Cardiac Cycle: • Sequence of mechanical events that occurs during one heartbeat • The two atria contract simultaneously • A fraction of a second later, the two ventricles contract • Contraction of chamber=systole, is followed by relaxation=diastole
Cardiac Cycle: • In diastole (relaxed), the atria continually receive blood from the veins-filling • Blood builds up and the pressure increases, causing atrial systole (contraction), forcing the AV valves open=blood passes into ventricles
Cardiac Cycle: • The atria then relax and ventricular systole occurs, closing the AV valves and opening the semilunar valves • Blood then pumps into the aorta and pulmonary artery • As the ventricles empty and pressure drops=some backflow of blood occurs=closes the semilunar valves
Heart Sounds : • The events of the cardiac cycle create normal heart sounds • Lubb-dubb • First sound (lubb)=caused by closure of AV valves during ventricular systole • Second sound (dubb)=from closure of the aortic and pulmonary semilunar valves • “Wooshing” sound indicates murmur from incomplete valve closure • Watch this video:
Cardiac Output: • Amount of blood ejected from the left ventricle in one minute • Calculated by multiplying stroke volume x HR • Stroke volume=amount of blood ejected by a ventricle in one contraction • Depends upon contractility, pre-and- afterload • Average=60-80 ml/beat, 5-6 ml/min
Cardiac Output •
Cardiac Output and Exercise: • During exercise, venous return increases and stretches the ventricular myocardium=causes it to contract more forcefully • Referred to as Starlings Law of the Heart • Stroke volume increases, more blood is pumped per beat, HR increases and thus cardiac output increases (up to 4 x and even more for athletes)
Ejection Fraction: • Measures how well the ventricle is functioning • Measures how much blood is pumped by the left ventricle in one contraction • Normal: 60% • Lower values=ventricle not pumping sufficiently and that too much blood remains in ventricle • Norm remaining values: 50-60 ml
Preload and Afterload: • Preload: how much blood is filling the ventricles • Can be increased in hypervolemia • Afterload: force at which the heart has to contract in order to eject blood • Hypertension can increase afterload • Narrowed valves can increase afterload
Regulation of Heart Rate: • Heart generates its own electrical impulse, beginning at the SA node • But the nervous system can change the HR in response to environmental circumstances • Either increase or decrease HR • BP drop or decrease in O2 sat can increase HR
Hormones and the Heart: • Epinephrine: • Secreted by adrenal medulla during stress • increases HR and force of contraction=increases cardiac output and systolic BP • Aldosterone: • Produced by adrenal cortex • Helps regulate blood levels of K and Na, both needed for electrical activity
Hormones and the Heart: • Atria secrete their own hormone=atrial natriuretic peptide • Increases secretion of Na by kidneys •
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- Fall '15