The Cardiovascular System The Heart Chapter 20

Location The heart lies in the mediastinum.

Pericardium and Layers of Heart Wall

Chambers and Sulci

Chambers and Sulci

Right Atrium

Right Ventricle

Left Atrium

Left Ventricle

Anterior Heart

Myocardial Thickness and Function Thickness of myocardium varies according to the function of the chamber

Atrioventricular Valves

Blood Circulation Blood flow Blue: deoxygenated Red: oxygenated

Coronary Circulation

Coronary Artery Disease

Coronary Artery Disease

Coronary Artery Disease

Cardiac Muscle Histology

Conducting System Autorhythmic cells – self excitable Reduced permeability of K+, but no change in permeability to Na+. Na+ continues to diffuse in. Unstable resting potential – continuously depolarizes, drifting slowly toward threshold. (pacemaker potential)

Physiology of Contraction Potentials initiated by conducting fibers stimulate contractile fibers.

Electrocardiogram (ECG/EKG)

EKG

Cardiac Rhythm and Rate Normal heart rate: 60-100 beats/min Avg. heart beat at rest: 70-72 beats/min Sinus tachycardia: > 100 beats/min Sinus bradycardia: < 60 beats/min

Cardiac Rhythm and Rate Arrhythmias: abnormal rhythm resulting from a defect in the heart conduction system. Fibrillation: rapid and irregular contractions. Can be atrial or ventricular. Defibrillation: electrical shock to depolarize myocardium. Ectopic Focus: a region of the heart, other than the conducting system, that causes and abnormal depolarization. Junctional rhythm: AV node becomes pacemaker Heart block: an arrhythmia that occurs when electrical pathway between the atria and ventricle is blocked.

EKG

Cardiac Cycle Systole: contraction Diastole: relaxation

Heart sounds Lubb - AV valves close Dupp - semilunar valves close

Stroke volume = end diastolic volume - end systolic volume Stroke Volume (SV) Volume of blood pumped out by a ventricle with each beat Stroke volume = end diastolic volume - end systolic volume SV = EDV - ESV EDV = Amount of blood that collects in a ventricle during diastole ESV = Amount of blood remaining in a ventricle after contraction SV depends on amount of stretch produced by venous return. Averages about 70 ml/beat Anything that influences heart rate or blood volume influences venous return and therefore SV

Cardiac output = Heart rate x Stroke volume Cardiac Output (CO) amount of blood pumped by each ventricle in one minute Cardiac output = Heart rate x Stroke volume CO = HR x SV HR = beats per minute SV = volume of blood pumped out by a ventricle with each beat

Factors Affecting SV Preload Degree of stretch on the heart before it contracts Greater preload increases the force of contraction Frank-Starling law of the heart – the more the heart fills with blood during diastole, the greater the force of contraction during systole Preload proportional to end-diastolic volume (EDV) 2 factors determine EDV Duration of ventricular diastole Venous return – volume of blood returning to right ventricle

Factors Affecting SV Contractility Strength of contraction at any given preload Positive inotropic agents increase contractility Often promote Ca2+ inflow during cardiac action potential Increases stroke volume Epinephrine, norepinephrine, digitalis Negative inotropic agents decrease contractility Anoxia, acidosis, some anesthetics, and increased K+ in interstitial fluid

Factors Affecting SV Afterload Pressure that must be overcome before a semilunar valve can open Increase in afterload causes stroke volume to decrease Blood remains in ventricle at the end of systole Hypertension and atherosclerosis increase afterload

Regulation of Heart Rate 1. Autonomic regulation 2. Chemical regulation 3. Other Factors

Cardiac Output Change stroke volume and/or heart rate

Homeostatic Imbalance of CO Congestive heart failure - occurs when the pumping ability of the heart is inadequate to provide normal circulation to meet body needs. Pulmonary congestion - left side failure Peripheral congestion - right side failure