under Conditions of Rest and Exercise

Condition

Blood Flow (ml/min)

Mechanism

Rest

1,000

High adrenergic sympathetic stimulation of vascular alpha receptors, causing vasoconstriction

Beginning exercise

Increased

Dilation of arterioles in skeletal muscles due to cholinergic sympathetic nerve activity and stimulation of beta-adrenergic receptors by the hormone epinephrine

Heavy exercise

20,000

Fall in alpha-adrenergic activity Increased sympathetic cholinergic activity

Increased metabolic rate of exercising muscles, producing intrinsic vasodilation

25 L/min

20 L/min

25 L/min

20 L/min

Rocky Fitness Changes Throughout

0.75 L/min

0.75 L/min

5 L/min

5 L/min

Cardiac output = 5 L/min

â–  Figure 14.19 The distribution of blood flow (cardiac output) during rest and heavy exercise. At rest, the cardiac output is 5 L per minute (bottom of fgure);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 (= 750 ml/min), whereas during exercise it receives 3% to 4% of 25 L per minute (0.03 25 = 750 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%.

however, can increase only up to a maximum value (table 14.6), which is determined mainly by a person's age. In well-trained athletes, the stroke volume can also increase significantly, allowing these individuals to achieve cardiac outputs during strenuous exercise up to six or seven times greater than their resting values. This high cardiac output results in increased oxygen delivery to the exercising muscles; this is the major reason for the much higher than average maximal oxygen uptake (VO2max) of elite athletes (chapter 12).

In most people, the increase in stroke volume that occurs during exercise will not exceed 35%. The fact that the stroke volume can increase at all during exercise may at first be surprising, given that the heart has less time to fill with blood between beats when it is pumping faster. Despite the faster beat, however, the end-diastolic volume during exercise is not decreased. This is because the venous return is aided by the improved action of the skeletal muscle pumps and by increased respiratory movements during exercise (fig. 14.20). Since the end-diastolic volume is not significantly changed during exercise, any increase in stroke volume that occurs must be due to an increase in the proportion of blood ejected per stroke.

The proportion of the end-diastolic volume ejected per stroke can increase from 60% at rest to as much as 90% dur

Table 14.6 Relationship Between Age and Average Maximum Cardiac Rate*

Age

Maximum Cardiac Rate

20-29

190 beats/min

30-39

160 beats/min

40-49

150 beats/min

50-59

140 beats/min

60+

130 beats/min

*Maximum cardiac rate can be estimated by subtracting your age from 220.

ing heavy exercise. This increased ejection fraction is produced by the increased contractility that results from sympathoadrenal stimulation. There also may be a decrease in total peripheral resistance as a result of vasodilation in the exercising skeletal muscles, which decreases the afterload and thus further augments the increase in stroke volume. The cardiovascular changes that occur during exercise are summarized in table 14.7.

Endurance training often results in a lowering of the resting cardiac rate and an increase in the resting stroke volume. The lowering of the resting cardiac rate results from a greater degree of inhibition of the SA node by the vagus nerve. The increased resting stroke volume is believed to be due to an increase in blood volume; indeed, studies have shown that the blood volume can increase by about 500 ml after only 8 days

Cardiac output t Blood flow to skeletal muscles

t Cardiac rate t Stroke volume

Blood Pressure Health

Blood Pressure Health

Your heart pumps blood throughout your body using a network of tubing called arteries and capillaries which return the blood back to your heart via your veins. Blood pressure is the force of the blood pushing against the walls of your arteries as your heart beats.Learn more...

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