A healthy young adult stands up from a supine (lying down) position. Which of th

Question

A healthy young adult stands up from a supine (lying down) position. Which of the following sets of cardiovascular changes is most likely to occur? A(n) _____ in heart rate and a(n) ______ in total peripheral resistance.

a. increase/increase

b. increase/decrease

c. decrease/decrease

d. decrease/increase

In the absence of any compensatory responses by any parts of the body, a decrease in blood volume leads to

a. vasoconstriction

b. vasodilation

c. an increase in cardiac output

d. decrease in hemocrit

Explanation / Answer

The correct answer is option (a) increase/increase.

Baroreceptor reflex responds to the fall in Mean Arterial Pressure (MAP) in part by increasing Heart Rate.

The baroreceptor reflex increases Total Peripheral Resistance to increase MAP back towards normal.

Thus, both HR and TPR increase when a man stands up from a supine position.

If there are compensatory mechanisms, then vasoconstriction would be the right answer. But, in the given case there are no compensatory mechanisms. Therefore, the right answer is option (d) decrease in hemocrit. The decrease in blood volume eventually decreases the number of red blood cells, decreasing the hemocrit.

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