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+ influx 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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