stroke volume and heart rate regulation Flashcards
explain the effects of the sympathetic nervous system in increasing heart rate:
- SNS activate the release of noradrenaline and adrenaline from the adrenal medulla
- Act on ß1 receptors (adrenergic) on the SA node
- Increase the slope of the pacemaker potential so it reaches the threshold quicker
-> tachycardia (depolarisation)
-> Increase heart rate
explain the effects of the parasympathetic nervous system in decreasing heart rate:
- Vagus nerves release acetylcholine
- Act on muscarinic receptors on the SA node
- Decreases slope of the pacemaker potential so it reaches threshold slower
-> bradycardia (hyperpolarisation)
-> Decreases heart rate
explain the effects of parasympathetic system on the heart
affects heart rate
little effect on contractility
because the vagus does not innervate the ventricular muscle (only the top and the nodes)
what is afterload?
the load against which the muscle tries to contract.
How difficult it is for the heart to pump out the blood.
what is preload?
the initial stretching of the cardiac myocytes before contraction
affected by EDV (how full the ventricle is before it starts contracting)
define contractility:
describes how strong a contraction is produced for any given preload or afterload.
It is affected by the sympathetic system.
what is the law that relates to the preload of stroke volume?
Starling’s law: the energy of contraction (of cardiac muscle fibres) is proportional to the initial length of the cardiac muscle fibres
length-tension relationship -> higher preload
= bigger strength of contraction
define inotropic and chronotropic:
Inotropic → increase contractility
Chronotropic → increase heart rate
what is afterload set by?
by the arterial pressure against which the blood is expelled (this in turn depends on the Total Peripheral Resistance).
roles of venules/veins and arterioles:
Venules/veins - (venous return) are capacitance vessels that affect preload
Arterioles are resistance (TPR) vessels that affect afterload
what effect does hypercalcemia have on the curve?
shifts curve up and left, has the same effect as sympathetic stimulation
what effect does hypocalcaemia have on the curve?
shifts curve down and right
what effect does ischaemia have on the curve?
(that side of the heart will pump less strongly) – shifts curve down and right
what effect does barbiturates (or other drugs) have on the curve?
shifts curve down and right. (if used too much)
why do you have a low stroke volume if you had a myocardial infarction?
the curve will be shift down and you will end up with a smaller SV for any given EDV
what is the equation for cardiac output?
CO = HR x SV
what effect does heart rate have on stroke volume and cardiac output?
increasing HR higher and higher, decreases SV lower and lower = CO stays the same.
