CVS 6 Control of the CVS Flashcards
What happens when Total Peripheral Resistance falls at a given cardiac output?
Arterial pressure falls
Venous pressure rises
What happens when Total Peripheral Resistance rises at a given cardiac output?
Arterial pressure rises
Venous pressure falls
If cardiac output falls at a given TPR what happens?
arterial pressure falls
venous pressure rises
If cardiac output rises at a given TPR what happens?
arterial pressure rises
venous pressure falls
How is the system kept stable with changes in venous and arterial pressures?
The system is demand led
TPR changes in response to metabolic demand
This alters venous and arterial pressures
These determine the CO
What are the end diastolic and end systolic volumes?
The volume of blood in the ventricles at the end of diastole/systole
What is the stroke volume?
The difference between end systolic and end diastolic volumes
What determines how much the ventricles fill during diastole?
The venous pressure
The ventricles fill until the walls stretch exerting a pressure that equals the venous pressure
What is the relationship between end diastolic volume and venous pressure?
The higher the venous pressure, the more the heart fills in diastole. The graph plotted to show this is called the ventricular compliance curve
What is pre-load?
The end-diastolic stretch of the myocardium determined by venous pressure
What is after-load?
The force necessary to expel blood into the arteries
What is Starling’s law of the heart?
The more the heart fills, the harder it contracts and so the greater the stroke volume.
MORE IN -> MORE OUT
Explain the shape of the starling curve
(It is an n shape. x axis-venous pressure y axis- stroke vol)
The more the heart fills due to increased venous pressure, the harder it contracts giving bigger sroke vol up to a limit when heart becomes overfilled and myocardium is overstretched
What is the contractility of the heart?
This is the gradient of the starling curve.
It is not the force of contraction of the heart, it is the stroke volume you get for a given venous pressure.
What nervous activity increases contactility?
Sympathetic.
This means sympathetic activity makes the heart more susceptible to venous pressure
What is autonomic outflow to the heart controlled by?
Signals from baroreceptors located in the arch of the aorta and carotid sinus which sense arterial pressure and send signals to the medulla
How do changes in venous pressure affect cardiac output?
Decrease in venous pressure- decreased CO as decreased preload
Increase in venous pressure- increased CO as increased preload
How do changes in arterial pressure affect cardiac output?
Increased arterial pressure- decreased CO as increased afterload
Decreased arterial pressure- increased CO as decreased afterload
What is the response of the CVS to eating a meal?
Increased activity of gut leads to release of metabolites and local vasodilation
TPR falls causing arterial pressure to fall, venous pressure to rise
The rise in venous pressure causes rise in CO
The fall in arterial pressure triggers rise in heart rate and CO
Extra pumping of heart reduces venous pressure and raises arterial pressure
Demand met- system stable
What is the response of the CVS to exercise?
Enormous increase in demand
‘Muscle pumping’ forces extra blood back to heart
Overfilling of ventricles prevented by a rise in heart rate
When venous pressure starts to rise, heart rate already high
Stroke volume kept down but CO increased
Demand met- system stable
What would happen in exercise if the heart rate didnt rise?
Venous pressure would rise greatly and arterial pressure would fall greatly.
The large venous inc pushes starlings curve onto flat part and ventricles overfilled
Risk of pulmonary oedema as left heart may not be able to match the output of the right heart causing blood accumulation in the lungs
What is the response of the CVS to standing up?
On standing, blood pools in the superficial veins of the legs (great/short saphenous veins) due to gravity
Central venous pressure falls
CO falls due to fall in venous pressure
Arterial pressure falls
Both arterial and venous pressure falling
Baroreceptors detect fall in arterial pressure
Raise HR but venous pressure still low
TPR increased to defend arterial pressure (skin, gut)
What happens when the Baroreceptor reflex doesn’t work when standing up?
postural hypotension
What is the response of the CVS to haemorrhage?
Reduced blood vol lowers venous pressure
So CO falls
Arterial pressure falls
Baroreceptors detect fall in arterial pressure, HR rises, TPR increased
Rise in HR lowers venous pressure further making problem worse
HR can become very high
Venous pressure must be increased to solve
Venoconstriction
Blood transfusion to replace lost volume
What is the response of the CVS to a long term increase in blood volume?
Blood volume is under control of kidney
If blood volume increases for days there is an increase in venous pressure
CO rises
Arterial pressure rises
More blood perfuses tissues which auto-regulate and increase TPR
Arterial pressure rises further and stays up
