cardiovascular mechanics Flashcards
what happens when a ventricular cell receives an electrical event?
Ca2+ influx, Ca2+ release
then contraction
what is the coupling of the electrical event and the contractile event?
excitation-contraction coupling
what does the contraction of cardiac muscle depend on?
influx of calcium from outside the cell to the inside
what is the difference between cardiac muscle and skeletal muscle when placed in saline solution?
skeletal muscle would still contract
cardiac muscle needs calcium in the solution so will not contract
what is the size of a ventricular cell?
150um long and 15um wide
what are T-tubules?
finger-like invaginations of the ventricular cell surface
how large are T-tubules?
200nm in diameter
how much space between each T-tubule? and why?
2um apart
so each T-tubule lies alongside each Z-line of every myofibril
allows surface depolarisation to be carried deep into the cell
what is the sarcoplasmic reticulum for?
calcium store for the muscle cells
where is the sarcoplasmic reticulum found?
above the myofilaments
order the organelles in terms of their volume in cells of the heart
myofibrils(46%) mitochondria(36%) sarcoplasmic reticulum(4%) nucleus(2%) others(12%)
what happens at the end of sarcoplasmic reticulum?
wrap around the T-tubules
what is LTCC?
L-type calcium channels
what are LTCCs closely linked to?
a cluster of ryanodine receptors (SR calcium release channel)
what senses the depolarisation from calcium influx?
L-type calcium channel it opens up allowing calcium to diffuse into the cytosol
what are the roles of the calcium that enter the cytosol?
to bind to SR calcium release channels
can feed to the myofilaments
what happens when calcium binds to the SR calcium release channel?
it opens up allowing calcium to leave from the sarcoplasmic reticulum and enter the cytosol
what happens to the calcium leaving the sarcoplasmic reticulum?
binds to troponin on the myofilaments and activates contraction
how is relaxation brought about?
calcium pumped against its concentration gradient via SR calcium ATPase using ATP (recycle calcium)
how does calcium efflux and why?
same amount of calcium that enters must leave
uses sodium/calcium exchange system
sodium moves in providing energy to expel calcium from the cell
what is the relationship between cytoplasmic calcium concentration and force?
increase in calcium increases force produced
sigmoidal(S shaped graph)
what is the relationship between muscle length and force produced?
increase in length increases force produced
active force drops after a certain muscle length
passive force constantly increases with muscle length
why is cardiac muscle more resistant to stretch and less compliant than skeletal muscle?
due to properties of the ECM and cytoskeleton
what is the difference between cardiac muscle and skeletal muscle in terms of over stretching?
cannot over stretch cardiac muscle (only works on ascending limb of muscle length)
what are the types of muscle contraction in cardiac muscle?
isometric
isotonic
what is isometric contraction?
muscle fibres do not change length but pressure increases in both ventricles
what is isotonic contraction?
shortening of fibres and blood is ejected from ventricles
what is the preload?
weight that stretches muscle before it contracts
what is the afterload?
weight not apparent to muscle in resting state; only encountered when muscle has started to contract
what happens to force as preload increases?
it increases
what happens to muscle shortening as after load increases?
amount of shortening is reduced and velocity
what happens to muscle shortening as preload increases?
it increases shortening and velocity
in cardiac muscle what is the preload?
ventricle filling with blood
what is the afterload in cardiac muscle?
pressure in the aorta
what is preload dependant upon?
venous return
what are measures of preload?
end-diastolic pressure and volume aswell as right atrial pressure
what happens to the amount isotonic shortening with increased afterload?
it decreases
what is a measure of afterload?
diastolic blood pressure
what is starlings law (frank-starling relationship)
as filling of heart increases the force of contraction is also increased
increased diastolic fibre length increases ventricular contraction
what are the consequences of starlings law?
ventricles pump greater stroke volume so that, at equilibrium cardiac output exactly balances the augmented venous return
(balance between amount of blood pumped and the amount that enters the heart)
why is the Frank-Starling relationship?
changes in the number of myofilament cross bridges that interact
changes to the calcium sensitivity of the myofilaments
what happens to troponin C (TnC) with longer sarcomere length? (hypothesis 1)
affinity for calcium is increased because of a conformational change
less calcium required for same force
what happens when we stretch muscle cells? (hypothesis 2)
we increase their length but decrease the space between actin and myosin filaments (increases probability of cross bridges
lattice spacing
what is stroke work?
work done by heart to eject blood under pressure into the aorta and pulmonary artery
what is the equation for stroke work?
volume of blood ejected during each stroke(SV) X pressure at which the blood is ejected(P)
what is the law of LaPlace?
when the pressure within a cylinder is held constant, the tension on its walls increases with increasing radius
what is the equation of wall tension?
wall tension (T)= pressure in vessel(P) X radius of vessel( R) when incorporating wall thickness divide by h
what is the comparison of pressure in either side of the heart?
right side is lower in pressure than the left
how is tension maintained with different pressures?
change in radius of ventricles
less radius in left ventricles
what often happens to failing hearts (radius)?
they become dilated and spherical
