mechanics Flashcards
pressure volume loops: draw cardiac pressure-volume loops and explain the influence of preload and afterload
define preload
weight that stretches muscle before stimulated to contract
define afterload
weight not apparent to muscle in resting state, only encountered after starting to contract
isometric-preload relationship
greater preload leads to greater force because it stretches before contraction
isotonic-afterload relationship
greater afterload leads to reduced shortening and velocity (heavier weight = reduced shortening but same force)
preload-afterload relationship
greater preload leads to greater shortening for given afterload
in vivo correlates of preload
blood filling in diastole stretches resting ventricular walls, and stretch determines the preload before ejection (dependent on venous return); mre blood leads to greater contraction
how is preload measured in vivo
wall stress at end of diastole so end-diastolic volume, end-diastolic pressure, right atrial pressure
in vivo correlates of afterload
load against which left ventricle ejects blood after opening of aortic valve (pressure of blood in arteries); any increase decreases isotonic shortening and velocity
how is afterload measured in vivo
wall stress during systole so diastolic blood pressure
effect of hypertension on ventricular work
higher diastolic blood pressure (afterload) means ventricle has to work harder to expel blood as shortening reduced; must reduce otherwise blood flows backwards
what does a pressure-volume loop show
how left ventricular pressure and volume change in cardiac cycle
what signifies preload
A - end-diastolic volume; blood stretching muscle fibres
what signifies afterload
B - diastolic blood pressure; blood pressure in aorta
in a pressure-volume loop, what valves open and close at A, B, C and D
A: mitral valve closes, B: aortic valve opens, C: aortic valve closes, D: mitral valve opens
what happens during diastole (D → A)
atrial pressure > ventricular pressure, so blood enters ventricles, increasing volume
why does the pressure increase slightly during diastole
because end-diastole volume is increasing, so preload is also
what happens at A
mitral valve closes because ventricular pressure > atrial pressure
what happens between A and B
isovolumetric contraction (where walls contract but volume remains constant)
what happens at B
aortic valve opens because ventricular pressure > aortic pressure
what happens during systole (B → C)
blood ejected so volume falls, but pressure increases as fibres shorten
what happens at C
aortic valve closes
what happens between C and D
isovolumetric relaxation
diagram of pressure-volume loop
benjis
effect on pressure-volume loop of increasing preload (left ventricular filling)
larger volume of blood (A and B shift right), causing ventricle fibres to contract more strongly and quicker to produce a larger stroke volume
effect on pressure-volume loop of decreasing preload (left ventricular filling)
smaller volume of blood (A and B shift left)
effect on pressure-volume loop of increasing afterload
smaller stroke volume (C and D shift right) as ventricle walls won’t shorten as much, reducing force and velocity at which blood ejected against higher diastolic pressure (C and B shift up)
effect on pressure-volume loop of decreasing afterload
larger stroke volume (C and D shift left) as ventricle walls will rapidly shorten, producing a larger force and velocity at which blood ejected against lower diastolic pressure (C and B shift down)
diagram of how preload and afterload affect pressure-volume loop
benjis
define contractility
force of contraction of myocardium
what measures contractility
ejection fraction
what stimulation increases contractility
sympathetic
effect of increasing contractility on pressure-volume loop
greater gradient of tangent to C
effect of decreasing contractility on pressure-volume loop
reduced gradient of tangent to C
diagram of contractility in pressure-volume loop
benjis
