preload, afterload and contractility Flashcards
what affects the amount of blood pumped by the heart for each cardiac cycle
stroke volume
what affects the amount of blood delivered to the tissues
- stroke volume
- heart rate
- vasculare tone
what is cardiac output
the amount of blood pumped in 1 minute
- usually close to blood volume (blood usually circulates every minute)
- at rest, tissue oxygen delivery exceeds oxygen consumption (2:1)
cardiac output = heart rate x stroke volume
what happens if the cardiac output from the left and right ventricles arent matched
normal heart automatically pumps our whatever blooc volume is put into it
stroke volume must be matched (volume entering lungs = volume entering systemic circulation otherwise:
- increase in pressure on venous side
- leading to oedema (pulmonary or peripheral hence:
- tightly regulated by a range of mechanisms
how is stroke volume regulated
by preload (end diastolic volume), afterload (resistance to ventricular ejection) and contractility (end systolic colume)
stroke volume = end diastolic volume - end systolic volume
what is preload
dependent of venous return of blood
- if low: ventricular filling is reduced thus stroke volume is reduced (ex. hemorrahge)
- if high: ventricular filling is increased hence stroke volume increases (ex. exercise)
is stretch of cardiac muscle prior to contraction
descrieb the frank-starling mechanism
- generally, a linear relationship between preload and stroke volume
- ventricular muscle stretching leads to stronger contractile force
describe the length-tension relationship
increased preload leads to increased exposure of myosin to actin leading to increased cross bridge formation leading to increased force of contraction
what are the limits to the frank-starling mechanism
- excessive stretching causes a decrease in cross bridge formation
- laplace’s law: in a large sphere more wall tension is required to generate the same internal as it does in a small sphere as governed by pressure=tension/radius
clinically: as the heart fills up with more blood, then the muscle will find itself at an increasing mechanical disadvantage. thus, the chambers will become more difficult to empty (=> DCM, because bigger has to work harder to maintain pressure)
what factors influence preload
- filling time of the heart (low heart rates -> longer period for ventricular filling = greater distension of the ventricle
- venous return: i.e pressure difference between venous system and atrium
how does the skeletal muscle pump increase venous return
contraction of skeletal muscle leads to compression of veins leading to the blood being forced toward heart. this increases preload (more blood to stretch cardiac muscles)
describe how the respiratory pump increases venous return
inspiration: diaphragm moves caudally, increasing abdominal pressure and reducing thorax pressure increasing abdominal return of blood. this increases preload
how does sympathetic control of venous return increase preload
- venous system acts as reservoir for blood
- sympathetic stimulation of the venous system causes venous vasoconstriction, increases venous pressure. this leads to increased preload
how does the SNS influence stoke volume
increases contractility thus increases strength of contraction at any given preload
how does contractility effect stroke volume
increased contractility = greater force of contraction = increases amount of blood being pushed out = increase stroke volume = increased cardiac output