Exercise and Integrated Cardiovascular Response

Question 1

Whole body O2 consumption (QO2)

Answer 1

• Rest: 250 ml O2 / min
• Exercise: 3-5 L / min
• QO2 is permitted through a parallel increase in cardiac output

Question 2

Triggers for CV Responses During Exercise

• Central command
• Reflexes from skeletal muscle

Answer 2

• Central command
  
  • Increases HR, contractile force, & vasoconstriction before physical activity is initiated
  • Exhibit similar changes in BP & HR when imagine exercise as when muscle contraction actually occurs
• Reflexes from skeletal muscle also elicit increases in CO & vasoconstriction
  
  • Involved muscle afferents
    
    • Unmyelinated fibers that are sensitive to chemicals produced by contracting muscle
    • Small myelinated fibers that respond to pressure changes during muscle contraction
  • Decrease baroreceptor reflex gain
    
    • Triggered by inputs from muscle receptors & central command
    • Signals –> nucleus tractus solaritus –> inhibit neurons that receive baroreceptor signals

Question 3

BP During Exercise

Answer 3

• MAP = CO * TPR
  
  • CO increases a lot during exercise
  • MAP increases only moderatly during exercise
• Diastolic vs. systolic pressure during exercise
  
  • Diastolic pressure doesn’t increase in proportion to systolic pressure
    
    • Diastolic pressure may drop
  • Decreased vascular compliance –> drop in BP during diastole
  • Decreased TPR –> rapid runoff of arterial blood into capillaries –> large drop in pressure during diastole
• Decreased peripheral resistance
  
  • Limits the increase in BP during exercise
  • Vessels dilate to increase perfusion & oxygen delivery
  • Increase in blood flow / perfusoin to skeletal, cadiac muscle, & skin (to dissipate heat)
  • Decrease in blood flow / perfusion to digestive system & kidneys

Question 4

Factors contributing to exercise-related CV responses: cardiac output

• Effects of exercise on HR, contractility, & CO
• Ways to increase HR
• Max limit on HR
• Moderately increased HR & stroke volume
• Stroke volume & cardiac output
• Factors that contribute less to enhancing venous return

Answer 4

• Exercise –> increased HR & contractility –> increased CO
• Ways to increase HR
  
  • Decrease PNS
  • Increase SNS
    
    • Binding of NE or Epi on autorhythmic cells –> increase firing rate –> increase HR
    • Catecholamines increase conduction through the AV node, bundle of His, & Purkinje fibers
• Max limit on HR
  
  • Heart beats too rapidly (170 bpm) –> insufficient filling time
  • Limit is imposed by delay in conduction through AV node & long refractory period of myocardial cells
    
    • Ensures heart won’t beat too rapidly
  • Increase in HR alone can’t explain increase in CO
• Why moderately increased HR doesn’t diminish stroke volume
  
  • Most ventricular filling occurs early during diastole just after the AV valves open
    
    • Mitigates effects of a shortened diastolic period
  • Increased HR –> increased contraction intensity
    
    • Enhances ventricular filling
  • Elastic recoil of prevoius ventricular contraction draws blood into relaxing chamber when residual volume is small
  • Bowditch effect: automatically increases contractility as HR increases
• Stroke volume must increase to increase CO
  
  • Binding of catecholamines to beta-receptors –> increased contractility
  • Starling’s Law of the Heart: skeletal muscle contractions –> increased venous return –> increased cardiac output
• Factors that contribute less to enhancing venous return
  
  • Increased respiratory muscle contractions suck blood into thorax from abdomen
  • Contraction of smooth muscle within veins

Question 5

Factors contributing to exercise-related CV responses: changes in blood flow

• SNS outflow during exercise
  
  • Increased to…
  • Decreased to…
• Effects of vasoconstriction vs. Epi
• Major factor contributing ot muscle vasodilation during exercise

Answer 5

• Increased SNS outflow to blood vessels during exercise
  
  • Not the same in every vascular bed
  • Increased SNS outflow to…
    
    • Viscera
    • Skeletal muscle
  • Decreased SNS outflow to…
    
    • Skin (would inhibit cooling of the body)
    • Digestive organs
• Effects of muscle vasoconstriction is opposed by the release of Epi from the adrenal medulla
  
  • Epi binding to beta2 receptors in skeletal muscle promotes vasodilation
• Major factor contributing to muscle vasodilation during exercise: release of paracrines from exercising muscle
  
  • H+ from acids (lactic acid), K+, adenosine, & CO2 –> vasodilation
  • Regulates blood flow during exercise
  • Ensures only muscles that need a large blood flow to meet metabolic needs will have increased perfusion

Question 6

What happens in a trained athlete to potentiate the ability to exercise

• Result of training
• Marathoner vs. nonathlete

Answer 6

• Training –> increase heart muscle –> increase chamber size –> increase stroke volume
  
  • HR decreases so MAP will remain normal
  • “Reserve capacity” to increase CO is higher
• Marathoner vs. nonathlete
  
  • Decreased resting HR
  • Increased stroke volume
  • During max exercise, HR can rise as much as in a nonathlete while stroke volume is much larger
  • Increased vascularity –> more efficient oxygen delviery to tissues

Question 7

Enhanced oxygen delivery to muscle during exercise

Answer 7

• Most oxygen in bloodstream is bound to hemoglobin
  
  • Local changes in pH & temperature facilitate shedding of oxygen from hemoglobin
• Increased levels of vasodilating paracrine factors –> relaxed precapillary sphincters –> increased number of open capillaries
  
  • Increases surface area available for exchange of oxygen b/n blood & tissues
  • CaO2 - CvO2 is larger for working muscles

Question 8

CV drift during exercise

• ~10 mins of exercise in a warm environment
• Sweating
• Hyperthermia

Answer 8

• ~10 mins of exercise in a warm environment
  
  • –> decreased stroke volume, CO, & BP
  • –> increased TPR & HR
• Sweating –> dehydration –> loss of blood volume
  
  • Decreased venous return –> decreased Starling forces –> decreased CO
  • Increased SNS –> increase HR (to compensate)
• Hyperthermia enhances muscular activity
  
  • Increase SNS –> increase HR –> overly increased HR –> not adequate ventricular filling –> decrease stroke volume

Question 9

Exercise vs. Moderate Hemorrhage for the following Conditions

• Baroreceptor activity
• Muscle paracrine factors
• Angiotensin II levels
• Vasopressin levels
• Epinephrine levels
• Vasoconstrictor activity
• Diameter of gut arterioles
• Diameter of brain arterioles
• Blood pressure
• Heart rate

Answer 9

• Baroreceptor activity
  
  • Exercise: elevated BP –> increase, CNS –> decrease
  • Hemorrhage: decrease BP –> increaes hormones like ANG-2 –> increase
• Muscle paracrine factors
  
  • Exercise: extreme increase in working muscle
  • Hemorrhage: vasoconstriction –> blood doesn’t get adequate blood flow –> increase
• Angiotensin II levels
  
  • Exercise: reduced renal blood flow –> increased SNS –> increase
  • Hemorrhage: hypotension –> extreme increase
• Vasopressin levels
  
  • ​Exercise: depends on volume & salt loss from sweating
  • Hemorrhage: decrease blood flow –> increase
• Epinephrine levels
  
  • ​Exercise: increase –> vasodilation
  • Hemorrhage: extreme increase –> vasoconstriction b/c high Epi affect alpha receptors
• Vasoconstrictor activity
  
  • ​Exercise: increase –> prevent TPR from dropping
  • Hemorrhage: increase –> raise BP
• Diameter of gut arterioles
  
  • ​Exercise: constricted
  • Hemorrhage: constricted
• Diameter of brain arterioles
  
  • ​Exercise: no difference / normal
  • Hemorrhage: no difference / normal
• Blood pressure
  
  • ​Exercise: slightly increase MAP, increase systolic
  • Hemorrhage: no difference / normal
• Heart rate
  
  • Exercise: increase
  • Hemorrhage: increase

Question 10

Exercise vs. Hemorrhage

Answer 10

• Similarities
  
  • Increased HR
  • Increased vasoconstriction within visceral organ vasculature
  • Increased circulating Epi
• Differences
  
  • Higher Epi levels during hemorrhage –> bind alpha receptors
    
    • Exercise: vasodilatoin
    • Hemorrhage: vasoconstriction
  • Higher antiogensin-2 during hemorrhage
    
    • Muscle arterioles will constrict, despite paracrines
    • Increase during exercise is just moderate

Question 11

CV Adaptations During Spaceflight

• Pressure in arteries
• Venous circulation
• Compensatory mechanisms
• Post-spaceflight orthostatic intolerance
• Earth analog of CV adaptatoins during spaceflight

Answer 11

• Pressure in arteries
  
  • Increases as blood goes down w/ gravity
  • Decreases as blood goes up against gravity
  • Earth: 70mmHg in carotid artery, 160mmHg in legs
  • Space: 90mmHg in both carotid artery & legs
• Venous circulation
  
  • Earth: venous pooling in legs, little venous pooling in upper body b/c gravity facilitates drainage
  • Space: increase venous return from the lower body, decrease venous return from head b/c no gravity to drive fluid down
• Compensatory mechanisms
  
  • Increase venous return –> activate atrial stretch receptors –> increase secretion of atrial natriuretic hormone
  • Increase BP at carotid sinus –> activate baroreceptors –> decrease baroreceptor reflex
  • Increased natriuretic peptides + activated baroreceptors –> decrease vasopression –> inactivate renin-angiotensin system –> decrease plasma aldosterone –> decrease plasma volume
  • Net accumulation of fluid in upper body –> net loss of fluid from CV system
    
    • When astronaut returns to earth –> bulk of fluid returns to lower body –> hypovolemia
• Post-spaceflight orthostatic intolerance
  
  • Can’t stand w/o syncope
  • Loss of plasma volume –> down-regulation of baroreceptor reflex
  • Atrophy of muscles of lower body
    
    • In 0-gravity, standing provides no loading of muscles –> decrease skeletal muscle pumping
• Earth analog of CV adaptations during spaceflight
  
  • Prolonged bedrest induces similar fluid shifts & muscle wasting
  • However, getting out of bed for 1 hour per day circumvents the physiological changes resulting from prolonged bedrest