Circulatory Responses to Exercise

Copyright Information

  • Copyright © 2021 McGraw-Hill Education. All rights reserved.

  • No reproduction or distribution without the prior written consent of McGraw-Hill Education.

Presentation Overview

  • Prepared by: Scott K. Powers, Ph.D., Ed.D., Edward T. Howley, Ph.D., and John Quindry, Ph.D.

  • Topic: Circulatory responses to exercise

  • From "EXERCISE PHYSIOLOGY Theory and Application to Fitness and Performance, 11th Edition".

Lecture Outline

  • Key Topics:

    • Organization of the Circulatory System

    • Heart: Myocardium and Cardiac Cycle

    • Cardiac Output

    • Hemodynamics

    • Changes in Oxygen Delivery to Muscle During Exercise

    • Circulatory Responses to Exercise

    • Regulation of Cardiovascular Adjustments to Exercise

The Circulatory System

  • Works in conjunction with the pulmonary system, referred to as the cardiopulmonary or cardiorespiratory system.

Purposes of the Cardiorespiratory System

  • Transport O2 and nutrients to tissues.

  • Removal of CO2 and metabolic wastes from tissues.

  • Regulation of body temperature through blood flow adjustments.

Major Adjustments of Blood Flow During Exercise

  • Increased cardiac output.

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

Components of the Circulatory System

  • Heart:

    • Primary function is to create pressure to pump blood.

  • Arteries and Arterioles:

    • Carry blood away from the heart.

  • Capillaries:

    • Sites for the exchange of oxygen (O2), carbon dioxide (CO2), and nutrients with tissues.

  • Veins and Venules:

    • Carry blood toward the heart.

Heart Structure and Circuits

Heart Structure

  • Myocardium: Consists of three layers:

    • Epicardium: The lubricating outer layer.

    • Myocardium: Muscle layer responsible for heart contraction.

    • Endocardium: Protective inner lining.

  • Blood supply is provided via coronary arteries due to high oxygen and nutrient demand.

  • Myocardial infarction (MI):

    • Occurs due to blockage in coronary blood flow, causing cell damage.

    • Exercise training is beneficial, providing protection against heart damage during an MI.

Circuits of the Heart

  • Pulmonary Circuit:

    • Right side of the heart pumps deoxygenated blood to the lungs via pulmonary arteries.

    • Returns oxygenated blood to the left side of the heart via pulmonary veins.

  • Systemic Circuit:

    • Left side of the heart pumps oxygenated blood to the body via arteries.

    • Returns deoxygenated blood back to the right side of the heart via veins.

Cardiac Cycle

Phases of the Cardiac Cycle

  • Systole:

    • The contraction phase, where blood is ejected from ventricles (about 2/3 of blood is expelled per beat).

  • Diastole:

    • The relaxation phase, where the heart fills with blood.

Time Distribution:

  • At rest, diastolic time is longer than systolic time.

  • During exercise, both systole and diastole durations are shorter.

Pressure Changes During the Cardiac Cycle

  • Diastole:

    • Low pressure in ventricles while filling with blood from atria.

    • Atrioventricular (AV) valves open when ventricular pressure < atrial pressure.

  • Systole:

    • Pressure rises in ventricles as blood is ejected into circulation.

    • Semilunar valves open when ventricular pressure > aortic pressure.

Heart Sounds

  • First heart sound: Closing of AV valves.

  • Second heart sound: Closing of aortic and pulmonary valves.

Cardiac Output and Blood Pressure

Determinants of Mean Arterial Pressure (MAP)

  • MAP is influenced by:

    • Cardiac Output (CO): defined as the product of heart rate (HR) and stroke volume (SV).

    • ext{MAP} = ext{Cardiac Output} imes ext{Total Vascular Resistance}

    • Total Vascular Resistance varies with vessel length, blood viscosity, and vessel radius.

Regulation of Blood Pressure

  • Short-term regulation:

    • Managed by the sympathetic nervous system and baroreceptors in the aorta and carotid arteries.

    • Increase in blood pressure leads to decreased sympathetic nervous system activity and vice versa.

  • Long-term regulation:

    • Managed by the kidneys through blood volume control.

Cardiac Mechanics

Intrinsic Control of Heart Rate

  • SA Node: Initiates contraction signals; consists of pacemaker cells in the upper posterior right atrial wall.

    • Generates its own action potentials through slow depolarization.

    • Signal spreads from SA node via right and left atria to the AV node to stimulate contraction.

  • AV Node:

    • Located in the right atrial wall near the center of the heart.

    • Delays and relays the signal to the ventricles to ensure coordinated contraction.

    • Serves as a passage to the AV bundle, then divides into right and left bundle branches leading to Purkinje fibers that stimulate ventricular contraction.

Extrinsic Control of Heart Rate

  • Parasympathetic Nervous System:

    • Hyperpolarizes conduction cells, decreasing heart rate by inhibiting the SA node.

  • Sympathetic Nervous System:

    • Increases the rate of depolarization, thereby increasing heart rate by stimulating the SA node.

  • Drugs:

    • Atropine: Blocks parasympathetic activity.

    • Propranolol: Blocks sympathetic activity.

    • Important to consider which drugs would be used in treating hypertension.

Responses to Exercise

Cardiac Output During Exercise

  • Cardiac output significantly increases due to:

    • Increased heart rate resulting from sympathetic activation of the SA and AV nodes.

    • Increased venous return and greater ventricular filling (end-diastolic volume).

    • Reduced aortic blood pressure due to vasodilation.

    • Enhanced strength of ventricular contraction facilitated by epinephrine and norepinephrine which increase calcium availability.

Influences on Stroke Volume During Exercise

  • End-Diastolic Volume (EDV):

    • The Frank-Starling mechanism states that greater EDV leads to a more forceful contraction due to the stretch of ventricles, which is dependent on venous return.

  • Venous return is increased by:

    • Venoconstriction of veins via sympathetic nervous system (SNS).

    • Skeletal muscle pump, which pushes blood toward the heart aided by one-way valves and the respiratory pump, which changes thoracic pressure.

Oxygen Delivery During Exercise

  • Equation: Total oxygen consumption can be expressed as:

    • ext{VO}2 = ext{Cardiac Output} imes ext{A-V O}2 ext{ difference}.

  • Oxygen demand increases by 15x to 25x more than at rest.

  • Increased oxygen delivery is achieved through:

    • Increased cardiac output and redistribution of blood flow from inactive organs to working skeletal muscles.

  • Trained individuals can achieve higher cardiac outputs than untrained individuals.

Changes in Blood Flow During Exercise

  • Blood flow to working skeletal muscle increases significantly during maximal exercise.

  • At rest, only about 15-20% of cardiac output goes to muscles, while this can increase to 80-85% during intense exercise.

  • Blood flow to less active organs (liver, kidneys, GI tract) decreases significantly (to 20-30% of resting values).

Regulation of Blood Flow During Exercise

  • Skeletal Muscle Vasodilation:

    • Autoregulation allows for increased blood flow to meet the muscular metabolic demand.

  • Sympathetic Nervous System:

    • Causes vasoconstriction of visibly inactive tissues.

  • Nitric Oxide:

    • Acts as a significant vasodilator produced in the endothelium of arterioles, promotes muscle blood flow during exercise.

Summary of Cardiovascular Responses to Exercise

Key Points

  • Heart rate and cardiac output increase linearly with exercise intensity and reach a plateau at maximum exertion (100% VO2 max).

  • Mean arterial pressure rises linearly, systolic pressure increases, while diastolic pressure remains relatively constant, indicating the work exerted by the heart.

  • Prolonged exercise leads to a gradual decrease in stroke volume due to dehydration and reduced plasma volume, while heart rate gradually increases, a phenomenon known as cardiovascular drift.

Visual Aids

  • Figures referenced in the lecture provide visual interpretation of the changes in cardiovascular variables during exercise, muscle and splanchnic blood flow, as well as the physiological adaptations shown through beta-blockers in heart rate control.

Conclusion

  • This concludes the overview of circulatory responses to exercise, focusing on the organization and function of the circulatory system, cardiovascular changes during physical activity, and relevant physiological mechanisms. The content helps establish a foundational understanding of exercise physiology and its applications for health and performance.