Regulation Of Cardiac Function Flashcards
What is EDV?
End diastolic volume
It is the amount on blood in the ventricles at the end of diastole, just before contraction
A larger volume will produce a greater SV
How do you calculate CO ?
HR * SV
What is ESV ?
End systolic volume
It is the volume of blood left in the ventricles after contraction
A low ESV will produce a greater SV
How do you calculate ejection fraction ?
SV/EDV
At rest only half of the blood is pumped out of the heart
What does EDV depend on ?
The venous return- a larger venous return causes an increase in EDV
Filling time - longer in diastole the more filling time and greater EDV
What is preload ?
It’s the same as EDV
What does ESV depend on ?
Afterload - pressure in the aorta which the ventricle is pumping against - a large pressure in the aorta requires a greater pumping force so a greater pressure will increase ESV
Strength of ventricular contraction - the greater the contraction the smaller the ESV
What happens to cross bridge formation when cardiac muscle stretches ?
It increases the number of cross bridges because resting sarcomeres length is closer to 1.8 micrometers so there is more overlap causing greater force of contraction
What is the frank starling law of the heart ?
as the EDV increases the ventricles stretch more which increases the SV therefore the greater the degree of stretch the greater the force of contraction
It is an intrinsic mechanism for increasing heart
What happens if the right side of the heart of failing to pump blood properly ?
Causes peripheral edema
What happens if the left side of the heart is failing to pump properly?
It causes a build up of blood and fluid at the lungs causing ingestive heart failure causing breathlessness
Why do people with hypertension have to make their hearts work harder ?
Due to hypertension there is a increased after load which means the heart has to overcome and larger pressure and if it doesn’t then ESV is increased which reduces CO and SV
Explain sympathetic stimulation of the heart
Sympathetic post ganglionic fibres release noradrenaline onto the heart and this binds to beta adrenoreceptors causing an increased heart rate and force of contraction leading to increased CO
This is an extrinsic mechanism
What happens when noradrenaline binds to beta-1 receptors on the heart ?
Associated with Gs protein Activated G protein activates adenylate Cyclase This increases cAMP This increases protein kinase A Protein kinase A phosphorylates targets
What are the target proteins which protein kinase A phosphorylates upon activation of beta-1 receptors by noradrenaline ?
1) L-type calcium channels - increase influx of calcium ions
2) RyRs - makes these channels more sensitive to calcium so it increases calcium release from SR.
3) phospholambam - causes faster uptake of calcium into SR so there is more calcium ready to release again
4) troponin-I - decreases its affinity for calcium ions allowing calcium ions to be released more quickly so more calcium ions bind in a shorter period of time
Where do the post ganglionic sympathetic fibres emerge from ?
Cervical and superior thoracic ganglia
Where does the sympathetic system originate from ?
Medulla
Courses via the cardio accelerators centre
Where do the parasympathetic fibres originate from and where do they synapse ?
Medulla
Course via the cardio inhibitory centre
Synapse in cardiac plexus from vagus nerve
What are the effects of sympathetic activity on the SAN ?
Causes an increase in cAMP
This increases the sodium influx
Also causes a reduction in potassium permeability by shutting off delayed potassium channels
This makes the max diastolic pressure more positive so it is easier to reach threshold
What effect does the sympathetic system have on the ventricles ?
It shorten systole so that more systole can occur per unit time
Also increases force of contraction of the myocytes
What happens to the heart when parasympathetic stimulation is increased ?
Post ganglionic fibres release acetylcholine which acts on M2 receptors
Decreases funny current so decreases sodium influx
This inhibits adenylate Cyclase
Reducing cAMP
Pacemaker potential is slowed
Also increases potassium permeability causing hyperpolarisation of max diastolic potential
How is blood supplied to the heart ?
By coronary circulation
When does most of the coronary blood flow occur and how much as a percentage ?
During diastole
70%
This is because during systole he coronary vessels are compressed
Why is the 30% of coronary blood flow during systole so important ?
Because during exercise the amount of time spent in systole is longer due to faster heart rate
What happens to the coronary blood vessels during exercise ?
They dilate to increase blood flow to the heart
Requires more oxygen and nutrients
How are the coronary vessels dilated during exercise?
The increased metabolic activity causes an increase in adenosine which causes vasodilation of the coronary vessels causing increased blood flow
Where do preganglionic parasympathetic neurons synapse to postganglionic neurons ?
Cardiac plexus
Where do preganglionic sympathetic neurons synapse with post ganglionic neurons ?
Sympathetic chain
