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Chapter 14: The Heart as A Pump (Electrical Signals Coordinate Contraction…
Chapter 14: The Heart as A Pump
Electrical Signals Coordinate Contraction
Sinoatrial node: autorhythmic cells in the right atrium that serve as the main pacemaker of the heart
Internodal pathway: connects the SA node to the AV node
Atrioventricular node: a group of autorhythmic cells near the floor of the right atrium
Purkinje fibers: specialized conducting cells that transmit electrical signals down the bundle of His in the ventricles
Electrical signals need to be conducted downward so the ventricles contract from the bottom up
AV node delay: slower conduction of signals through the nodal cells
Pacemakers Set the Heart Rate
Cells follow the fastest pacemaker, which should be the SA node
Complete heart block: conduction of electrical signals from the atria to the ventricles through the AV node is disrupted and the ventricles contract slower than the atria
The Electrocardiogram Reflects Electrical Activity
ECG: show summed electrical activity generated by all cells of the heart
Waves: deflections above or below the baseline
P wave: first wave which corresponds to depolarization of the atria
QRS Complex: represents the progressive wave of ventricular depolarization (atrial repolarization is also incorporated)
T wave: final wave which represents the repolarization of the ventricles
Segments: sections of baseline between two waves
Intervals: combinations of waves and segments
Direction of the EKG reflects the direction of current flow relative to the axis of the lead
Cardiac cycle: single contraction-relaxation cycle which consists of both electrical and mechanical events
The Heart Contracts and Relaxes during a Cardiac Cycle
Diastole: relaxation of cardiac muscle
Systole: contraction of heart muscle
atrial and ventricular diastole:arrow_right:completion of ventricular filling (atrial systole):arrow_right:early ventricular contraction and the first heart sound (lub):arrow_right:ventricular ejection:arrow_right:ventricular relaxation and the second heart sound (dub)
Pressure-Volume Curves Represent One Cardiac Cycle
End-diastolic volume (EDV): maximum volume of blood in the heart at the end of ventricular relaxation
Isovolumic relaxation: relaxation which occurs without a change in blood volume
Multiple Factors Affect Stroke Volume
Stroke Volume: amount of blood pumped by one ventricle during a contraction (measured in mm/beat)
Contractility: intrinsic ability of a cardiac muscle fiber to contract at any given fiber length and is a function of Ca2+ interaction with the contractile filaments
Preload: degree of myocardial stretch before contraction begins; represents the load placed on cardiac muscles before they contract
Frank-Starling law of the heart: within physiological limits, the heart pumps all the blood that returns to it
Venous return: Amount of blood that enters the heart from the venous circulation
Skeletal Muscle Pump: skeletal muscle contractions that squeeze veins, compressing them and pushing blood toward the heart
Respiratory Pump: thoracic cavity enlarges and develops pressure which draws blood into the vena cava
Constriction of veins by sympathetic activity
Cardiac Output: volume of blood pumped by one ventricle in a given period of time
The Autonomic Division Modulates Heart Rate
Parasympathetic activity slows heart rate
ACh activates muscarinic cholinergic receptors that influence K+ and Ca2+ channels in the pacemaker cell; decreased Ca2+ permeability slows the rate at which the pacemaker potential depoarlizes and the cell takes longer to reach threshold
Dominates tonic control
Sympathetic activity speeds up the heart rate
Catecholamines norepinephrine and epinephrine increase ion flow through If an Ca2+ channels; rate of pacemaker depolarization is sped up due to rapid cation entry and the cell can reach threshold faster
Both autonomic branches alter the rate of conduction through the AV node
Contractility is Controlled by the Nervous and Endocrine Systems
Ionotropic agent: any chemical that affects contractility
Ionotropic effect: influence of an ionotropic agent
Phospholamban: regulatory protein which increases Ca2+ storage
Catecholamines increase the force and shorten the duration of cardiac contraction
Cardiac glycosides enhance contractility and can be used as a remedy for heart failure
EDV and Arterial Blood Pressure Determine Afterload
Afterload: combined load of EDV and arterial resistance during ventricular contraction
Ejection Fraction: percentage of EDV ejected with one contraction