The Heart

Part of the Cardiovascular SystemThe transport system that keeps blood circulating

Structure and Function

250-350 grams Within the thorax, in the mediastinum 2/3 lies to the left of midsternal line Base – broad flat posterior surface is

directed toward right shoulder Apex–points inferiorly toward left hip

Homeostatic Imbalance Pericarditis – inflammation of the

pericardium Hinders production of serous fluid and

roughens serous membrane surfaces “Creaking sound” when heart rubs

against pericardial sac, pain deep to sternum

Severe cases – cardiac tamponade “heart plug” caused by fluid compressing the heart

Conduction System of Heart

Rhythmical electrical activity causes continuous beating

Autorhythmic cells are self-excitable and repeatedly generate action potentials that trigger contractions (heart continues to beat after nerves cut)

Pacemaker that forms conduction system

Pacemaker

Cardiac chambers stimulated in coordinated manner to pump blood

Autonomic nervous system and hormones modify but don’t establish the rhythm

Conduction of Impulse

Cardiac excitation begins in the sinoatrial (SA) node in right atrial wall inferior to opening of sup. vena cava

Action potential propagates throughout both atria and atria contract

Action potential reaches atrioventricular (AV) node → bundle of His (connection between atria and ventricles)

Conduction

→ right and left bundle branches toward the apex → Purkinje fibers → throughout ventricles

About 200 milliseconds after the atria contract, the ventricles contract

Homeostatic Imbalance

Angina pectoris or “choked chest” Thoracic pain caused by fleeting

deficiency in blood delivery to myocardium

Serious, prolonged blockage can lead to heart attack or coronary (myocardial infarction MI) because blockage results in noncontractile scar tissue

Homeostatic Imbalance Valve deformities

Incompetent valve – forces heart to repump same blood; improper closure and blood back flows

Valvular stenosis – valve flap becomes stiff and constricts the opening (scar tissue after endocarditis or Ca2+ salt deposit) Both increase work load and weaken heart Faulty valve is usually the mitral (bicuspid)

Homeostatic Imbalance Heart region deprived of blood becomes

ischemic Cells begin to metabolize anaerobically

producing lactic acid Increased acidity inhibits cardiac cells’

ability to produce ATP (ATP needed to pump Ca2+ into extracellular space)

Increased H+ and Ca2+ inside cell causes gap junctions to close

Forces action potentials to find alternate routes (if area is large leads to heart attack)

Homeostatic Imbalance Defect in intrinsic conduction system

Irregular heart rhythms (arrhythmias) Uncoordinated atrial and ventricular contractions Fibrillation: rapid and irregular or out of phase

contractions Heart rhythm taken away from the SA node

(pacemaker) by rapid activity in other heart regions

Defibrillation – electrically shocking the heart which stops the chaotic twitching by depolarizing the entire myocardium

Homeostatic Imbalance Defective SA node (Sinoatrial node)

Ectopic focus – abnormal pacemaker takes over pacing of heart

AV node may take over (atrioventricular)

Only route for impulse transmission from atria to ventricles is through AV node Damage to AV node = heart block and

interferes with the ability of ventricles to receive pacing impulses

Homeostatic Imbalance Heart murmurs are abnormal heart

sounds Healthy blood flow is silent in adults Sounds may indicate valve problems

Incompetent valve = swishy sound with backflow through the partially open valve after the valve has closed

Stenotic valve = “narrowed” high-pitch sound or click when valve should be wide open during systole

Homeostatic Imbalance Reduced Ca2+ blood levels (hypocalcemia)

depress the heart Hypercalcemia prolongs the plateau phase of the

action potential that can lead to spastic heart Excess Na+ (hypernatremia) inhibits

transport of Ca2+ into cardiac cells and blocks heart contractions

Excess K+ (hyperkalemia) interferes with depolarization by lowering resting potential –may lead to heart block and cardiac arrest Hypokalemia is also life threatening because

heart beats feebly

Homeostatic Imbalance Tachycardia “heart hurry” – abnormally

fast heartbeat – more than 100 beats/min Elevated body temperature Stress Drugs Heart disease Promotes fibrillation and is pathological

Bradycardia – less than 60 beats/min Low body temperature Drugs Parasympathetic nervous activation Warning of brain edema after head trauma

Homeostatic Imbalance Congestive heart failure (CHF)

Pumping efficiency (CO) is so low that blood circulation is inadequate

Progressively worsens, reflects weakening of myocardium caused by Coronary artherosclerosis (clogging of vessels

with fatty buildup) Increasing hypoxia due to insufficient O2

Persistent high blood pressure The myocardium must exert more force to open the

aortic valve and pump out the same amount of blood (becomes weaker)

Homeostatic Imbalance

CHF (continued) Multiple myocardial infarcts

MI’s depress pumping efficiency because dead heart cells are replaced by scar tissue

Dilated cardiomyopathy (DCM) Ventricles stretch and become flabby and

myocardium deteriorates Cause often unknown Contractility is impaired, CO is poor and

condition progressively worsens

Homeostatic Imbalance Pulmonary congestion

Left side of heart fails Lungs become engorged with blood Leads to pulmonary edema Can cause suffocation

Peripheral congestion Right side of heart fails Blood stagnates in organs Inadequate oxygen and nutrients Edema pronounced in extremities

Homeostatic Imbalance

Seriously weakened heart is irreparable

Treatment involves: Conserving heart energy with digitalis

which reduces heart rate Removing excess fluids with diuretics Use drugs to lower blood pressure and

reduce afterload Transplants