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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
• 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
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
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.
Blood Flow Regulation
• Flow = pressure gradient x vessel radius4
vessel length x viscosity
• Blood flow Resistance Factors1. Viscosity or blood thickness
2. Length of conducting tube
3. 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
S ym p ath e tic :n o rep in ep h rin e
(ad ren erg ic )vasocon s tric to r
S ym p ath e ticace tylch o lin e(ch o lin e rg ic )
vasod ila tion in m u sc le
L oca l M etab o litesm ore p ow erfu l th an
sym p ath e ticvasocon s tric to rs
N eu ra lF ac to rs
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 ExerciseConditions Activator ResponsePreexercise
“anticipatory”response
Activation of motorcortex & higherbrain.
HR, myocardialcontractility; vaso-dilation in muscle
Exercise
Continued sympa-thetic cholinergicoutflow; alterationsin local metabolicconditions
Further dilations ofmuscle vasculature
Continued sympa-thetic adrenergicoutflow
Concomitant con-striction of vascula-ture in inactivetissues