As the posture of the body changes – for example, in going from lying down to st

Question

As the posture of the body changes – for example, in going from lying down to standing up – the effects of gravity on cardiovascular function change significantly. Nevertheless, mean arterial pressure is maintained more or less constantly. In this problem, we consider the phenomenon and the mechanisms that generate it.

Consider a normal individual who quickly goes from a supine (lying down) posture to an erect one. Predict the changes (increase/decrease/no change) that will occur in the following cardiovascular parameters.
A. stroke volume ?

B. heart rate?

C. cardiac output?

D. total peripheral resistance?

E. arterial pressure in foot?

F. venous pressure in foot?

G. arterial pressure in cerebral blood vessels?

Explanation / Answer

stroke volume
decrease
The absolute pressures below heart are increased, leading to increased venous volume below heart (outside of the thoracic cavity). The peripheral venous volume is increased by a portion of the LV output (i.e., the CO) that fills the veins as they stretch. Therefore, the quantity of blood returning to the heart, the venous return (VR) is transiently less than the CO. The volume of blood in the central veins, pulmonary circulation and the HEART must therefore fall. Thus, SV is down because filling volume is down. Activation of baroreceptor reflex increases CO and this also further decreases CVP and SV.

heart rate
increase
Baroreceptor reflex responds to fall in MAP in part by increasing HR.
cardiac output
decrease
CO falls on going erect (see discussion of SV above). The baroreceptor reflex increases HR, but in the new steady state the CO is still less than it was before standing (the reflex response, the increase in HR, is not large enough to fully restore CO to its recumbent value).

total peripheral resistance
increase
The baroreceptor reflex increases TPR to increase MAP back towards normal.
press. at root of aorta
decrease (slight)
MAP = CO x TPR; CO decreases more than TPR increases. (The reflex doesn't raise TPR enough to restore MAP to normal.)

arterial press. in foot
increase
Pressures below the heart are increased because of the added hydrostatic pressure created by the vertical columns of fluid (blood) between the heart and the feet.
venous press. in foot
increase
Venous pressure below heart is increased because of the hydrostatic column.

capillary press. in foot
increase
Pressures below the heart are increased. (Both arterial and venous pressures are up; see above.)
arterial press. in cerebral blood vessel
decrease
Pressures above the heart are less than the pressure at the aorta because of the vertical column "pulling down" the pressure in the head.

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