2.2 Cardiorespiratory Responses to Acute Exercise

The Fick Principle & Cardiovascular Delivery

To satisfy the exponential rise in metabolic oxygen demand during exercise, the cardiorespiratory system must orchestrate rapid, highly integrated adjustments. Central hemodynamic delivery and peripheral tissue oxygen utilization are mathematically unified by the Fick equation:

$\dot{V}\text{O}_2 = Q \times a\text{-}v\text{O}_2\text{ difference} = (\text{HR} \times \text{SV}) \times (C_a\text{O}_2 - C_v\text{O}_2)$

Where:

• $\dot{V}\text{O}_2$ represents whole-body oxygen consumption in milliliters per minute (mL/min) or relative to body mass (mL/kg/min).
• $Q$ represents cardiac output in liters per minute (L/min), the product of heart rate (HR) in beats per minute (bpm) and stroke volume (SV) in milliliters per beat (mL/beat).
• $a\text{-}v\text{O}_2\text{ difference}$ represents the arterial-venous oxygen difference, the concentration of oxygen in systemic arterial blood ($C_a\text{O}_2$) minus mixed venous blood ($C_v\text{O}_2$) entering the right atrium.

At rest, a healthy 70 kg individual exhibits an oxygen consumption of approximately 250 mL/min (equivalent to 1 Metabolic Equivalent of Task [MET], defined as $3.5\text{ mL/kg/min}$). During maximal aerobic exertion, $\dot{V}\text{O}_2$ can increase 10- to 15-fold in sedentary adults and over 20-fold in elite endurance athletes, driven by coordinated increases in both central cardiac delivery ($Q$) and peripheral tissue extraction ($a\text{-}v\text{O}_2\text{ diff}$).

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Cardiac Output ($Q$) & Stroke Volume Dynamics

Resting cardiac output averages approximately 4.5 to 5.0 L/min in both untrained individuals and elite athletes. During maximal graded exercise, $Q$ increases to 20 to 22 L/min in healthy young adults, whereas world-class endurance athletes can achieve maximal cardiac outputs of 35 to 40 L/min.

Stroke Volume Mechanics & The Frank-Starling Law

Stroke volume is the volume of blood ejected from the left ventricle per contraction ($\text{SV} = \text{End-Diastolic Volume [EDV]} - \text{End-Systolic Volume [ESV]}$). At rest, SV ranges from 60 to 80 mL/beat in untrained adults, and can exceed 100 to 120 mL/beat in aerobically trained athletes. Acute regulation of stroke volume is governed by three factors:

1. Preload (End-Diastolic Volume): Governed by the Frank-Starling law of the heart, an increase in venous return stretches the myocardial fibers of the ventricular wall prior to contraction. This elongation optimizes the overlap of actin and myosin filaments and increases troponin C sensitivity to $\text{Ca}^{2+}$, generating greater contractile tension and a larger stroke volume.
2. Myocardial Contractility (Inotropic State): Independent of resting fiber stretch, sympathetic nervous system stimulation via epinephrine and norepinephrine binds to myocardial $\beta_1$-adrenergic receptors. This triggers cyclic AMP-mediated phosphorylation of L-type calcium channels and phospholamban, enhancing sarcoplasmic calcium influx and accelerating cross-bridge cycling.
3. Afterload: The mean aortic arterial pressure against which the left ventricle must pump to open the aortic semilunar valve. Lower peripheral resistance promotes ventricular emptying and reduces ESV.

Venous return during dynamic exercise is augmented by three physiological mechanisms: the skeletal muscle pump (rhythmic muscular contractions compressing deep veins, with one-way venous valves preventing backflow), the respiratory pump (negative intrathoracic pressure during inspiration drawing venous blood into the right atrium), and sympathetic venoconstriction (splanchnic and cutaneous venous mobilization).

The Stroke Volume Plateau

In untrained and recreationally active individuals, stroke volume increases progressively during upright dynamic exercise up to approximately 40% to 50% of $\dot{V}\text{O}_2\text{max}$ (corresponding to heart rates of 110 to 120 bpm). Beyond this threshold, SV reaches a plateau.

Stroke Volume
(mL/beat)
140 ┤                         *   *   *   * (Plateau: Elite Athlete Continues Rise)
120 ┤                 *   *   *   *   *   * (Untrained Plateau at ~40-50% VO2max)
100 ┤         *   *
 80 ┤ *   *
 60 ┤
    └───┴───────┴───────┴───────┴───────┴───────
       Rest    20      40      60      80     100 % VO2max

The primary physiological explanation for this plateau is the tachycardia associated with escalating exercise intensities. As heart rate climbs, the duration of ventricular diastole (the ventricular filling phase) is truncated dramatically. Despite elevated venous return, shortened diastolic filling time limits further increases in EDV. In contrast, elite endurance athletes frequently exhibit a continuous, progressive increase in SV up to maximal exertion due to enlarged ventricular dimensions, heightened myocardial compliance, and rapid active diastolic relaxation.

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Heart Rate Dynamics & Blood Pressure Regulation

Heart Rate Responses

Resting heart rate in healthy adults typically ranges from 60 to 80 bpm, while endurance athletes frequently display resting bradycardia (<50 bpm) resulting from augmented vagal (parasympathetic) tone and expanded stroke volume. Immediately prior to exercise, an anticipatory rise in heart rate occurs, initiated by central command in the cerebral cortex, which withdraws parasympathetic restraint and triggers sympathetic outflow.

During progressive dynamic exercise, heart rate increases in a direct, linear relationship with workload and $\dot{V}\text{O}_2$ until approaching maximal capacity ($\text{HR}_\text{max}$). Maximal heart rate is primarily age-dependent and relatively resistant to exercise training. Two widely utilized predictive formulas include:

• Standard Fox Formula: $\text{HR}_\text{max} = 220 - \text{age}$ (standard error $\pm 10\text{ to }12\text{ bpm}$)
• Tanaka Formula: $\text{HR}_\text{max} = 208 - (0.7 \times \text{age})$ (less age bias; the formula CSEP-PATH has used since its 2019 Second Edition)

Dynamic Aerobic vs Resistance Blood Pressure Responses

Systemic arterial blood pressure reflects the balance between cardiac output and Total Peripheral Resistance (TPR) ($\text{Mean Arterial Pressure [MAP]} = Q \times \text{TPR}$):

Pressure
(mmHg)
 220 ┤                             * (Peak Systolic: 180-220 mmHg)
 180 ┤                     *
 140 ┤             *       
 120 ┤ * (Resting SBP)
     ───────────────────────────────────────
  80 ┤ * ─ ─ ─ ─ ─ * ─ ─ ─ * ─ ─ ─ * ─ ─ ─ * (Diastolic: Stable / Slight Drop ±10 mmHg)
  60 ┤
     └───┴─────────┴───────┴───────┴───────┴
        Rest      25      50      75     100 % VO2max

During progressive dynamic aerobic exercise (running, cycling):

• Systolic Blood Pressure (SBP): Increases in direct linear proportion to workload, rising from ~120 mmHg at rest to peak values of 180 to 220 mmHg at maximal effort. This rise is driven by dramatic increases in stroke volume and left ventricular contractile force.
• Diastolic Blood Pressure (DBP): Remains relatively stable or exhibits a slight reduction ($\pm 10\text{ mmHg}$ from baseline). Although sympathetic outflow produces systemic vasoconstriction in non-exercising viscera and kidneys, working skeletal muscle beds undergo profound local metabolic vasodilation (functional sympatholysis). Locally produced metabolites (adenosine, nitric oxide, $\text{K}^+$, $\text{H}^+$, $\text{CO}_2$) relax vascular smooth muscle in active arterioles, dramatically lowering TPR.

In stark contrast, heavy resistance exercise (e.g., leg press or deadlift) involves high-load sustained muscular contractions that physically compress intramuscular blood vessels, sharply escalating afterload. When combined with the Valsalva maneuver (forced expiration against a closed glottis), intrathoracic pressure surges dramatically. This produces transient blood pressure spikes exceeding 300/200 mmHg. CSEP practice standards instruct trainers to teach clients continuous rhythmic breathing (exhaling during the concentric sticking point, inhaling during eccentric lowering) to prevent extreme blood pressure excursions and transient syncope.

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Arterial-Venous Oxygen Difference & Pulmonary Ventilation

Peripheral Oxygen Extraction ($a\text{-}v\text{O}_2\text{ diff}$)

The $a\text{-}v\text{O}_2\text{ difference}$ quantifies the volume of oxygen extracted by peripheral tissues from each 100 mL of blood passing through systemic microcirculation.

• Resting Extraction: Systemic arterial blood carrying normal hemoglobin concentration (~15 g/dL) is saturated with oxygen at approximately 97% to 98%, holding ~20 mL $\text{O}_2$ per 100 mL of blood ($C_a\text{O}_2$). Mixed venous blood returning to the right atrium contains approximately 15 to 16 mL $\text{O}_2$ per 100 mL of blood ($C_v\text{O}_2$). Thus, resting $a\text{-}v\text{O}_2\text{ diff}$ is approximately 4 to 5 mL $\text{O}_2$/dL, reflecting a baseline extraction fraction of ~20% to 25%.
• Maximal Exercise Extraction: While arterial oxygen concentration remains stable or increases slightly due to hemoconcentration (~20 to 21 mL $\text{O}_2$/dL), intense mitochondrial consumption in contracting muscle drives mixed venous oxygen content down to 2 to 4 mL $\text{O}_2$/dL. Consequently, the maximal $a\text{-}v\text{O}_2\text{ diff}$ widens to 15 to 18 mL $\text{O}_2$/dL, an extraction rate of nearly 85% in active capillary beds.

This dramatic widening of the extraction gradient is facilitated by the Bohr effect: elevated intracellular temperature, hypercapnia (increased $\text{PCO}_2$), and cellular acidosis (decreased pH) shift the oxyhemoglobin dissociation curve down and to the right, weakening hemoglobin's affinity for oxygen and promoting rapid offloading at active tissues.

Pulmonary Ventilation & Ventilatory Thresholds

Minute ventilation ($\dot{V}_E$) represents total expired air per minute, calculated as the product of breathing frequency ($f_b$) and tidal volume ($V_T$):

$\dot{V}_E = f_b \times V_T$

At rest, $\dot{V}_E$ averages ~6 L/min ($12\text{ breaths/min} \times 0.5\text{ L}$). During heavy exertion, $\dot{V}_E$ can exceed 100 to 150 L/min in healthy adults and over 200 L/min in elite athletes. Early during incremental exercise, ventilation increases linearly with $\dot{V}\text{O}_2$, driven primarily by expansions in tidal volume. At higher intensities, tidal volume reaches a mechanical limit (~50% to 60% of vital capacity), and further ventilatory increases are driven almost entirely by respiratory frequency.

During a graded cardiorespiratory test, two distinct ventilatory threshold transitions appear:

1. Ventilatory Threshold 1 (VT1 / Aerobic Threshold): The exercise intensity where ventilation begins to rise non-linearly relative to oxygen consumption ($\dot{V}_E/\dot{V}\text{O}_2$ increases) while remaining linear relative to carbon dioxide production ($\dot{V}_E/\dot{V}\text{CO}_2$ remains constant). VT1 coincides with the first accumulation of blood lactate (~2 mmol/L). The excess hydrogen ions are buffered by plasma bicarbonate ($\text{H}^+ + \text{HCO}_3^- \leftrightarrow \text{H}_2\text{CO}_3 \leftrightarrow \text{CO}_2 + \text{H}_2\text{O}$), and the resulting extra $\text{CO}_2$ stimulates medullary chemoreceptors to accelerate ventilation. VT1 marks the upper boundary of the comfortable talk test.
2. Ventilatory Threshold 2 (VT2 / Respiratory Compensation Point): At higher workloads, unbuffered metabolic acidosis stimulates peripheral carotid chemoreceptors, driving hyperventilation. Here, ventilation rises disproportionately to both oxygen consumption and carbon dioxide production (both $\dot{V}_E/\dot{V}\text{O}_2$ and $\dot{V}_E/\dot{V}\text{CO}_2$ increase sharply). Beyond VT2, sustained speech is impossible, and exercise capacity is severely constrained.