20B11: Exam Report
Describe the changes in the circulatory system that occur during exercise.
22% of candidates passed this question.
This is an applied physiology question. Better answers categorised the changes in some manner and included a measure of the degree of change as applicable (e.g., what increases, what decreases and what may stay the same). The question was to describe the changes so that the detail behind the mechanisms enabling these changes to occur was expected (e.g., neurohumoral, local factors). Marks were also awarded for any regional variation that occurs.
G5i / 20B11: Describe the changes in the circulatory system that occur during exercise.
Definition
Exercise = strenuous physical activity where there is increased metabolic activity leading to increased requirement for energy and oxygen
CVS Parameters
CVS changes to accommodate and compensate for such increase in metabolism
Parameter
Degree of Change
Mechanism of Change
Cardiac Output
Degree of Change
Increase up to 4-5 folds
CO plateaus at near-maximal workload
Mechanism of Change
Cardiac output can increase up to 20L/min
Due to both increase in HR (mediated by SNS activation) and SV and decreased afterload
With increasing workload HR increases but SV decreases due
Heart Rate
Degree of Change
Increases
Max HR = 220-age
Mechanism of Change
Increases due to SNS activation -> increased circulating catecholamines
Stroke Volume
Degree of Change
Increases
Mechanism of Change
Initially increases – up to 40-60% of the person’s VO2 max
After which it reaches a plateau 2’ to increased heart rate
Contractility
Degree of Change
Increases
Mechanism of Change
Increased HR -> increased contractility (treppe effect)
Activation of sympathetic nervous system -> increased contractility
CVP
Degree of Change
Increases
Mechanism of Change
Cardiac output and venous return are equally matched
Increased blood returned to the heart 2’ to increased cardiac
SVR
Degree of Change
Decreases
Mechanism of Change
Vasodilation 2’ to muscle hypoxia and accumulation of metabolic byproducts
Increased concentration of vasoactive mediators – NO/ATP/prostaglandins
Activation of B2 receptor in skeletal muscles -> further vasodilation
SBP
Degree of Change
Increases
Mechanism of Change
Increases due to increase in stroke volume and Velocity of
DBP
Degree of Change
Decreases
Mechanism of Change
Due to decrease in peripheral vascular resistance
MAP
Degree of Change
Increase
Mechanism of Change
Increases slightly
Increase in cardiac output > decrease in peripheral vascular
Pulse Pressure
Degree of Change
Widens
Mechanism of Change
Due to changes seen in SBP & DBP
Coronary Blood Flow
Degree of Change
Increases
Mechanism of Change
Due to increase in perfusion pressure of the coronary arteries + coronary vasodilation as mediated by increased catecholamines
Flow is coupled with increased metabolism
Skeletal Muscle Blood Flow
Degree of Change
Increases
Mechanism of Change
Increased muscle activity -> increased o2 demand
⦁ Regional vasodilation
– Increased local concentration of metabolites ie Co2, lactate, K+
– Vasoactive mediator released by endothelium ie NO, ATP
– Sympathetic stimulation -> B2 adrenoceptor activation
Overall: decrease in vascular resistance to muscles -> increased blood flow
Corresponding vasoconstriction of viscera and skin
Author: Zoe Guo