Intrinsic Control of Cardiac Function
Intrinsic control of cardiac function by heart rate (Treppe effect)
Intrinsic control of cardiac function by Frank-Starling mechanism
The length dependent activation
Concepts of preload, afterload and contractility
Physiological Factors of the CVS:
MAP = CO x TPR
Controls originating from within the heart: intrinsic
Stroke volume (dictated by preload, afterload, contractility)
Input from outside the heart: extrinsic
Hormonal and nervous control
Cardiac cycle: closed loop
Both of these operate continuously to adjust the heart and
vessels to maintain MAP
CO = SV x HR
Intrinsic Control of Heart Rate:
Overall intrinsic control of heart rate seems very limited
Extrinsic parasympathetic and sympathetic nervous system control
seems to be much more important
Increased heart rate = increased developed force/pressure
Treppe effect: Bowditch effect: positive pressure/force-frequency
relationship: myocardial contraction increases with frequency.
Mechanism: Increase in Ca per unit time (increase in Na due to
slower Na/K pump).
Negative force/frequency relationship is the hallmark of heart
failure. o Increases HR
o Stronger contraction
o Faster contraction
Intrinsic control of stroke volume:
Determined by pre-load (input), afterload (resistance to output), and
contractility (force produced).
Increase in preload produces a larger stroke volume: ‘starling’s law of
the heart’ or the ‘Frank-Starling mechanism’
Increase in venous return = ventricle is filled more = increased
Increased end-diastolic volume (EDV) consequently the ventricle
is stretched- increase in sarcomere length therefore increase in
contraction which leads to an increased stroke volume.
The heart regulates ventricular output in response to ventricular filling.
Frank-Starling mechanism: increased volume = increased length of
cardiomyocytes = produces more force.
This is called length-dependent activation: need less Ca to produce
larger forces (compared to a shorter cardiomyocyte).
At the same Ca concentration at different lengths, you have different
amounts of force produced.
Overlap of actin/m