Heart Physiology

Heart Physiology

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Cardiac Conduction System· ability of cardiac muscle to depolarize and contract is intrinsic

does NOT depend on nervous systemcomposed of autorhythmic cells

1. pacemaker potentialno stable resting potentialinstead, Na+ channels are

opened after hyperpolarization allowing potential to drift toward threshold

threshold potential = -40mV2. depolarization

Ca2+ channels open and Ca2+ rushes into cells3. repolarization

K+ channels open and K+ rushes out of cell

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Electrical System1. Sinoatrial (SA) node

pacemaker determines heart rate70-80 per minute

2. Atrioventricular (AV) nodeimpulse pauses about 0.1 sec to

allow atria to finish contracting3. Atrioventricular bundle

electrical connection between atria and ventricles4. R and L bundle branches5. Purkinje fibers

highly branchedresponsible for ventricular contraction

In a healthy heart, it takes 0.22 sec from SA node stimulation to end of ventricular depolarization!

Changing the Rhythm · cardiac center located in the medulla oblongata

sympathetic nervous system speeds up heart rate and force (cardioacceleratory center)

parasympathetic nervous system slow down heart rate and force (cardioinhibitory center)· factors that affect heart rate

anxiety/stress and activity speed up ratemeditation, yoga, sleep slow down rate

Electrocardiogram (ECG)· aka electrokardiogram (EKG)· recording of the electrical changes in the myocardium· P wave - atrial depolarization

quickly followed by atrial contraction· QRS complex - ventricular depolarization

quickly followed by ventricular contractionatrial repolarization obscured by QRS complex

· T wave - ventricular repolarization

Arrhythmias

fibrillation

tachycardia

bradycardia

Heart Physiology

Heart Sounds· associated with closing valves· lub-dub, pause, lub-dub, pause, and so on

lub - AV valves (tricuspid and bicuspid) closedub - SL valves (pulmonary and aortic) closepause - heart resting between beats

Cardiac Cycle· aka "heartbeat"· mechanical actions of the heart· each heartbeat (cycle) blood is forced out of ventricles· average adult pumps about 5 L/min· stroke volume - volume of blood pumped by each ventricle per heartbeat · cardiac output - volume of blood pumped per minute by the heart (both ventricles)

multiple stroke volume (mm) by heart rate (beats/min)

· systole - contraction of heart muscle· diastole - relaxation of heart muscle· steps

1. atria contract forcing blood into relaxed ventricles (AV valves open, SL valves closed)

2. ventricles contract forcing blood into pulmonary trunk and aorta (AV valves closed; SL valves open)

3. both atria and ventricles relax and atria begin to fill passively with blood (all valves closed)

Cardiac Cycle (cont.) Cardiac Cycle