Regulation of Stroke Volume and Heart Rate Flashcards
How is heart rate regulated?
Neural
How is stroke volume regulated?
- Preload
- Afterload
- Neural
- Pathological
How does the sympathetic nervous system cause tachycardia?
- Sympathetic nerves release noradrenaline
- Plus circulating adrenaline from adrenal medulla
- Both act on B1 receptors on sinoatrial node
- Increases slope of the pacemaker potential
- Increases heart rate
How does the parasympathetic nervous system cause bradycardia?
- Vagus releases acetylcholine
- Acts on muscarinic receptors on the sinoatrial node
- Hyperpolarises cells and decreases slope of pacemaker potential
- Decreases heart rate
What does Starling’s law state?
The energy of contraction is proportional to the initial length of the cardiac muscle fibre
How does tension vary with preload?
- If the muscle is stretched above optimum there there will be a slight decrease in tension as less cross bridges formed
- If the muscle is not stretched then the actin filaments will start to interfere with each other and there will not be as many successful cross bridges formed so less tension
In vivo, what is preload affected by?
End diastolic volume
How does increased venous return lead to increased stroke volume?
Increased EDV
How does decreased venous return lead to decreased stroke volume?
Decreased EDV
How is self-regulation of stroke volume maintained?
Stroke volumes of the left and right ventricles are matched
What is afterload?
The load against which the muscle tries to contract
What is aortic pressure affected by?
- How much blood is pushed into the aorta (CO)
- How easy it is for that blood to get out of the aorta (TPR)
Why does SV decrease if TPR increases? Use aorta as an example
- TPR increases
- Aortic pressure increases
- The ventricle will have to work harder to push open the aortic valve
- It will have less energy left to do the useful bit of ejecting blood
- SV will decrease
- More energy is wasted building up pressure to open the aortic valve
What will happen to afterload if the arterioles are constricted?
Increase
What will happen to afterload if the arterioles are dilated?
Decrease
In vivo, what is afterload dependent on?
- Arterial pressure against which the blood is expelled
- This is dependent on TPR
What vessels affect preload?
Venules/veins= the capacitance vessels
What vessels affect afterload?
Arterioles= resistance vessels
How does the sympathetic nervous system increase strength of contraction?
- Sympathetic nerves release noradrenaline
- Plus circulating adrenaline from adrenal medulla
- Both act on B1 receptors on the myocytes
- Increases contractility
- Gives stronger but shorter contractions
What effect does the parasympathetic system have on strength of contraction?
- Little effect
- Probably because the vagus does not innervate the ventricular muscle
How does hypercalcaemia effect the SV against EDV curve?
Shifts curve up and left
How does hypocalcaemia effect the SV against EDV curve?
Shifts curve down and right
How does ischaemia effect the SV against EDV curve?
Shifts curve down and right
How does the heart compensate for reduced pumping ability?
- It works around a bigger EDV.
- This results in lower ejection fraction and reduced exercise capacity
How do barbiturates effect the SV against EDV curve?
Shifts down and right
What is CO equal to?
HR x SV
Why does increasing HR with an electronic pacemaker cause a small increase in CO but SV decreases?
- Shortened cardiac interval cuts into rapid filling phase
- Reduced end diastolic volume reduces preload
- Reduces stroke volume
Physiological increases in HR are accompanied by what to offset the effects of Starling’s Law?
- Heart rate increases
- Contractility increases
- Venous return increases
- TPR falls
- CO in
What 2 things contribute to an increase in HR?
- Decreased vagal tone
- Increased sympathetic tone
How does contractility increase and what is the result?
- Via increased sympathetic tone
- Alters inotropic state and shortens systole
What 3 factors contribute to an increase in venous return?
- Vasoconstriction
- Skeletal/ respiratory pump
- Maintenance of preload
What causes TPR tod fall and what is the result?
- Due to arteriolar dilatation in muscle, skin and heart
- Reduces afterload
