2 Regulation of cardiac contraction Flashcards
What are the steps in excitation-contraction coupling? (cardiac cell)
AP from adjacent cell travels along sarcolemma then down T-tubule
AP triggers v.g Ca2+ channels open and Ca2+ enters cell
Entry of CA2+ triggers release of Ca2+ from SR
Ca2+ bind troponin to initiate contraction
How does relaxation of contracted muscle occur? (cardiac cell)
Ca2+ unbinds from troponin
Ca2+ is pumped back into SR for storage (ATP is needed)
Ca2+ is exchanged with Na+
Na+ gradient is maintained by ATPase pump
What gradient does blood move via?
From areas of high pressure to low pressure
Contraction of the heart produces the pressure
Outline of the cardiac cycle
Late ventricular diastole
Isovolumetric ventricular contraction
Ventricular ejection
Isovolumetric ventricular relaxation
Ventricular diastole
How is the ventricle filled?
Up to 70% of blood fills it PASSIVELY
Then the pressure becomes less and less so blood fills slower
Need atrial contraction to FULLY fill ventricle
Why is atrial dysfunction not life ending?
Because it only fills the heart 30% more
What are the normal volumes of SV, EDV and ESV?
SV ~ 80mL
EDV ~ 130mL
ESV ~ 50mL
Define cardiac output
Flow rate out of one side of the heart = Amount of blood the heart pumps each minute
~5L per min at REST
Define heart rate
Number of contraction per min
Define stroke volume
Volume pumped by a ventricle in one contraction
Define venous return
Flow rate INTO heart
What components control stroke volume?
Venous return (contractility)
What is the Frank-Starlin Law?
Within physiological limit, the heart pumps ALL the blood it receives
Increased venous return stretches ventricles = increases force production until CO matches venous return
What is contractility regulated by?
Sympathetic nervous system
What is a change in contractility defined as?
Change in the work performed by the heart
That is NOT brought about be a change in initial fibre length (increases the force without stretching it)
What is the effect of norepinephrine on cardiac output?
Increases cardiac output without changing venous return
Doesn’t change volume of blood heart receives = just increases force of contraction (contractility)
What is the action of sympathetic stimulation?
Sympathetic nerve stimulation increases cardiac contractility
At rest, heart is under sympathetic tone
What does parasympathetic stimulation have little effect on?
Contractility
What is the difference between contractility and Starling’s Law on regulating the heart?
They both increase stroke volume but differently
Starlings increases the venous return
Contractility decreases the ESV because it forces more blood out in each pump
What is intrinsic regulation of cardiac contraction?
The heart’s ability to regulate its contraction strength and rhythm without external neural or hormonal influences
What is extrinsic regulation of cardiac contraction?
Extrinsic regulations are not part of the organism’s inherent structure but can impact its functions
Give examples of intrinsic regulation of cardiac contraction
Venous return produces intrinsic regulation
Intrinsic regulation gives a constant ejection fraction
Give examples of extrinsic regulation of cardiac contraction
Autonomic nerves
Hormones
Drugs
Can alter force of contraction
What do inotropic agents do?
Agents that alter contractility
What does increased inotropy cause?
Increase in ejection fraction because of more contractility
Name an inotropic hormone
Adrenaline
What part of the heart do sympathetic nerves innervate?
Whole heart
What -topic affects do sympathetic stimulation have on the heart?
POSITIVE chronotropic and inotropic actions
What -topic affects do parasympathetic stimulation have on the heart?
Negative chronotropic affect (slows heart)
Little inotropic action = cannot weaken contractility
