2. Control Of Cardiac Output Flashcards
What is afterload?
The load the heart must eject blood against
What is afterload roughly equivalent to?
Aortic pressure
What is preload?
The amount the ventricles are stretched (filled) in diastole.
Related to EDV and CVP.
What is stroke volume?
The difference between EDV and ESV.
What is cardiac output the product of?
HR x SV
What is end diastolic volume determined by?
The filling of the LV.
The ventricles fill in diastole as the venous pressure drives blood into them.
Fills until intraventricular pressure matches venous pressure.
The higher the venous pressure, the more the ventricle will fill in diastole.
What is stroke volume determined by?
How much the ventricle contracts during systole.
Mechanical and chemical factors will influence this.
Which 2 mechanical factors influence stroke volume?
Preload - determined by venous pressure
Afterload - pressure in arteries which heart has to overcome
Increased venous pressure will lead to increased stroke volume. True or false?
True
Why does a decrease in TPR result in increased stroke volume?
Reduction of afterload (pressure in arteries), so it’s easier for the heart to expel the blood into the aorta.
If TPR is high, what is the effect on stroke volume?
Decrease stroke volume because it is much harder to eject the blood.
Lower stroke volume but higher pressure will be generated.
Which 2 mechanical mechanisms will increase stroke volume?
Increasing venous pressure or decreasing total peripheral resistance.
What factors may affect the contractility of the heart and the force of contraction?
Neurotransmitters
Hormones
Drugs
How does increasing sympathetic activity increase stroke volume?
Noradrenaline and adrenaline have a positive inotropic effect, so will increase contractility.
Rises in ________ pressure increase ______________ and therefore increase ___________.
venous
Stroke volume
Cardiac output
Falls in arterial pressure _________ stroke volume therefore __________ cardiac output.
Increase
Increase
If TPR falls, what will happen to arterial and venous pressure?
Arterial pressure will decrease
Venous pressure will rise
How does the heart response to a decrease in arterial pressure?
Increases heart rate and stroke volume (SNS) and therefore cardiac output.
This will increase arterial pressure and reduce venous pressure to restore status quo.
What is total peripheral resistance?
Resistance to blood flow offered by all the systemic vasculature
Which blood vessels offer the greatest resistance?
Arterioles
What happens to the pressure of blood as it flows through resistance?
Pressure decreases
How do arterioles increase resistance?
By contracting
How does contraction of an arteriole affect the pressure of the arterial and capillaries/venous side?
Pressure in the capillary/venous side will fall
Pressure in the arterial side will rise.
What will happen to the arterial and venous pressure if TPR falls and CO is unchanged?
Decrease arterial pressure
Increased venous pressure
What will happen if TPR increases and CO is unchanged?
Increased arterial pressure
Decreased venous pressure
What will happen if CO increases and TPR is unchanged?
Increased arterial pressure
Decreased venous pressure
What will happen if CO decreases and TPR is unchanged?
Decreased arterial pressure
Increased venous pressure
How does the heart increase blood flow to a particular area?
Precapillary sphincters will dilate, decreasing peripheral resistance.
How does the heart ‘see’ changes in demand?
Arterial blood pressure
Central venous pressure
It response to changes in these by intrinsic and extrinsic mechanisms.
How can you calculate CO?
CO = SV x HR
How can you calculate SV?
SV = EDV - ESV
Increase SV by increase in EDV or decreasing ESV
What is the typical stroke volume?
70 ml
67% of normal EDV
What relationship does the ventricular compliance curve illustrate?
The higher the venous pressure, the more the heart fills.
What is the Frank-Starling law of the heart?
If you stretch the fibres of the heart before contracting, it will contract harder.
(Increasing preload increases CO)
How does compliance effect the extent of ventricle filling?
Rescued compliance will reduce filling.
Increased compliance will increase filling.
What ensures that both sides of the heart remain balanced?
Starlings law
Increased SV with increased filling is an INTRINSIC control mechanism.
Ensures same output in both sides.
Why is it important that the heart is balanced ?
The same volume of blood that is pumped to the body must also be pumped to the lungs.
What extrinsic factors alter contractility?
Sympathetic stimulation- NA and adrenaline
How does increasing arteriolar resistance decrease CO?
Increased arterial resistance which will increase aortic pressure (after load)
Also decreases venous pressure which reduces filling of the heart.
Whwr are the 2 main factors which determine how hard the ventricle contracts?
- EDV
CONTRACTILITY
How does the heart respond to a decrease in arterial BP?
Reduce parasympathetic NS activity and stimulate sympathetic NS to imcrease HR and contractility.
