Exercise
Summary of CVS changes during exercise
- No change in cerebral blood flow
- Increase in overall muscle blood flow
* Vasodilation + capillary recruitment in active muscles
* Vasoconstriction in inactive muscles
- Increased oxygen offloading at tissues
- Increase in cardiac output
- Fluid shift to ISF and increased lymph flow
- Skin vasodilation and sweat production
- BP increases more in isometric than in isotonic exercises
- Stroke volume and cardiac output increase more in isotonic than isometric exercise.
Muscle blood flow
At rest, blood flow is low (2-4mL/100g/min)
When muscle tension is >10% maximum
--> Compression of the vessels
When muscle tension is >70% of maximum
--> Blood flow stops
At the start of exercise
Blood flow sometimes increase at or before the start of exercise
Initial rise may be due to neural factors
--> sympathetic vasodilator system involved
Systemic vs local
Neural and local mechanism oppose each other.
[BL8:p247]
In inactive muscles, increased sympathetic activity causes vasoconstriction
In exercising muscles, local mechanism (vasodilatory) dominates
Local mechanism of vasodilation
- Decrease in tissue pO2
- Increase in tissue pCO2
- Accumulation of K+ and other metabolites
- Increase in local temperature
10-100 fold increase in the number of open capillaries
--> Recruitment
Systemic CVS changes
Systemic response to exercise depends on whether muscle contractions are primarily isometric or isotonic
Isometric exercises
Heart rate increases at the start
- Largely due to decreased cardiac vagal tone
- Some contribution by increased cardiac sympathetic stimulation
Total peripheral resistance increased
- Partly due to contracted muscles
--> Both SBP and DBP increase sharply
However,
Stroke volume remain relatively unchanged
Isotonic exercises
HR also increases
Marked increase in stroke volume
Net decrease in total peripheral resistance
Thus,
--> SBP increases moderately
--> DBP remains steady or even falls
Changes in cardiac output
[BL8:p218;WG21:p637]
Increased in venous return and cardiac output during exercise is mainly due to:
- Reduction in peripheral vascular resistance
- Positive inotropic effect of sympathetic stimulation
- Auxiliary pumping action of the contracting skeletal muscles
- Increase in effective blood volume
* Mobilisation of blood from viscera
* Venoconstriction by sympathetic stimulation
NB:
- Changes in heart rate is permissive, but not causative
* i.e. Increased HR allows increase in CO, but does not cause the increase
Fluid shift and lymph flow
Fluid transudation into interstitial spaces because
- Accumulation of metabolite in ISF decreases osmotic gradient
- Increase in hydrostatic pressure
Thus,
- Lymph flow is also greatly increased
Changes in oxygen dissociation
Decreased pH and increased temperature
--> Shift oxygen dissociation curve to RIGHT
(i.e. Oxygen affinity decreased)
2,3-DPG also increases
--> Further decreases oxygen affinity
Overall, right shift in ODC
--> Increased offloading of oxygen at tissue
--> Increased A-V O2 difference
Thermoregulation
Hypothalamic temperature rises
--> Stimulate anterior hypothalamus
Thus,
- Skin vessels dilation
* Decreased sympathetic tone in cutaneous vessels
* Bradykinin produced as a result of sweat gland stimulation --> vasodilation
- Sweat production
* Stimulation of cholinergic sympathetic nervous system