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:
- 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]