Cardiovascular Response to Exercise

Introduction – CV adjustments during exercise due to:

(1) central command

CNS activation of the sympathetic system
reflexes activated intramuscularly
carried by myelinated (group III) or unmyelinated (group IV, may be chemoreceptors) afferent nerves to CNS; efferent nerves are sympathetic fibers to heart and vessels

(2) baroreceptors
(3) reflexes originating in skeletal muscle
(4) local chemical factors

Mild to Moderate exercise

inhibits parasympathetic (vagal) and increases sympathetic Þ increase HR and contractility Þ increased CO

peripheral resistance: constriction in some areas allows dilation in others

vasoconstriction in kidney, splanchnic regions, inactive muscle via sympathetic stim
flow to brain remains constant
flow to active muscles and myocardium increases

vasodilation from metabolites (K, adenosine, pH)

flow to skin initially decreases, then increases, then decreases at maximum O₂ consumption
only a few caps perfused at rest, almost all during exercise

hydrostatic pressure increases, lymph flow increases

cardiac output

despite high HR, SV only increases 10-35%
diastolic pressure is almost constant, systolic pressure increases, mostly due to increases in SV

venous return

sympathetic constriction of capacitance vessels
return aided by pumping of muscles and respiratory system
blood volume doesn’t rise in humans, may even fall during exercise

fluid loss from sweating, fluid circulating in active areas, etc
counteracted by: interstitial pressure increase; hydrostatic pressure decrease in splanchnic areas leads to fluid movement into blood stream; reduced urine formation

efficient pumping through right atrium, etc, means that central venous pressure almost constant
Frank Starling mech only comes into play at heavy exercise

arterial pressure

despite decrease in resistance, pressure increases so increased CO is greater than decreased TPR
BP maintenance and increase made possible by vasoconstriction in inactive areas

Severe Exercise

HR hits max; SV plateaus and often drops; BP drops; dehydration

vasoconstriction in skin to compensate, leads to heat retention

rise in pH of tissues from increased lactic acid production

Endurance Training

lower resting HR from higher vagal and lower sympathetic tone

max HR is still same, but attained at a higher level of performance

endurance training increases SV without thickening walls of LV(low HR means more filling time)

strength training thickens walls without changing volume

HR decrease and SV increase leads to same CO

lower peripheral resistance (TPR) in muscle from more capillaries and arterioles

with major blood loss, lower TPR is compensated for by larger CO to maintain mean arterial pressure (BP) and flow (flow = pressure x resistance ; mean arterial pressure = CO x TPR)