sem 2: preload and afterload Flashcards
what is cardiac output
volume of blood ejected per minute
CO= SV x HR
CO= BP/TPR
what 4 factors control stroke volume
heart rate
contractility
preload
afterload
what is preload
strecthing of the heart at rest
linked to starlings law
what is afterload
stressing of the heart wall
linked to laplaces law
what is HR controlled by
autonomic nerves
what is contractility controlled by
sympathetics
what is energy of contraction
the amount of energy required to generate stroke volume
what is starlings law
the enrgy of contraction of cardiac muscle is proportional to the muscle fibre length at rest
what is good measure of preload
CVP (Pressure in great veins)
in a stretched muscle what is the less interferance of
mechanical interference - more crossbridge formation - increased sensitivity to Ca2+ ions
what does after load oppose
ejection
what is afterload determined by
wall stress
energy of contraction needed to overcome it
what is laplaces law
P = 2T/r
in a larger heart is there more or less wall stress
more
what is wall tension composed of
T= wall stress(s) x wall thickness(w)
in a smaller radius heart:
greater wall curvature
more wall stress directed to the center
less afterload
better ejection
what does laplaces law say about arterial blood pressure
increased arterial blood pressure will lead to increased wall stress and increased afterload
two causes of heart failure
volume overloaded( e.g. MI, cardiomyopathies, mitral valve regurgitation) pressure overloaded- e.g. hypertension, aortic stenosis
what does heat failure lead to
increased wall stress because of equation
how does the heart compensate with increased wall stress
hypertrophy
greater myocyte side- so wall stress disappated over larger area
what is the bad thing about hyptertrophy
greater O2 demand, ultimately decreases contractility
