Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Differences in Contraction Mechanisms  Heart has autorhythmicity (approx. 1%)  Heart contracts as a unit or not  Absolute refractory period is longer in cardiac cells  Action potential lasts longer providing sustained contraction

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Similarities in contracting mechanisms  1. depolarization – Na+ moves into cell  2. propagation of depolarization via T tubules  3. Ca 2+ released from sarcoplasm  4. cross bridges form

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Energy requirements  Heart has more mitochondria  Heart relies on aerobic respiration  Heart readily switches metabolic pathways to use whatever nutrient supply is available  Biggest danger to heart is lack of oxygen  Lack of blood (ischemia)-can ultimately lead to heart attack

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings The autorhythmic cells (the 1%)  Responsible for the rhythmic contractions of the heart  Contain special ion channels that allow for Ca2+ influx which produces the action potential and not the Na+  Cells found in Sinoatrial Node (right atrial wall just below entry of superior vena cava.) generates impulses 75 times per minute – called the pacemaker

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Autorhythmic cells (the 1%)  AV node –located in interatrial septum above the tricuspid valve  Impulses move from AV node to AV bundle which conducts the electrical connection between atria and ventricle  Impulse moves to Purkinje fibers which conduct impulses to the rest of the bulk of the heart

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Electrocardiography page )  Electrical activity is recorded by electrocardiogram (ECG)  P wave corresponds to depolarization of SA node  QRS complex corresponds to ventricular depolarization  T wave corresponds to ventricular repolarization  Atrial repolarization record is masked by the larger QRS complex

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Electrocardiography Figure 18.16

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Heart Sounds Figure 18.19

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Heart Sounds  Heart sounds (lub-dup) are associated with closing of heart valves  First sound occurs as AV valves close and signifies beginning of systole  Second sound occurs when SL valves close at the beginning of ventricular diastole

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Cardiac Cycle  Cardiac cycle refers to all events associated with blood flow through the heart  Systole – contraction of heart muscle  Diastole – relaxation of heart muscle

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Cardiac Output (CO) and Reserve  CO is the amount of blood pumped by each ventricle in one minute  CO is the product of heart rate (HR) and stroke volume (SV)  HR is the number of heart beats per minute  SV is the amount of blood pumped out by a ventricle with each beat  Cardiac reserve is the difference between resting and maximal CO

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Cardiac Output: Example  CO (ml/min) = HR (75 beats/min) x SV (70 ml/beat)  CO = 5250 ml/min (5.25 L/min)

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Extrinsic Factors Influencing Stroke Volume  Agents/factors that decrease contractility include:  Acidosis  Increased extracellular K +  Calcium channel blockers

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings  Sympathetic nervous system (SNS) stimulation is activated by stress, anxiety, excitement, or exercise  Parasympathetic nervous system (PNS) stimulation is mediated by acetylcholine and opposes the SNS Regulation of Heart Rate: Autonomic Nervous System

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Chemical Regulation of the Heart  The hormones epinephrine and thyroxine increase heart rate  Intra- and extracellular ion concentrations must be maintained for normal heart function – specifically Ca2+ and K+

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Congestive Heart Failure (CHF)  Congestive heart failure – when the CO is so low that blood circulation is inadequate to meet tissue needs  Coronary atherosclerosis – fatty buildup in coronary vessels  Persistent high blood pressure – above 90 mm Hg in the aortic diastole phase  Multiple myocardial infarcts – dead cells replaced by scar tissue

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Homeostatic Imbalance  Tachycardia – abnormally fast heart rate (110 beats/min.)  Bradycardia – abnormally slow heart rate (60 beats/min.) ~low body temperature ~certain drugs ~endurance exercise

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Examples of Congenital Heart Defects Figure 18.25

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Age-Related Changes Affecting the Heart  Sclerosis and thickening of valve flaps  Decline in cardiac reserve  Fibrosis of cardiac muscle  Atherosclerosis