Hematocrit
• hematocrit is the percentage of whole blood
which is composed of solid material
– cells, platelets etc
• the blood is composed primarily of water
(~55 %) called plasma
– the hematocrit would be 45
• can vary between 40 and 50
Pressure Difference Drives Blood
Flow in the Systemic Circuit
Pressure Changes Across the Systemic
Circulation
Why the pressure change?
• Blood flow = change in pressure / resistance
• increases in pressure at the beginning or
decreases in pressure at the end will
increase blood flow
• this could result in increased resistance to
compensate (homeostasis)
Resistance
• the most important factor determining blood
flow is resistance
• the most important factor determining
resistance is the radius of the vessel
• Resistance = Length X viscosity / radius4
Cardiac Output during Exercise
• Q increases in direct proportion to the
metabolic rate required to perform task
• linear relationship between Q and VO2
• remember... Q = HR x SV
Stroke Volume and Heart Rate
during Exercise
• in untrained or moderately trained
individuals stroke volume plateaus ~ 40%
VO2 max
• at work rates > 40% VO2 max, Q increases
by HR alone
• See fig 9.17
Changes in Cardiovascular Variables
During Exercise
The Fick Equation
• VO2 = Q x (a-vO2 diff)
• VO2 is equal to the product of cardiac
output and arterial-mixed venous difference
• an increase in either Q or a-vO2 difference
will result in an increase in VO2max
Redistribution of Blood Flow
• Increased blood flow to working skeletal
muscle
• Reduced blood flow to less active organs
– Liver, kidneys, GI tract
Changes in Muscle and Splanchnic
Blood Flow During Exercise
Increased Blood Flow to Skeletal
Muscle During Exercise
• Withdrawal of sympathetic vasoconstriction
• Autoregulation
– Blood flow increased to meet metabolic
demands of tissue
– O2 tension, CO2 tension, pH, potassium,
adenosine, nitric oxide
Redistribution of Blood Flow During
Exercise
Circulatory Responses to
Exercise
• Heart rate and blood pressure
• Depend on:
– Type, intensity, and duration of exercise
– Environmental condition
– Emotional influence
Transition From Rest  Exercise
and Exercise  Recovery
• Rapid increase in HR, SV, cardiac output
• Plateau in submaximal exercise
• Recovery depends on:
– Duration and intensity of exercise
– Training state of subject
Cardiovascular Responses during
Transitions
Incremental Exercise
• Heart rate and cardiac output
– Increases linearly with increasing work rate
– Reaches plateau at 100% VO2max
• Systolic blood pressure
– Increases with increasing work rate
• Double product
– Increases linearly with exercise intensity
– Indicates the work of the heart
Double product = heart rate x systolic BP
Arm vs. Leg Exercise
• At the same oxygen uptake arm work
results in higher:
– Heart rate
• Due to higher sympathetic stimulation
– Blood pressure
• Due to vasoconstriction of large inactive muscle
mass
.
Heart Rate and Blood Pressure During
Arm and Leg Exercise
Prolonged Exercise
• Cardiac output is maintained
– Gradual decrease in stroke volume
– Gradual increase in heart rate
• Cardiovascular drift
– Due to dehydration and increased skin blood
flow (rising body temperature)
.
HR, SV, and CO During
Prolonged Exercise
Summary of Cardiovascular
Adjustments to Exercise
Summary of Cardiovascular
Control During Exercise
• Initial signal to “drive” cardiovascular
system comes from higher brain centers
• Fine-tuned by feedback from:
– Chemoreceptors
– Mechanoreceptors
– Baroreceptors
A Summary of Cardiovascular Control
During Exercise