2006A09 Outline the systemic cardiovascular response to exercise.

 

List:

        Response to isotonic exercise

        Response to isometric exercise

        Adaptations to isotonic exercise

        Adaptations to isometric exercise

 

Isotonic exercise:

Example

   Jogging

Energy production

   Mostly aerobic

Aim

   ↑O2 and nutrient delivery to skeletal muscle and heart

   DO2 = cardiac output x CaO2

   CVS response: ↑CO

   Resp response: ↑CaO2

Autonomic response

   ↑SNS output and ↓PSNS output

o Signals from cortex

o Sensory nerves from muscles

o Metabolites produced by active muscle

Organ blood flow

   Heart: 250 -> 1000mL/min

o Mainly due to metabolic autoregulation

o ↓pO2, ↑pCO2/H+/K+/adenosine/lactate -> coronary vasodilation

   Skeletal muscle: 1.2->20L/min

o Metabolic autoregulation (as above)

   Brain: 750mL/min unchanged

o Myogenic autoregulation (↑BP -> ↑stretch -> reflex contraction -> ↓flow)

o Metabolic autoregulation (as above)

   Kidney: 1100mL/min -> 250mL/min

o SNS α1

   Splanchnic: massive ↓

o SNS α1

   Cutaneous:

o Mild intensity: ↓Q (SNS at α1)

o Moderate: ↑Q (SNS at mAChR; for heat loss)

o Severe: ↓Q (↑↑SNS at α1 -> redirection to skeletal muscle; failure of heat loss)

Preload

*Note ↑↑preload drives ↑↑cardiac output*

   Skeletal muscle vasodilation -> ↑venous return (VR)

   SNS α1 -> constrict capacitance vessels -> ↑MSFP

   Skeletal muscle pump -> ↑MSFP

   Thoracic pump -> intermittent ↓CVP -> ↑VR

   Ventricular contraction -> atrial elongation -> ↓CVP -> ↑VR

Heart rate

   Dose-dependent increase up to 220-(age x 0.7)

   ↑SNS β1, β2: ↑ICF [Ca2+] -> ↑HR

   Bainbridge effect: ↑venous return -> ↑RA stretch -> reflex ↑HR

Contractility

   SNS β1, β2: ↑ICF [Ca2+] -> ↑contractility

   Bowditch effect: ↑HR -> ↓time for Na+ efflux by Na+K+ATPase -> ↓Ca2+ efflux by Na+/Ca2+ exchanger -> ↑contractility

Afterload

   Skeletal muscle vasodilatation -> ↓↓

o Metabolic autoreg (most important)

   SNS:

o ↑Synaptic ACh at mAChR (at onset of exercise)

o ↑Circulating adrenaline at β2

Stroke volume

   Increase mainly between rest and mild-moderate work rate

   Due to ↑preload

Cardiac output

   ↑ up to 5x

   Mostly due to ↑preload

   Also ↓afterload, ↑HR, ↑contractility

Blood pressure

   Changes:

o ↑SBP: due to ↑SV

o ↓DBP: due to ↓afterload

o mAP: minimal change

   Causes:

o ↑Cardiac output

o Right shift of baroreceptor reflex

o Partly offset by ↓SVR

Oxyhaemoglobin dissociation curve

   Bohr effect: ↑pCO2, ↑[H+] -> right shift

   At tissues: ↑temp/↓pO2/↑H+ -> ↑2,3-DPG -> right shift

   Hence ↑O2 extraction

(mentioned by the examiner)

 

Isometric exercise:

Example

   Deadlift

Energy production

   Mostly anaerobic

Aim

   During reps: prevent ↑↑BP (responses as follows)

   Between reps: as for aerobic (responses as for isometric)

Autonomic response

   Baroreceptor reflex (BR):

o ↑↑SVR -> ↑mAP -> ↑stretch in aortic and carotid sinuses

o ↓SNS and ↓PSNS output

o ↓HR, ↓contractility, vasodilatation

Organ blood flow

   Similar redirection to aerobic exercise, but to a lesser extent

Preload

   Valsalva -> ↑intrathoracic pressure -> ↓preload

Heart rate

   ↓ due to baroreceptor reflex

Contractility

   ↑: due to Anrep effect: ↑afterload -> stretch activation of Na+/H+ channel -> ↓Ca2+ efflux by Na+/Ca2+ exchanger -> ↑ICF [Ca2+]

   ↓: due to baroreceptor reflex

Afterload

   ↑ due to:

o Skeletal muscle contraction

o Valsalva manoeuvre

   Partly offset by baroreceptor reflex

Stroke volume

   ↓ due to:

o ↑Afterload

o Baroreceptor reflex

Cardiac output

   ↓ due to:

o ↑Afterload

o Baroreceptor reflex

Blood pressure

   ↑SBP, DBP, mAP

   Due to ↑SVR

Oxyhaemoglobin dissociation curve

   Lesser extent

 

Adaptation to isotonic exercise:

Heart

   ↑Mitochondria, ↑TCA cycle and fatty acid oxidative enzymes

   Eccentric hypertrophy (due to ↑preload)

   ↑SV, ↑max cardiac output

   ↓Resting HR

Blood vessels

   Angiogenesis in skeletal muscles

Blood

   ↓pO2 in renal interstitium -> ↑EPO -> ↑erythropoiesis -> ↑CaO2 -> DO2 in exercise

Skeletal muscle

   ↑Mitochondria

   ↑Glycogen synthesis

   ↑Enzymes for TCA cycle, fatty acid oxidation

 

Adaptation to isometric exercise:

Heart

   Concentric hypertrophy (due to ↑afterload)

(comes at the cost of ↓LV compliance)

Blood vessels

   Minimal

Blood

   Minimal

Skeletal muscle

   Hypertrophy

   ↑Glycolytic enzymes

 

 

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