Regulation of stroke volume & heart rate Flashcards
How does the sympathetic nervous system regulate heart rate?
- Sympathetic nerves release noradrenaline
- plus circulating adrenaline from adrenal medulla
- Both act on B1-receptors on the surface of sinoatrial node
- Increases heart rate = tachycardia
What is Tachycardia?
- Heart rate that exceeds the normal resting rate.
- In general, a resting heart rate over 100 beats per minute is accepted as tachycardia in adults.
How does the parasympathetic nervous system regulate heart rate?
- Vagus releases ACh
- acts on muscarinic receptors in sinoatrial node
- Hyperpolarises cells
- Decreases heart rate = bradycardia
What is Bradycardia?
It is a slower than normal heart rate
What is normal stroke volume?
60-120 ml/beat
What 3 things affect stroke volume?
- Contractility
- Preload
- Afterload
What does Starling’s law state?
The energy of contraction is proportional to the initial length of the cardiac muscle fibre.
What is Preload?
Preload is the end diastolic volume that stretches the right or left ventricle of the heart to its greatest dimensions under variable physiologic demand.
- It is how much volume is in the ventricles at the end of diastole : EDV.
What volume is preload affected by, in vivo?
End diastolic volume
What are factors determining cardiac Preload? (to increase it)
- Increased EDV
- Increased atrial contractility
- Increased aortic pressure
- Increased atrial contractility
- Decreased heart rate
- Increased central venous pressure
Which valve closes after diastole?
The mitral valve
What is Afterload?
It is the pressure required to open the aortic valve.
- It is the load against which the muscle tries to contract
What is total (systemic) peripheral resistance?
The total resistance to flow of blood in the systemic circuit
What occurs to Aortic pressure, if total peripheral resistance increases?
If TPR increases, aortic pressure will increase, the ventricle will have to work harder to push open the aortic valve..
- Stroke volume will decrease.
What is required for the aortic valve to open?
The pressure in the ventricle needs to be greater than the pressure distal to the aortic valve.
In vivo, how is afterload set?
- Afterload is set by the arterial pressure against which the blood is expelled - this in turn depends on the Total Peripheral Resistance.
Which vessels affect preload?
Capacitance vessels
e.g. venules/veins
Which vessels affect afterload?
Resistance vessels
e.g. Arterioles
What is contractility?
The inherent vigor of contraction of the heart muscles during systole.
How does the sympathetic system affect the regulation of stroke volume?
- Sympathetic nerves releasing noradrenaline
- plus circulating adrenaline from adrenal medulla
- Both act on ß1-receptors on the myocytes
- Increases contractility (an inotropic effect)
gives stronger, but shorter contraction
Does the parasympathetic nervous system affect regulation of stroke volume?
- Little effect
- Probably because the vagus does not innervate the ventricular muscle.
How does Hypercalcemia affect the stroke volume-EDV curve?
Shifts curve up and left
How does Hypocalcemia affect the stroke volume-EDV curve?
Shifts curve down and right
How does Ischaemia affect the stroke volume-EDV curve?
Shifts curve down and right
Ischaemia - loss of blood supply to the cardiac muscle.
How will the heart compensate for a reduced pumping ability?
By working around a bigger EDV.
- Results in lower ejection fraction, and reduced exercise capacity.
How do Barbiturates affect the stroke volume-EDV curve?
Shifts curve down and right
- A barbiturate is a drug that acts as a central nervous system depressant.
Equation to calculate cardiac output
Cardiac output = Heart rate x Stroke volume
Increasing heart rate with an electronic pacemaker, causes a small increase in CO, but then SV(stroke volume) decreases, Why?
- Shortened cardiac interval cuts into the rapid filling phase.
- Reduced end diastolic volume reduces preload
- So by Sterling’s law, reduces stroke volume.
Mechanisms that cause heart rate to decrease for control of cardiac output
- via decreased vagal tone
- & increased sympathetic tone
How does contractility increase for control of cardiac output?
- via increased sympathetic tone
- alters inotropic state & shortens systole
How does venous return increase for control of cardiac output?
- Via venoconstriction (via a1 receptors)
- & skeletal/respiratory pumps
- maintains preload
How does total peripheral resistance fall (TPR) to control cardiac output?
- Due to arteriolar dilation in muscle, skin & heart (via b2 receptors)
- reduces afterload
How much does Cardiac output increase during exercise?
4-6 times
