Chapter 09: Circulatory Responses to Exercise

Circulatory Responses to Exercise

Organization of the Circulatory System

  • Works with the Pulmonary System: Often referred to as the cardiopulmonary or cardiorespiratory system.

  • Purposes of the Cardiorespiratory System:

    • Transport O_2 and nutrients to tissues.

    • Removal of CO_2 and wastes from tissues.

    • Regulation of body temperature.

  • Two Major Adjustments of Blood Flow During Exercise:

    • Increased cardiac output.

    • Redistribution of blood flow from inactive organs to active muscle.

Clinical Applications 9.1: Exercise Training Protects the Heart

  • Regular Exercise is Cardioprotective:

    • Reduces the incidence of heart attacks.

    • Improves survival from a heart attack.

  • Mechanisms for Reduced Myocardial Damage from Heart Attack:

    • Improvements in the heart's antioxidant capacity.

    • Improved function of ATP-sensitive potassium channels.

The Cardiac Cycle

  • Systole (Contraction Phase):

    • Ejection of blood.

    • Approximately \frac{2}{3} of blood is ejected from ventricles per beat.

  • Diastole (Relaxation Phase):

    • Filling with blood.

  • At Rest: Diastolic time is longer than systolic time.

  • During Exercise: The duration of both systole and diastole are shorter.

Factors That Influence Arterial Blood Pressure

  • Determinants of Mean Arterial Pressure (MAP):

    • MAP = Cardiac Output \times Total Vascular Resistance

  • Short-Term Regulation:

    • Sympathetic Nervous System (SNS):

      • Baroreceptors in the aorta and carotid arteries detect changes in blood pressure.

      • Increase in BP leads to decreased SNS activity.

      • Decrease in BP leads to increased SNS activity.

  • Long-Term Regulation:

    • Kidneys: Regulate blood pressure via control of blood volume.

Cardiac Output (Q)

  • Definition: The amount of blood pumped by the heart each minute.

  • Formula: Q = HR \times SV

    • Heart Rate (HR): Number of beats per minute.

    • Stroke Volume (SV): Amount of blood ejected in each beat.

  • Dependence: Cardiac output depends on training state and sex.

  • Typical Values (Examples for College-Age Subjects):

    • Rest:

      • Untrained male (70 kg): HR = 72 bpm, SV = 70 ml/beat, Q = 5.00 L/min

      • Untrained female (50 kg): HR = 75 bpm, SV = 60 ml/beat, Q = 4.50 L/min

      • Trained male: HR = 50 bpm, SV = 100 ml/beat, Q = 5.00 L/min

      • Trained female: HR = 55 bpm, SV = 80 ml/beat, Q = 4.40 L/min

    • Maximal Exercise:

      • Untrained male: HR = 200 bpm, SV = 110 ml/beat, Q = 22.0 L/min

      • Untrained female: HR = 200 bpm, SV = 90 ml/beat, Q = 18.0 L/min

      • Trained male: HR = 190 bpm, SV = 180 ml/beat, Q = 34.2 L/min

      • Trained female: HR = 190 bpm, SV = 125 ml/beat, Q = 23.8 L/min

Regulation of Heart Rate (HR)

  • Parasympathetic Nervous System (PNS):

    • Via the vagus nerve.

    • Slows HR by inhibiting the SA (sinoatrial) and AV (atrioventricular) nodes.

    • Low resting HR is primarily due to parasympathetic tone.

  • Sympathetic Nervous System (SNS):

    • Via cardiac accelerator nerves.

    • Increases HR by stimulating the SA and AV nodes.

  • Increase in HR at Onset of Exercise:

    • Initial Increase: Primarily due to parasympathetic withdrawal (up to approximately 100 bpm).

    • Later Increase: Due to increased SNS outflow.

Heart Rate Variability (HRV)

  • Definition: The time between heart beats (standard deviation of the R-R interval).

  • Indicates: Balance between the SNS and PNS, known as sympathovagal balance.

  • Significance:

    • A wide variation in HRV is considered "healthy."

    • Low HRV is a predictor of cardiovascular morbidity and mortality, especially in patients with existing cardiovascular disease.

Regulation of Stroke Volume (SV)

  • End-Diastolic Volume (EDV):

    • Definition: Volume of blood in the ventricles at the end of diastole (referred to as "preload").

    • Frank-Starling Mechanism: Greater EDV results in a more forceful contraction due to the stretch of the ventricles.

    • Dependent on Venous Return: Venous return must increase for EDV to increase.

  • Average Aortic Blood Pressure:

    • Definition: The pressure the heart must pump against to eject blood (referred to as "afterload"). This is essentially the mean arterial pressure.

  • Strength of Ventricular Contraction (Contractility):

    • Enhanced by: Circulating epinephrine and norepinephrine, and direct sympathetic stimulation of the heart.

Venous Return

  • Increased by:

    • Venoconstriction: Achieved via the SNS.

    • Skeletal Muscle Pump: Rhythmic skeletal muscle contractions force blood in the extremities toward the heart. One-way valves in veins prevent backflow of blood.

    • Respiratory Pump: Changes in thoracic pressure during breathing pull blood toward the heart.

Physical Characteristics of Blood

  • Plasma:

    • Liquid portion of blood.

    • Contains ions, proteins, and hormones.

  • Cells:

    • Red Blood Cells: Contain hemoglobin to carry oxygen.

    • White Blood Cells: Important in preventing infection.

    • Platelets: Important in blood clotting.

  • Hematocrit: Percentage of blood composed of cells.

Oxygen Delivery During Exercise

  • Increased Oxygen Demand: Oxygen demand by muscles increases significantly during exercise, by a factor of 15x to 25x greater than at rest.

  • Increased Oxygen Delivery Accomplished by:

    • Increased cardiac output.

    • Redistribution of blood flow from inactive organs to working skeletal muscle.

Changes in Cardiac Output During Exercise

  • Cardiac Output Increases Due to:

    • Increased HR: Increases linearly with work rate.

      • Max HR Formulas:

        • For adults: Max HR = 220 - age (years)

        • For children: Max HR = 208 - (0.7 \times age (years))

    • Increased SV: Increases initially then plateaus at 40 \% to 60 \% of VO_2 max in untrained subjects.

      • No Plateau in Highly Trained Subjects: Due to improved ventricular filling, leading to an increase in EDV and SV even at high HR. In untrained subjects, filling time decreases at high HR, reducing EDV and SV.

Arteriovenous Oxygen Content (a-vO_2 difference)

  • Definition: Amount of O_2 that is taken up from 100 ml of blood.

  • During Exercise: Increases due to higher O_2 uptake in tissues (used for oxidative ATP production).

  • Fick Equation: Represents the relationship between cardiac output (Q), a-vO2 difference, and VO2 (oxygen consumption).

Redistribution of Blood Flow During Exercise

  • Increased Blood Flow to Working Skeletal Muscle:

    • At rest: 15 \% to 20 \% of cardiac output goes to muscle.

    • During maximal exercise: Increases to 80 \% to 85 \% of cardiac output.

  • Decreased Blood Flow to Less Active Organs: (e.g., liver, kidneys, GI tract).

  • Dependence: Redistribution depends on metabolic rate (exercise intensity).

Regulation of Local Blood Flow During Exercise

  • Skeletal Muscle Vasodilation (Autoregulation):

    • Blood flow is increased to meet the metabolic demands of the tissue.

    • Triggered by changes in O2 tension, CO2 tension, nitric oxide, potassium, adenosine, and pH.

  • Vasoconstriction to Visceral Organs and Inactive Tissues:

    • Mediated by SNS vasoconstriction.

    • Blood flow can be reduced to 20 \% to 30 \% of resting values.

  • Vasoactive Regulators and Factors:

    • Regulators: Include nitric oxide, prostaglandins, ATP, adenosine, endothelial derived hyperpolarizing factor, etc.

    • Factors: Produced in the endothelium or arterioles; promote smooth muscle relaxation, leading to vasodilation and increased blood flow.

Circulatory Responses to Exercise: Influencing Factors

  • Changes in heart rate and blood pressure depend on:

    • Type, intensity, and duration of exercise.

    • Arm versus leg exercise.

    • Environmental conditions (e.g., hot/humid versus cool).

Pressure and Volume Responses to Exercise

  • SV is elevated because of increased EDV and decreased ESV (end-systolic volume).

  • Time reductions in the cardiac cycle mean that intraventricular contractions are faster.

  • Intraventricular pressures are also increased due to elevated afterload during exercise.

Emotional Influence on Exercise Responses

  • Elevated HR and BP in emotionally charged environments due to increases in SNS activity.

  • Can increase pre-exercise HR and BP.

  • Does not typically increase peak HR or BP during exercise.

Transition from Rest to Exercise and Exercise to Recovery

  • At the Onset of Exercise:

    • Rapid increase in HR, SV, and cardiac output.

    • Plateau in submaximal (below lactate threshold) exercise.

  • During Recovery:

    • Decrease in HR, SV, and cardiac output toward resting levels.

    • Recovery rate depends on: duration and intensity of exercise, and the training state of the subject.

Incremental Exercise

  • Heart Rate and Cardiac Output: Increase linearly with increasing work rate and reach a plateau at 100 \% of VO_2 max.

  • Blood Pressure:

    • Mean arterial pressure increases linearly.

    • Systolic BP increases.

    • Diastolic BP remains fairly constant.

  • Double Product (Rate-Pressure Product):

    • Increases linearly with exercise intensity.

    • Indicates the work of the heart.

Arm Versus Leg Exercise

  • At the same oxygen uptake, arm work results in higher:

    • Heart Rate: Due to higher sympathetic stimulation.

    • Blood Pressure: Due to vasoconstriction of a large inactive muscle mass.

Intermittent Exercise

  • Recovery of Heart Rate and Blood Pressure between Bouts Depends on:

    • Fitness level.

    • Temperature and humidity.

    • Duration and intensity of exercise.

  • Heavy-Intensity Intermittent Exercise: Near maximal HR values are possible.

Prolonged Exercise

  • Cardiac Output is Maintained.

  • Gradual Decrease in Stroke Volume: Due to dehydration and reduced plasma volume.

  • Gradual Increase in Heart Rate: During prolonged exercise (particularly in the heat) – this phenomenon is known as cardiovascular drift.

Clinical Applications 9.3: Sudden Cardiac Death During Exercise

  • Uncommon Occurrence: Approximately 1/200,000 youth athletes.

  • Cause: Abnormal, lethal heart rhythms.

  • Causes in Children and Adolescents:

    • Genetic anomalies of coronary arteries.

    • Cardiomyopathy.

    • Myocarditis.

  • Causes in Adults:

    • Coronary artery disease.

    • Cardiomyopathy.

  • Prevention: A medical exam can help identify individuals at risk.

Central Command Theory

  • Initial Signal: The initial signal to "drive" the cardiovascular system comes from higher brain centers, due to centrally generated motor signals.

  • Fine-Tuning by Feedback from:

    • Heart mechanoreceptors.

    • Muscle chemoreceptors (sensitive to muscle metabolites – part of the exercise pressor reflex).

    • Muscle mechanoreceptors (sensitive to the force and speed of muscular movement – part of the exercise pressor reflex).

    • Baroreceptors (sensitive to changes in arterial blood pressure).