Regulation of Stroke Volume and Heart Rate Flashcards
Explain how the parasympathetic nervous system decreases heart rate
Parasympathetic input via the Vagus nerve transmits Ach which acts on the muscarinic receptors on the SA node = Bradycardia
How does the parasympathetic nervous system alter the pacemaker cells within the heart ?
The cells hyperpolarise and decrease in rate of polarisation
Explain how the sympathetic nervous system increases heart rate
Sympathetic nerves release nor-adrenaline which act on the beta-1 receptors on the SA Node = Tachycardia
How does the sympathetic nervous system alter pacemaker cells within the heart ?
Increases the rate of depolarisation
What effect does the parasympathetic nervous system have on stroke volume and why ?
Little to no effect
The parasympathetic nerves only act on the SA Node via the vagus nerve not the ventricles therefore do not effect contractility
What effect doe the sympathetic nervous system have on stroke volume & why ?
Increases the strength of contractions (contractility)
This is due to the sympathetic input acting not only on the SA Node but also the ventricles.
Define preload
pre-load is the relationship between the stroke volume and the end diastolic volume.
What is stroke volume ?
The volume of blood ejected after the filling phase
What is end diastolic volume ?
The filling phase/ the volume of blood filling the ventricles
Explain how pre-load affects an increase in stroke volume
If more blood is being pumped into the ventricle then the EDV will increase and therefore the amount of blood pumped out will also increase therefore SV will increase.
Explain how pre-load effects a decrease in stroke volume
If less blood is being pumped into the ventricle then he EDV will decrease and therefore there will be less blood to pump out therefore SV will also decrease
Define after-load
The force the muscle is trying to work against
Explain how after-load affects stroke volume
If pressure in the arterioles increases then that pressure will also increase in the aorta & ventricle. This increase in pressure means more energy is exerted by the heart in order to push blood through the aortic valve and for that reason blood is ejection speed and volume is decreased therefore SV is decreased.
Name 4 pathological causes that will also alter SV
1: Hypercalcaemia - stronger contractions
2: Hypocalcaemia - weaker contractions
3: Ischaemia - weaker contractions
4: Barbiturates - weaker contractions
Explain how during exercise an increase in HR and SV affects cardiac output
1: increase in HR due to suppressed vagal input and increased sympathetic input.
2: increase in contractility & venous return via skeletal & resp, pumps that maintain EDV.
3: Increased contractility shortens systole which leaves more time for filling phase.
4: Decrease in TPR due to dilation of arterioles leading to skin, muscles and heart reducing afterload.
5: Cardiac output increases
