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The Circulatory Responses to Exercise. Chapter 9 Powers & Howley 7 th Edition. The Circulatory System. Cardiopulmonary system: works to maintain oxygen and carbon dioxide homeostasis in body tissues The cardiovascular system includes the heart, vessels, and blood

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The Circulatory Responses to Exercise

Chapter 9

Powers & Howley

7th Edition

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The Circulatory System

  • Cardiopulmonary system: works to maintain oxygen and carbon dioxide homeostasis in body tissues

  • The cardiovascular system includes the heart, vessels, and blood

  • The purposes of the CV system are:

    • Transport of O2 to tissues and removal of wastes

    • Transport of nutrients to tissues

    • Regulation of body temperature

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Organization of the circulatory system

The heart is divided into four chambers (two pumps in one)

  • Right side: pumps “deoxygenated” blood to the lungs via the pulmonary circuit

  • Left side: pumps “oxygenated” blood to the various tissues via the systemic circulation

  • Heart wall composed of 3 tissues:

    • Epicardium: Outer layer

    • Myocardium: Middle layer (muscle responsible for contracting and forcing blood out of the heart)

    • Endocardium: Inner layer

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Electrical Activity of the Heart


Sinoatrial (SA) Node: Pacemaker of the heart

Atrioventricular (AV) Node: Located in floor of right atrium

Bundle of HIS, Bundle Branches, Purkinje Fibers

Electrocardiogram (ECG): A recording of the electrical changes that occur in the myocardium during the cardiac cycle

Contraction of chambers represented by P-wave, QRS complex, T-wave

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Cardiac Cycle

Cardiac Cycle - one complete sequence of contraction and relaxation of the heart

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The Heart - Terminology

  • Stroke Volume - amount of blood ejected from the ventricles with each beat of the heart

  • Heart rate = beats per minute

  • Cardiac Output - the amount of blood pumped per unit of time, in liters per minute

    • Q = SV  HR

  • Arterio-venous O2 difference (a-VO2 diff) - The amount of O2 that is taken up from 100 ml of blood by the tissue during one trip around the systemic circuit

  • Fick Equation: VO2 = Q x (a-VO2 diff)

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The Heart - Terminology

  • Blood Pressure: The force exerted by blood against the arterial walls, determined by the product of:

    • how much blood is pumped (Q)

    • resistance to blood flow (TPR)

  • SBP: the arterial pressure generated during ventricular systole (contraction phase)

  • DBP: the arterial pressure during ventricular diastole (relaxation phase)

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    Regulation of Cardiac Function

    Parasympathetic Nervous System

    • Causes HR to decrease

    • Parasympathetic tone:

      • Initial increase in HR during exercise is due to withdrawal of parasympathetic tone

        Sympathetic Nervous System

    • Causes HR to increase

    • Sympathetic Tone:

      • Increase tone => increased heart rate and increased force of contraction

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    Principles of Blood Flow

    • Pressure: Rate of blood flow is proportional to the pressure difference between the two ends of the vessel

      • Blood will flow from region of high pressure to a region of low pressure

    • Resistance: Blood flow is inversely proportional to resistance

      Length x viscosity


    • Major Factor determining resistance is the vascular diameter/radius *

      • Resistance is inversely proportional to the 4th power of the radius

    Resistance =

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    Cardiovascular Responses to Exercise

    Q, SV, HR, BP, a-VO2 diff

    • Relationship betweenVO2 and Q, HR, and SBP is essentially linear

    • Relationship between VO2 and SV (plateaus @ 40-50%), a-VO2 diff. (plateaus @ 40-50%), and DBP is nonlinear

      Arterio-venous O2 difference (a-VO2 diff)

    • Increases during exercise due to an increase in the amount of O2 taken up and used for oxidative production of ATP by skeletal muscle

      Redistribution of Blood Flow

    • Shifting of blood from inactive to active tissue

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    Integrated Response in Exercise

    Activation of motor cortex and higher areas of brain (central command)

    • Reciprocal inhibition of parasympathetic activity =>

      • Acceleration of heart rate (up to ~100 bpm)

    • Increase in sympathetic outflow =>

      • General visceral vasoconstriction

      • Increased myocardial contractility, hence increase in BP, SV & HR (Q )

        Regulation of local blood flow during muscle contraction & relaxation

    • Alterations in local / intrinsic metabolic conditions

      • due to increased production of local vasodilatory factors (i.e. hypoxia; decreased pH; increased PCO2, ADP, Mg++, Ca++, and temperature)

      • results in redistribution of blood flow

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    Integrated Response in Exercise

    Central command, cont.

    • Continued sympathetic outflow in conjunction with increases in epinephrine and norepinephrine

      • Continued constriction of vasculature in inactive tissues to maintain adequate perfusion pressure

    • Venous vessels stiffen to reduce their capacity

      • Facilitates venous return and maintains central blood volume

        Local regulation, cont.

  • Augmented local metabolic conditions

    • Further dilation of active muscle vasculature

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    Factors influencing Venous Return

    • Venoconstriction - Reducing the volume capacity of the veins to store blood

    • Muscle Pump - Mechanical action of rhythmic skeletal muscle contractions

    • One-way valves located in large veins

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    Arm versus Leg Exercise

    • HR and BP response to arm exercise is greater than during leg exercise

      • HR & BP during arm exercise exceeds expected values based on relative oxygen consumption

      • Greater sympathetic outflow to heart during arm work

      • Less arterioles dilated, therefore greater vascular resistance