Download CVS ADAPTATION DURING EXERCISE

CVS ADAPTATION DURING EXERCISE
LEARNING OBJECTIVES
At the end of this lecture the student should be able to:
• Outline the circulatory responses to various types of exercise
• Identify the factors that regulate local blood flow during exercise
• List & discuss those factors responsible for regulation of stroke
volume during exercise
• Discuss the regulation of cardiac output during exercise
•
• CIRCULATORY CHANGES DURING EXERCISE
• One major challenges to homeostasis posed by exercise is the
increased muscular demand for oxygen
• During heavy exercise, oxygen demands may  by 15 to 25 times
• Increase oxygen delivery can occur by increasing blood flow
• Two major adjustments of blood flow are;
–  cardiac output
– Redistribution of blood flow
BLOOD FLOW THROUGH THE MUSCLES
• During rest 3 to 4 ml/min/100 g of muscle.
• During extreme exercise in the well-conditioned athlete  can
increase 15- to 25-fold, rising to 50 to 80 ml/min/100 g of muscle
BLOOD FLOW DURING MUSCLE CONTRACTIONS
• During strong rhythmical muscular exercise the flow increases
and decreases with each muscle
contraction.
• At the end of the contractions blood
flow remains very high for a few
seconds but then fades toward
normal during the next few minutes.
• Decrease in blood flow
mechanical compression of
arteries by the contracted muscle
•
Due to sustained compression of the blood vessels, the blood flow
can be almost stopped,  also causes rapid weakening of the
contraction.
INCREASED BLOOD FLOW IN MUSCLE CAPILLARIES DURING
EXERCISE
• During rest, some muscle capillaries have little or no
flowing blood
• During strenuous exercise, all the capillaries open
• Increase blood flow increase surface area for
exchange  increase nutrient and oxygen supply
to muscle
CONTROL OF BLOOD FLOW THROUGH THE
SKELETAL MUSCLES
Local Regulation--- chemical metabolites causing vasodialtion
• Decreased Oxygen in Muscle Greatly Enhances Flow.
• Increase activity in exercising muscle increase in oxygen
utilization decrease in oxygen concentration causes local
arteriolar vasodilation and release of vasodilator substances
• The most important vasodilator substance is probably adenosine
but the effects wane off with time
• Other vasodilator factors
– (1) potassium ions
– (2) adenosine triphosphate (ATP)
– (3) lactic acid
– (4) carbon dioxide.
NERVOUS CONTROL OF MUSCLE BLOOD FLOW
Sympathetic Vasoconstrictor Nerves
• The sympathetic vasoconstrictor
nerve fibers secrete nor
epinephrine at their nerve
endings.
• can decrease blood flow through
resting muscles to as little as one
half to one third normal.
• Has physiologic importance in
circulatory shock and during other
hypotensive crisis
•
•
•
During strenous exercise muscles become unresponsive to
nor epinephrine’s vasoconstriction
During exercise two adrenal medullae secrete large amounts
of nor epinephrine and epinephrine
epinephrine,mainly acting on beta 2 adrenergic
receptor result in vasodilation
REDISTRIBUTION OF BLOOD FLOW
• Muscle blood flow to working skeletal muscle
• Splanchnic blood flow  to less active organs
– Liver, kidneys, GI tract
CIRCULATORY READJUSTMENT DURING
EXERCISE
• In addition to increase in muscle blood flow
following changes also occur to maximize oxygen
supply to muscle:
(1) mass discharge of the sympathetic nervous system
(2) increase in arterial pressure
(3) increase in cardiac output.
INCREASE IN SYMPATHETIC DISCHARGE
• Impulses travel from motor cortex to muscle to start
contraction
• Impulses also travel to vasomotor area to increase sympathetic
discharge and decrease parasympathetic discharge to
circulation
• Increase sympathetic impulses and decrease para sympathetic
impulses to heart increase heart rate and contractility i.e stroke
volume
• Constriction of arterioles increase in TPR increase BP and
decrease in flow to non muscular organ (except for brain and heart)
• Muscular arterioles are not constricted they are under control of
vasodilatory metabolite
• Constriction of large vein decrease in unstressed volume 
increase in mean systemic filling pressure increase in venous
return increase in cardiac output
INCREASE IN ARTERIAL PRESSURE DURING EXERCISE
• Result of
– vasoconstriction of the arterioles and small arteries in most
tissues of the body except the active muscles
– increased pumping activity of the heart
– a great increase in mean systemic filling pressure caused mainly
by venous contraction
• Magnitude of increase varies
depending upon type of exercise
•
•
•
When a person uses few muscles
in exercise  small number of
vessels are vasodilated
sympathetic stimulation has a
major role in increasing TPR and
BP
During massive exercise and running many muscles are
active most of the muscle vessels are dilated decreasing TPR
BP increases but to alesser extent
Increase in arterial BP not only increases flow in a direct relation,
but also streches vascular wall increase flow by additional
20percent
CARDIAC OUT PUT DURING EXERCISE
• Increases due to 2 reasons:
– Increase sympathetic drive to heart
– Increase in venous return
• Increase sympathetic stimulation:
– Increases cardiac output to about 2.5fold
by:
– Increase heart rate as high as 170190bpm
– Increase in contractility of heart
• Increase venous return:
– Venoconstriction leadto increase mean
systemic filling pressure
– Decrease resisitance in all vessels of
active muscle lead to decrease
resistance to venous return
CHANGES IN HEART RATE AND STROKE VOLUME
• The cardiac output increases from 5.5 L/min at rest to 30
L/min in the marathon runner by:
– Increasing stroke volume from 105 to 162 milliliters, about
50 %
– Increasing heart rate increases from 50 to 185 beats/min, an
increase of 270 per cent
• The heart rate increase accounts by far for a greater proportion of
the increase in cardiac output
• Stroke volume reaches plateu by the time cardiac output is only
half max.
• Any further increase in cardiac output must occur by increasing the
heart rate.
TRANSITION FROM REST  EXERCISE AND EXERCISE 
RECOVERY
• Rapid increase in HR, SV, cardiac output
• Plateau in submaximal (below lactate
threshold) exercise
• Recovery depends on:
– Duration and intensity of exercise
– Training state of subject
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
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
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)
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
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