Cardiovascular Responses to Exercise – Study Notes

Overview & Learning Outcomes

• Understand acute cardiovascular responses to three exercise patterns
  • Transition from rest → constant-load submaximal work (steady-state) → recovery
  • Incremental (progressive intensity) exercise to $\text{VO}_2\,\max$
  • Prolonged sub-maximal exercise (≥30 min)
• Be able to graph (with correct units & approximate values) and explain control of:
  • Heart rate (HR, beats·min$^{-1}$)
  • Stroke volume (SV, mL·beat$^{-1}$)
  • Cardiac output (Q, L·min$^{-1}$)
  • Blood pressures – systolic (SBP), diastolic (DBP) & mean arterial pressure (MAP, mmHg)
• Explain redistribution of blood flow from rest to exercise and the mechanisms behind it (sympathetic tone, local vasodilators, precapillary sphincters).
• Compare responses in males vs females and arm vs leg exercise.
• Calculate & interpret Rate Pressure Product (RPP; “double product”).

Key Definitions & Formulas

• $Q = HR \times SV$ (Fick principle for bulk flow)
• $HR_{\max} \;\text{(b·min}^{-1}\text{)} \approx 220 - \text{age (y)}$
• $SV = EDV - ESV$
• Ejection fraction $= \dfrac{SV}{EDV} \times 100$ (normal rest ≈ 60 – 70 %)
• $MAP = \dfrac{2\,DBP + SBP}{3}$
• Poiseuille’s Law for resistance to flow
  • $R = \dfrac{L\,\eta}{r^{4}}$
  • Vessel radius (r) is the most powerful determinant of total peripheral resistance (TPR).
• Rearranged haemodynamic relationships
  • $Q = \dfrac{MAP \times 80}{TPR}$
  • $TPR = \dfrac{MAP \times 80}{Q}$ (constant 80 converts mmHg·min·L$^{-1}$)
• Rate Pressure Product (RPP) – indirect myocardial work index
  • $RPP = HR \times SBP$ (dimensionless)

Big Picture – Why Circulate Blood During Exercise?

• Deliver $O_2$ & fuel substrates (glucose, fatty acids) from lungs & gut → working muscle mitochondria.
• Remove metabolic by-products $CO_2$, $H^+$, heat → lungs, skin, kidneys.
• Fast, flexible circulation supported by:
  • Pump (heart) – HR & SV adjustments
  • Pipes (arteries, arterioles, capillaries, veins) – diameter & compliance changes
  • Fluids (blood volume/viscosity)

Capillary Architecture & Flow Control

• Continuous, metarteriole–capillary beds with precapillary sphincters.
• Active tissue releases local vasodilators (↓PO$<em>2$, ↑P$</em>{CO_2}$, ↑K$^+$, adenosine, heat ➔ relax sphincters ➔ ↑regional blood flow).
• ‘Thoroughfare channels’ sustain minimal flow when sphincters closed.

Neural–Hormonal Control of the Heart

• Cardiovascular centre: medulla oblongata (input from higher brain, baro-, chemo-, proprio-receptors).
• Parasympathetic (vagus) ➔ SA/AV nodes ➔ ↓HR (dominant at rest).
• Sympathetic (cardiac accelerator nerves, adrenal catecholamines) ➔ SA/AV + myocardium ➔ ↑HR & ↑contractility.
• Rapid HR rise at exercise onset = vagal withdrawal; further rise = ↑sympathetic drive.

Heart Rate Responses

Single-intensity (steady-state)

• Sharp rise at onset, reaches steady state within ≈ 3 min once $O_2$ delivery meets metabolic demand.
• Magnitude proportional to absolute workload.
• Typical steady-state value 100–160 b·min$^{-1}$ for moderate work.
• Recovery: rapid early drop (parasympathetic re-activation) then slower to baseline.

Incremental exercise

• HR increases ~linearly with power/$\text{VO}<em>2$ until $HR</em>{max}$.
• Slightly attenuated slope in highly trained due to larger SV.

Prolonged exercise (cardiovascular drift)

• Despite constant load, HR drifts ↑ (~3–15 b·min$^{-1}$ per 10–30 min) especially in heat/humidity.
• Compensates for falling SV to maintain Q.

Stroke Volume (SV) & Its Regulation

Resting range: 60 – 100 mL (influenced by body size, sex, training status).
Determinants (Frank-Starling & inotropy)

• Preload (EDV) – ↑venous return via skeletal muscle/respiratory pumps & venoconstriction
• Afterload (aortic/arterial pressure) – small acute effect
• Contractility – sympathetic β-adrenergic stimulation ↑Ca$^{2+}$ availability
• Filling time (↔ HR)
  During exercise
• Venous return ↑ ➔ EDV ↑ ➔ SV ↑
• Sympathetic stimulation ↑contractility ➔ ↓ESV ➔ further ↑SV
• SV plateaus at ~40–60 % $\text{VO}_2\,\max$ in most untrained; highly trained may keep rising (larger LV volume & compliance).

Cardiac Output (Q) Responses

Steady-state

• Rises rapidly, plateaus within 2–3 min when Q ≈ metabolic requirement.
• Typical moderate work: 10–15 L·min$^{-1}$.
  Incremental
• Nearly linear ↑ driven mainly by HR after SV plateau.
• Untrained peak: ~20–25 L·min$^{-1}$; elite endurance male ≈ 30–35 L·min$^{-1}$.
  Prolonged
• Q maintained (or slight ↓ late) via HR ↑ offsetting SV ↓.

Blood Pressure Responses & Regulation

• SBP ↑ linearly with intensity (≈ 8–10 mmHg per MET) due to ↑Q.
• DBP: little change or slight ↓ (−0 to −10 mmHg) because peripheral vasodilation ↓TPR.
• Hence MAP ↑ modestly (≈ SBP ↑ partially counteracted by DBP ↓).
  Short-term regulation
• Baroreflex resets to higher operating point during exercise ➔ prevents excessive BP fluctuations.
  Long-term
• Kidney-mediated volume control; not acute.

Redistribution of Blood Flow

Rest (Q ≈ 5 L·min$^{-1}$)

• Skeletal muscle 15–20 % (≈ 1 L·min$^{-1}$)
• Splanchnic 24 %, Renal 19 %, Brain 13 %, Skin 9 %, Coronary 4 %
  Maximal exercise (Q ≈ 25 L·min$^{-1}$)
• Skeletal muscle 80–85 % (≈ 22 L·min$^{-1}$)
• Skin 2 % (heat stress may raise skin flow slightly), Splanchnic 1 %, Renal 4 %, Brain ~3 %, Others negligible.
  Mechanisms
• Global sympathetic vasoconstriction (α-adrenergic) to non-essential beds.
• Local metabolic vasodilation in active muscle overrides sympathetic tone (functional sympatholysis).
• Opening of more capillaries ↑surface area for gas exchange.

Rate Pressure Product (RPP)

• Surrogate for myocardial $O<em>2$ demand (closely related to coronary blood flow & MVO$</em>2$).
• Resting example: HR 75 b·min$^{-1}$ × SBP 110 mmHg = 8,250.
• Progressive exercise example (21-y female):
  • 25 % $\text{VO}_2$max ➔ 13,000
  • 50 % ➔ 22,400
  • 75 % ➔ 30,600
  • 100 % ➔ 42,000
• RPP ↑ in proportion to both HR & SBP; useful in cardiac rehab to keep workloads below angina threshold.

Responses to Different Exercise Patterns

Rest → Steady-State Submaximal

• HR, SV, Q & SBP all rise rapidly then plateau (~2–3 min).
• DBP unchanged/↓; MAP slight ↑.
  Incremental (Graded Test)
• HR & SBP: linear ↑ to maximal.
• SV: ↑ until ~40–60 % $\text{VO}_2$max then plateau (untrained) or continues (trained).
• Q: near-linear ↑ (slope steeper early; becomes HR-driven after SV plateau).
  Prolonged Constant-Load (>30 min)
• Cardiovascular drift: ↓SV (due to dehydration, ↓plasma volume, ↑body temp → skin circulation) + compensatory ↑HR, maintaining Q & SBP.

Arm vs Leg Exercise (Same Absolute Work Rate)

• ↑HR and ↑BP during arm ergometry vs cycle/leg because:
  • Smaller active muscle mass ➔ higher sympathetic outflow to maintain power ➔ greater vasoconstriction in inactive large beds ➔ ↑TPR.
• SV often lower (less muscle pump assistance).

Sex Differences

• $HR_{max}$ predicted by age; no intrinsic sex difference.
• Women: ↓SV (≈10–25 % lower) due to smaller heart size & lower blood volume ➔ slightly lower maximal Q.
• Men: higher SBP/DBP responses (greater sympathetic activation; androgen effects on arterial stiffness).
  Example values (college-age, 70 kg male vs 50 kg female)
• Rest: HR 72 vs 75 b·min$^{-1}$; SV 100 vs 70 mL; Q both ≈ 5 L·min$^{-1}$.
• Max untrained: HR 200 b·min$^{-1}$ (same); SV 110 vs 90 mL; Q 22 vs 18 L·min$^{-1}$.
• Trained endurance:
  • Male: HR 190; SV 180 mL; Q 34.2 L·min$^{-1}$.
  • Female: HR 190; SV 125 mL; Q 23.8 L·min$^{-1}$.

Cardiovascular Drift – Mechanistic Details

• Fluid loss (sweat) & plasma transudation → ↓central venous pressure → ↓EDV → ↓SV.
• Skin vasodilation for thermoregulation ↑venous compliance, ↓venous return.
• HR ↑ via baroreflex & ↑sympathetic activity to maintain MAP & Q.
• Performance implication: perceived effort ↑, may limit endurance in heat unless fluid intake & cooling strategies employed.

Concept Integration (Cardiac Output Map)

• Q depends simultaneously on:
  • HR (ANS modulation, intrinsic SA node rate)
  • SV determined by
  • Preload (venous return ↔ blood volume, muscle pump, respiratory pump, venoconstriction)
  • Contractility (SNS, circulating catecholamines, Frank-Starling)
  • Afterload (arterial pressure)
  • Filling time (inverse of HR)
• Blood pressure (MAP) = Q × TPR; TPR dynamically altered via arteriolar radius.
• Exercise sets off coordinated adjustments via central command, reflex feedback, and local metabolic control to match blood flow with metabolic demand while stabilising arterial pressure.

Worked Calculations & Quick Checks

1. Steady-state jog, 25-y male: HR 150 b·min$^{-1}$, SV 110 mL.
   • $Q = 150 \times 0.11 = 16.5\;\text{L·min}^{-1}$.
2. If SBP 180 mmHg at that workload:
   • $RPP = 150 \times 180 = 27{,}000$ (≈3.3-fold ↑ vs rest 8,000).
3. MAP when SBP 180 mmHg, DBP 70 mmHg:
   • $MAP = \dfrac{2(70)+180}{3} = \dfrac{320}{3} \approx 107\;\text{mmHg}$.
4. Estimate TPR:
   • $TPR = \dfrac{MAP \times 80}{Q} = \dfrac{107 \times 80}{16.5} \approx 519\;\text{dyn·s·cm}^{-5}$ (lower than rest ≈1,100, reflecting vasodilation).

Practical / Clinical Implications

• RPP monitoring in cardiac patients to keep exercise below ischaemic threshold.
• Understanding sex & modality differences informs prescription (e.g., arm ergometry elicits higher BP – caution in hypertensives).
• Hydration & cooling strategies mitigate cardiovascular drift and maintain performance.
• Knowledge of rapid HR recovery is used as marker of cardiovascular fitness (faster recovery = better vagal re-activation).