Regulation of Heart Rate and Stroke Volume Flashcards
(30 cards)
Describe the effect of the sympathetic system on HR
NA released and circulating adrenaline from adrenal medulla.
Act on Beta 1 receptors in sinoatrial node
Increase slope of pacemaker potential
Increased heart rate = tachycardia
What is the effect of the sympathetic system on HR in terms of the slope of the pacemaker potential
Increased slope
Describe the effect of the parasympathetic system on HR
ACh released
ACh acts on muscarinic receptors
Decrease slope of pacemaker potential
Decreased HR = bradycardia
What is the effect of the parasympathetic system on HR in terms of the slope of the pacemaker potential
Decreased slope
State the three physiological mechanisms that regulate stroke volume
Preload
Afterload
Neural (sympathetic and parasympathetic)
State Starling’s law
Energy of contraction proportional to initial length of cardiac muscle fibre
Define preload
End diastolic volume
How does starling’s law apply to preload and thus SV
Starling’s Law = Energy of contraction proportional to initial length of cardiac muscle fibre
Preload = EDV
Explanation: A big EDV means the cardiac muscle is very stretched initially so contraction strength is greater. A small EDV means cardiac muscle is not that stretched initially to contraction strength is smaller.
What vessels affect preload?
Venules and veins
Describe the effect of venous return and EDV/preload on SV
Increased venous return = increased EDV = increased SV
Decreased venous return = reduced EDV = reduced SV
Define afterload
Load against which heart must contract = total peripheral resistance (TPR)
What vessels affect afterload?
Arterioles
Describe the effect of TPR/afterload on SV
As TPR increases -> Aortic pressure increases -> More energy needed for ventricle to push aortic valve open -> less energy for ejection -> reduced SV
Describe the effect of the sympathetic system (neural) on SV
NA and adrenaline released
Act on Beta 1 receptors of myocytes
Inotropic effect - increase contractility
Stronger but shorter contraction
Does the parasympathetic system affect SV?
No
What does the parasympathetic system have little effect on SV?
Vagus nerve doesn’t innervate ventricular muscle
Name 4 factors that can pathologically affect SV
Hypercalcaemia
Hypocalcaemia
Ischaemia
Barbiturates
Describe the effect of hypercalcaemia on SV and the starling curve
More Ca2+ to form cross bridges.
Increased contractility
Curve shifts UP and TO LEFT
Describe the effect of hypocalcaemia on SV and the starling curve
Less Ca2+ to form cross bridges.
Decreased contractility
Curve shifts DOWN and TO RIGHT
Describe the effect of ischaemia on SV and the starling curve
Less blood supply to cardiac muscle.
Less energy for cardiac muscle to contract
Curve shifts DOWN and TO RIGHT
Describe the effect of barbiturates on the starling curve
Curve shifts DOWN and TO RIGHT
Describe how the heart compensates for reduced pumping ability
The heart works around its reduced EDV by reducing its ejection fraction which in turn reduces exercise capacity
Describe how cardiac output (CO) is controlled in an electronic pacemaker
Increased HR -> increased CO w/ small reduction in SV because the cardiac interval is shorter. Therefore part of the rapid filling phase is cut meaning reduced EDV and preload
List the physiological components that can increase CO
HR
Contractility
Venous Return
TPR
