Cardiovascular Regulation Exercise Physiology McArdle, Katch, and Katch, 4th ed.

Regulation of the Cardiovascular System Heart Rate Regulation Blood Flow Regulation

Heart Rate Regulation The heart has both intrinsic (situated within the heart) and extrinsic (originating outside the heart) regulation. Many myocardial cells have unique potential for spontaneous electrical activity (intrinsic rhythm). In normal heart, spontaneous electrical activity is limited to special region. Sinoatrial node serves as pacemaker.

Intrinsic Regulation of HR Sino atrial node: pacemaker

Intrinsic Regulation Depolarization muscle membrane creates an action potential or electrical impulse Impulse travels through the heart in an established pathway SA node →across atria →AV node →AV bundle →left & right bundle branches → Purkinjie fibers → Ventricles

Normal Route of Depolarization S-A Node  Atria A-V Node Bundle of His Purkinje Fibers Ventricles

Intrinsic Heart Rate SA node rate approximately 90 bpm Parasympathetic innervation slows rate referred to as parasympathetic tone training increases parasympathetic tone

Electrocardiogram The ECG is recorded by placing electrodes on the surface of the body that are connected to an amplifier and recorder. Each wave in the shape of the ECG is related to specific electrical change in heart. Purposes of ECG to monitor heart rate and diagnose rhythm.

Electrocardiogram Each wave of ECG related to specific electrical change in the heart P wave - atrial depolarization QRS complex - ventricular depolarization masks atrial repolarization T wave - ventricular repolarization

ECG Arrhythmias PACs- premature atrial contraction PVCs- premature ventricular contraction Ventricular fibrillation- cardiovert

Extrinsic Regulation of HR Neural Influences override intrinsic rhythm Sympathetic: catecholamines Epinephrine Norepinephrine Parasympathetic Acetylcholine Cortical Input Peripheral Input

Neural Regulation of HR Sympathetic influence Epinephrine  ↑HR (tachycardia) and ↑ contractility Norepinephrine  general vasoconstrictor Parasympathetic influence Acetylcholine→↓HR (bradycardia) Endurance (aerobic) trg. increases vagal dominance

Cardiac Accelerator Nerves Sympathetic Fibers Innervate SA node & ventricles Increase heart rate Increase contractility Increase pressure

Vagus Nerve Parasympathetic Nerve Innervates SA node & AV node Releases acetylcholine Slows heart rate Lowers pressure Strong vagal stimulation of the heart can actually stop the heart for a few seconds, but then the heart escapes. In addition, strong vagal stimulation can decrease strength of contraction by as much as 20 to 30%. Not a great decrease because vagal fibers are distributed mainly to atria and not much to ventricles.

Cortical Influences on Heart Rate Cerebral cortex impulses pass through cardiovascular control center in medulla oblongata. Emotional state affects cardiovascular response Cause heart rate to increase in anticipation of exercise

Peripheral Influences on HR Peripheral receptors monitor state of active muscle; modify vagal or sympathetic Chemoreceptors Monitor pCO2, H+, pO2 Mechanoreceptors Heart and skeletal muscle mechanical receptors Baroreceptors

Peripheral Influence on HR Baroreceptors in carotid sinus and aortic arch. ↑ pressure → ? HR & contractility ↓ pressure → ? HR & contractility

Blood Flow Regulation During exercise, local arterioles dilate and venous capacitance vessels constrict. Blood flow is regulated according to Poiseuille’s Law: Flow = pressure  resistance. Pwah-swe’ law

Blood Flow Regulation Flow = pressure gradient x vessel radius4 vessel length x viscosity Blood flow Resistance Factors Viscosity or blood thickness Length of conducting tube Radius of blood vessel

Blood Flow Regulation 1 of every 30 or 40 capillaries is open in muscle at rest Opening “dormant” capillaries during exercise Increases blood flow to muscle Reduces speed of blood flow Increases surface area for gas exchange

Local Factors Resulting in Dilation ↓ tissue O2 produces potent vasodilation in skeletal and cardiac muscle Increased temperature Elevated CO2 Lowered pH Increased ADP Nitric Oxide (NO) Ions of Mg+2 and K+ Acetylcholine

Blood Flow Neural Factors Sympathetic nerves (adrenergic): norepinephrine general vasoconstrictor Sympathetic nerves (cholingergic): acetylcholine vasodilation in skeletal and cardiac muscle.

Blood Flow Humoral Factors Sympathetic nerves to adrenal medulla causes release of epinephrine & norepinephrine into blood (humor).

Blood Flow Humoral Factors Sympathetic Nerves to Adrenal Medulla  epi & norepi in blood vasoconstriction except in skeletal muscle

Neural Factors of Flow Control

Integrated Response

Regulation from Rest to Exercise Rapid increase in heart rate, SV, cardiac output due to withdrawal of parasympathetic stimuli increased input from sympathetic nerves Continued increase in heart rate temperature increases feedback from proprioceptors accumulation of metabolites

Integrated Response in Exercise