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