3. Physiology
        3.2. Cardiovascular
            3.2.6. Special circumstances CVS response to standing

CVS response to standing

[Ref: KB2: p56; WG21:p633-634]

Response is similar to that of blood loss

Immediate response

Blood pooling in lower extremites
--> Venous return decreases
--> Cardiac output decreases
--> BP decreases

Sensed by carotid baroreceptors
--> Stimulation of vasomotor and cardiac centres in the medulla
--> Sympathetic stimulation

Thus, results are:

Intermediate term

Decreased stretching in volume receptor
--> Increase in circulating levels of renin and aldosterone

Muscle pump

If muscle pump mechanism is removed,

Cerebral perfusion

Due to hydrostatic effect
--> Decreasing cerebral perfusion pressure
--> CBF reduced

As a result
--> pO2 and pH decrease, pCO2 increase
--> Dilation of the cerebral vessels
--> Cerebral vascular resistance reduced
--> CBF partially restored

Overall, CBF reduces by 20% on standing

O2 extraction increases
--> CMRO2 stays the same

Volume and effects of gravity

When blood volume is low
--> Effects due to gravity are more marked

When blood volume is high
--> Effects are minimal



Other notes

BP in foot


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