Hematocrit
Dec 21, 2015
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