Physiology: Regulation of Stroke Volume and Heart Rate Flashcards
What effect does the sympathetic nervous system have on heart rate?
Increases heart rate causing tachycardia
What are the event occurring when the sympathetic system acts in heart rate?
Release of noradrenaline which act on B1 receptors on the the SA node
This increases the slope of pacemaker potential causing and increase in HR
What effect does the parasympathetic nervous system have on heart rate?
Decrease HR causing bradycardia
What events occur when the parasympathetic system acts on heart rate?
Vagus nerve releases Acetylcholine which acts on muscarinic receptors of the SA node
This hyperpolarises cells causing the decrease of the slope of the pacemaker potential decreasing the HR
Define Starlings Law
The energy of contraction is proportional to the initial length of the cardiac muscle fibre
What is Preload?
The volume of blood entering the heart from the inferior and superior Vena Cava
What is preload affected by?
End Diastolic Volume (EDV)
What does an increase in venous return cause to Stroke Volume?
Increase in venous return causes an increase in EDV which causes an increase in SV
What does an decrease in venous return cause to Stroke Volume?
Decrease in venous return causes a decrease in EDV which causes a decrease in SV
What is Afterload?
Load against which the muscle tries to contract
What effects the afterload?
Total Peripheral Resistance which is affected by how easily blood is able too pass through the arterioles. TPR sets arterial pressure
What effect does increasing the TPR on SV?
SV decreases as more energy is being used to open the aortic valves
What vessels effect the preload of the heart?
Capacitance Vessels
Veins and Venules
What vessels effect the afterload of the heart?
Resistance Vessels
Arterioles
What effect does the sympathetic system have on the SV produced by the heart?
Increases the contractility of the heart giving stronger and shorter contractions
What events occur causing the sympathetic system to have an effect on SV?
Sympathetic nerves release noradrenaline and adrenaline in the adrenal medulla act on B1 receptors on myocytes having an ionotropic effect.
This increases the contractility of the heart
What effect does the parasympathetic system have on SV of the heart?
Has little effect as the vagus nerve does not innervate the ventricular muscle
What effect does Hypercalcaemia have SV?
Increases the contractility
What effect does Hypocalcaemia have on SV
Decreases the contractility
What effect does ischaemia have on SV?
Decreases the contractility
What effect do barbiturates have on SV?
Decreases contractility
What is Cardiac Output?
HR x SV
Why does increasing the HR of an electronic pacemaker have a small increase in CO but decrease SV?
Shortens the rapid filling phase which occurs in the heart which reduces the EDV and therefore decreases SV
What is HR affected by?
Decreased by Vagal tone
Increased by Sympathetic tone
