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