CVS S6 - Control of the CVS Flashcards
Describe the effect on the CVS if metabolic demand of the body changes and cardiac output remains the same
If metabolic demand falls:
- TPR rises
- Arterial pressure rises
- Venous pressure falls
If metabolic demand rises:
- TPR falls
- Arterial pressure falls
- Venous pressure rises
Describe the effect on pressure in the veins and arteries if cardiac output is changed and TPR remains the same
If CO rises:
- Arterial pressure will rise
- Venous pressure will fall
If CO falls:
- Arterial pressure will fall
- Venous pressure will rise
How does the heart supply enough blood to the body at all times if cardiac output can fall or rise?
System is demand led
TPR changes in response to metabolic demand altering arterial and venous pressures and hence cardiac output
Cardiac output is therefore dependent on metabolic demand
How do changes in arterial and venous pressures form ‘signals’ to the heart?
Heart responds to increased venous and decrease arterial pressure by increasing cardiac output
Heart responds to decreased venous and increased arterial pressure by lowering cardiac output
In this way, changes in pressure lead to the heart being ‘signalled’ to meet demand
What is cardiac output the product of?
Stroke volume and heart rate
What is end diastolic volume?
The volume of blood in the ventricle at the end of diastole
What is end systolic volume?
The volume of blood left in the ventricle at the end of systole
What is stroke volume?
The difference between end diastolic and end systolic volume?
Describe the connections to the ventricle during diastole
What does this mean for ventricular filling?
During diastole the ventricle is connected to the veins and isolated from the arteries
Therefore the ventricle will fill until the walls stretch to the point that ventricular pressure matches venous pressure
Describe the relationship between venous pressure and ventricular filling
Higher the venous pressure the more the heart fills in diastole
What is the ventricular compliance curve?
Can you draw it?
Curve showing how end diastolic volume (x) is related to venous pressure (y)
Drawn as a shallow curve showing a positive relationship
Define the terms:
Pre-load
After-load
Pre-load is the end diastolic stretch in the myocardium, determined by venous pressure
After-load is the force needed to expel blood in the ventricles into the arteries
What is end systolic volume dependent on?
How hard the heart contracts
How hard it is to eject blood
Describe how end diastolic volume and stroke volume are related
The more the heart fills (the greater the end diastolic volume) the harder the ventricle will contract (up to a limit) and the bigger the stroke volume
This is Starlings’ law of the heart and can be summarised simply as:
- More in = more out
Can you draw the Starling curve?
What does it show us?
X = Venous pressure Y = Stroke volume Gradient = Contractility
Curve is a steep rise followed by a short fall after the apex
This shows us that the more the heart fills, the harder it contracts (greater stroke volume)
There is however a limit where the heart becomes over-filled and the myocardium is overstretched, resulting in a fall in stroke volume after a certain venous pressure
Define the term ‘contractility’
How is it determined?
The intrinsic ability of the myocardium to contract (ionotropic state)
The gradient of the Starling curve shows contractility
Delta stroke volume / Delta venous pressure
What effect does the sympathetic nervous system have on contractility?
Sympathetic activity makes the heart more susceptible to venous pressure, raising the gradient of the Starling curve
What effect does increased arterial pressure have on contractility and contractile force?
Raises contractility and hence contractile force by increasing sympathetic stimulation to the heart (via baroreceptors)
What is aortic impedance?
What effect does this have on arterial pressure?
The difficulty of ejecting blood from the ventricle, this mainly depends on TPR (rise in TPR raises aortic impedance)
This means the harder it is to eject blood the higher the pressure will rise in the arteries
What is the effect of a drop/rise in arterial pressure on the stroke volume?
If arterial pressure drops:
- End systolic volume will fall as there is decreased after-load, therefore stroke volume has risen
If arterial pressure rises:
- End systolic volume will rise as there is increased after-load, therefore stroke volume has fallen
What is the effect of a drop/rise in venous pressure on the stroke volume?
Drops:
- End diastolic volume will fall and stroke volume will fall due to decreased pre-load
Rises:
- End diastolic volume will fall and stroke volume will rise due to increased pre-load
Describe how falls and rises in arterial pressure affect the signals sent by the ANS to the heart and the changes this leads to
Baroreceptors sense the arterial pressure
Rise leads to:
- Heart rate falls by decreasing sympathetic and increasing parasympathetic activity
- Contractility falls due to reduced sympathetic activity
Fall leads to:
- Heart rate rises by increasing sympathetic and decreasing parasympathetic activity
- Contractility rises due to increased sympathetic activty
What is the Bainbridge reflex?
If venous pressure as sensed by baroceptors in the right atrium rises this will lead to reduced parasympathetic activity and a rise in heart rate
Give a detailed list of effects of an increase or decrease in arterial pressure on the heart
Hint: This summarises all the smaller cards in this deck related to the effects of arterial pressure on the heart
Increase:
- Increased end systolic volume and after-load
- Increased contractility
- Decreased heart rate and stroke volume
- Decreased cardiac output
Decrease:
- Decreased end systolic volume and after-load
- Decreased contractility
- Increased heart rate and stroke volume
- Increased cardiac output
Give a detailed list of effects of an increase or decrease in venous pressure on the heart
Hint: This summarises all the smaller cards in this deck related to the effects of venous pressure on the heart
Increase:
- Increased venous return
- Increased end diastolic volume and pre-load
- Increased contractile force (up to a limit)
- Increased heart rate and stroke volume
- Increased cardiac output
Decrease:
- Decreased venous return
- Decreased end systolic volume and pre-load
- Decreased contractile force
- Decreased heart rate and stroke volume
- Decreased cardiac output
Describe in detail the response of the CVS to eating a meal
Increased gut activity leads to a fall in TPR and subsequent fall in arterial pressure and rise in venous pressure
Rise in venous pressure triggers rise in cardiac output
Fall in arterial pressure triggers baroreceptors which signal the brain to increase sympathetic stimulation to the heart, raising heart rate and contractility and hence cardiac output rises
Extra pumping of the heart reduces venous pressure and raises arterial pressure
Demand met, system stable
Describe what would happen if heart rate were to increase with no other change
What does this tell us?
Initially cardiac output would rise, total peripheral resistance would however remain the same
The rise in cardiac output decreases venous pressure so stroke volume will fall
The cardiac output is back to its original value
This tells us the heart is driven by the circulation, not the other way around
What is the major effect of movement during exercise on the CVS?
Muscle pumping, which forces extra blood back to the heart
If muscle pumping was the only change during exercise (no response from the CVS) what would be the effect?
Venous pressure would rise greatly
Arterial pressure would fall greatly
These changes may be too big to cope with
What is the main problem the CVS must deal with during exercise?
This leads to the risk of?
The great increase in venous pressure that will lead to overfilling of the heart and push the heart towards the peak of the Starling curve
At the top of the Starling curve the left ventricular output cannot be further increased if the right ventricular output increases
Therefore there is a risk of blood accumulating in the lungs and pulmonary oedema may result
How does the CVS respond during exercise?
Why is this necessary?
An increase in heart rate, driven by the brain at the start of exercise
This means when venous pressure starts to rise heart rate is already high and stroke volume is kept down so the left and right cardiac outputs can be matched
What happens in the CVS when we stand up?
Blood pools in the superficial veins of the legs (great/short saphenous veins) due to gravity
Central venous pressure falls
Cardiac output falls as a result
Arterial pressure falls
Baroreceptors detect fall in arterial pressure
Increased sympathetic activity raises heart rate to raise arterial pressure
Venous pressure still low so TPR increases (skin/gut) to both defend arterial pressure and lower venous pressure
What is postural hypotension?
Sometimes the baroreceptor reflex which raises heart rate when arterial pressure falls as we stand up doesn’t work
Arterial pressure remains low, this is postural hypotension
What is the response of the CVS to haemorrhage?
Reduced blood volume lowers venous pressure
Cardiac output falls
Arterial pressure falls
Baroreceptors detect fall in arterial pressure and increase in heart rate results (this can become very high)
TPR is increased
Rise in heart rate lowers venous pressure further
Problem is now worse, not better
What is needed to restore the CVS to normal function following haemorrhage?
Veno-constriction to raise the venous pressure
Blood transfusion to replace lost volume
What is the response of the CVS to a long term increase in total blood volume?
Blood volume controlled by kidney
Increase for days = an increase in venous pressure
Cardiac output rises
Arterial pressure rises
More blood is forced through tissues, which auto-regulates and raises TPR
Arterial pressure rises further and stays up
