Physiology of the heart 2 Flashcards
Whats the residual volume?
When the heart contracts- doesnt eject all of the blood- leaves some behind
Whats the stroke volume and normal stroke volume?
When the heart contracts- delivers the stroke volume-
70mls approx
Whats the cardiac output?
How much blood the heart is delivering each minute
=heart rate x stroke volume
Whats the ejection fraction?
The % of blood ejected with each cardiac contraction
its a measure of the internal contractility of the heart
What are the factors that affect heart rate?
sympathetic nervous system- positively chronotropic
parasympathetic nervous system- negative chronotropic
circulating catecholamines- e.g adrenaline
drugs
Factors affecting stroke volume
intrinsic contractility- intracellular calcium, oxygen, free fatty acids, ATP
extrinsic contractility- pre load, after load, sympathetic activity
*parasympathetic nervous system has virtually no effect on contractility
What’s preload?
The filling pressure going into the ventricles- LVEDP
What’s afterload?
The pressure beyond the heart- in the circulation- resistance to ejection
What happens when calcium enters the cell?
Calcium enters the cell- attaches to ryanodine receptors on the sarcoplasmic reticulum- causes calcium to be released from the SR- binds to more receptors- second wave of calcium is responsible for contraction
Whats the stages of the ATP dependent actin-myosin dependent interaction?
ATP is hydrolyzed when the myosin head attaches to actin
ADP + Pi are bound to the myosin as the myosin head attached to actin
ADP + Pi are released causing the head to change position and the actin filament to move
Binding of ATP causes the myosin head to return to the resting potential
What is the Frank-Starling mechanism?
Increases in the filling pressure (LVEDP- which results in myocardial fibre stretching)-> increased force of contraction-> increased stroke volume and cardiac output
How does changes in venous return affect the frank-starling curve?
Increase the venous return- move up the curve- increase in the filling pressure- increase stroke volume-
We move up and down this curve on a minute to minute basis
Takes a small change in pressure to alter stroke volume
How does exercise affect venous return?
Self-regulating essentially
sympathetic stimulation takes control- increases heart rate
frank starling controls CO based on venous return at resting
How do changes in afterload and contractility affect the frank-starling curve?
increase contractility/ decrease in aferload= curve moves up and to the left - easier for the heart to eject- larger stroke volume
decreased contractility/ increase in afterload= operate on a lower curve thats flatter - have to have a higher pressure for the same cardiac output
*problems arise with high filling pressures- heart failure etc
How does sympathetic stimulation affect cardiac output?
Isoprenaline (drug equivalent to adrenaline)- stimulus causes a greater influx of calcium and a greater contraction
drug increases contractility
How does heart failure and therapy effect the frank-starling curve?
Failing heart= something intrinsically wrong- curve falls down to the right - much flatter
limit to how much stroke volume you can achieve in this heart (because of how flat the curve was)
LVEDP of 25mmHg- as the filling pressure increases - more symptoms and signs of heart failure
Diuretics used in HF- move down the curve- reduced filling pressure, fewer symptoms- small decrease in stroke volume
What happens if you drop below a certain CO level?
fatigued, poor exercise capacity, in a state of medical shock
How does high VEDP affect the right and left side of the heart?
right side= peripheral fluid collection, oedema
left side= back pressure can go into lungs- feeling breathless
filling pressure only needs to go up slightly for feelings of breathlessness to occur
Whats the dilemma with treating HF patients?
Want to taken them out of a zone of high filling pressures, but dont want to drop their stroke volume
How do we treat HF patients?
Diuretics- bring them down the curve- move them out of area of symptoms
Vasodilaters (ACE inhibitors)- reduce afterload, shift the curve up and left - can use diuretics as well
How does digoxin affect cardiac output?
Causes a greater influx of calcium- greater contraction
inhbits Na/K ATPase- greater intracellular calcium
