CVS response to standing

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:

• Peripheral vasoconstriction
  * Increase systemic vascular resistance
• Peripheral venoconstriction
  * Increasing venous return and cardiac output
  * Relatively little (because baroreceptors have less effect on capacitance vessles)
• Increased HR
• Increased contractility

Intermediate term

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

Muscle pump

• Carotid sinus reflex is more effective in causing vasoconstriction than venoconstriction
• Muscle pump minimise venous pooling in the lower limbs and is very significant in minimising BP drop

If muscle pump mechanism is removed,

• Blood pools in the leg and cerebral perfusion becomes inadequate
• Carotid sinus reflex alone is not effective in producing venoconstriction

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

Overall

[WG21:p633]

• Peripheral vascular resistance increased by 25%
• Central blood pool = Reduced by 400mL
• Central venous pressure = Reduced by 3 mmHg
• Cardiac output = Reduced by 25%
• Stroke volume = Reduced by 40%

Other notes

• Venous pressure in legs does not reach equilibrium value until almost 1 minute after standing
  [BL8:p221]
• In elderly, sympathetic response is slower and less effective

BP in foot

• BP in foot would be less than 30mmHg when walking
  * [WG21:p634]
• BP in foot would be 80~90mmHg when standing still
  * [BL8:p222]