Regulation of Stroke Volume and Heart Rate Flashcards
What is the effect of sympathetic nervous system on heart rate?
Increases HR - tachycardia
What is the effect of parasympathetic nervous system on heart rate?
Decreases HR - bradycardia
How does the sympathetic nervous system increase HR?
Nerves release noradrenaline plus circulating adrenaline from adrenal medulla
Acts on B1 receptors on sinoatrial node and increases slop of pacemaker potential and this decreases Cardiac interval
How is there an increase in slope of pacemaker potential?
Increase ion flow through funny Na+ channels and Ca2+ channels
How does parasympathetic nervous system decrease HR?
Vagus nerve releases acetylcholine and acts on muscarinic receptors on sinoatrial node
This hyperpolarises cells and decreases slope of pacemaker potential so decrease HR as increase of cardiac interval
What does Starlings Law state?
The energy of contraction is proportional to initial length of cardiac muscle fibre
What is preload?
Initial stretching of cardiac myocytes prior to contraction
What is peak tension on starling curve?
Optimal no. of cross-bridges forming from thin actin and thick myosin filaments
Means really strong muscle contraction
What is preload affected by?
End Diastolic Volume - volume of blood filled into ventricle before contraction
What is the effect of increased venous return?
Increase in EDV and therefore stroke volume
Ventricles are more stretched so bigger EDV and stronger contraction so bigger stroke volume
What is the effect of decreased venous return?
Decreased EDV and therefore decreased stroke volume
This is because ventricles are not really stretched and there is weak contraction
What is afterload?
Load against which the muscle tries to contract
What is stopping the left ventricle shortening?
Aortic valve being closed so enough pressure in LV so it can exceed aorta pressure and then the aortic valve will open
Also mitral valve
What is aortic pressure affected by?
Total Peripheral Resistance
Explain total peripheral resistance
How easy it is for blood to get out through the arterioles
What happens when TPR increases?
Aortic pressure will increase and the ventricle will have to work harder to push open the aortic valve so less energy left to do the useful bit of ejecting blood - stroke volume decreases
How does the sympathetic nervous system affect stroke volume?
Nerves release noradrenaline plus circulating adrenaline from adrenal medulla
Both act on B1 receptors on myocytes which increased contractibility - gives stronger but shorter contraction so stroke volume increased
What is the ionotropic effect that causes an increase in contractibility?
More release of Ca2+ within cardiomyocytes causing more cross-bridges to form and stronger contraction as Ca2+ taken up quicker
What is the effect of parasympathetic nervous system on stroke volume?
Little effect on contractibility as vagus nerve does not innervate the ventricular muscle
What vessels affect preload?
Venules and veins as capacitance vessels
What vessels affect afterload?
Arterioles as resistance vessels
What causes an increase diastolic volume?
If more blood is squeezed out of venules and veins
What does cardiac output determine?
How much blood and 02 to tissues
What is cardiac output equal to?
Heart rate x stroke volume
What effect does increasing HR have on cardiac output?
Causes small increase in cardiac output but stroke volume starts decreasing
How does increasing HR cause increase of CO and decrease of SV?
Shortened cardiac interval cuts into rapid filling phase and the reduced EDV reduces preload so cardiac muscle stretches less so SV will fall
What are serval things that offset physiological increases in HR - exercise?
HR increases
Contractibility increases
Venous return increases
Total peripheral resistance falls
Causes CO increase x 4-6 times
What causes increase of HR in control of CO?
Decreased vagal tone and increased sympathetic tone
What causes increase of contractibility in control of CO?
Increased sympathetic tone and alters ionotropic state and shortens cycle
What causes increase in venous return in control of CO?
Via vasoconstriction and skeletal/ resp. pumps which maintains preload
What causes decrease in total peripheral resistance in control of CO?
Due to arteriolar dilation in muscle, skin and heart which reduces afterload - making heart easier to pump
