CVS Control of the CVS Flashcards
Assuming cardiac output is constant, how does a change in the total peripheral resistance affect arterial and venous pressures?
If total peripheral resistance falls:
- arterial pressure will fall
- venous pressure will rise
If total peripheral resistance rises:
- arterial pressure will rise
- venous pressure will fall
Assuming total peripheral resistance is constant, how does a change in cardiac output affect arterial and venous pressures?
If cardiac output falls:
- arterial pressure will fall
- venous pressure will rise
If cardiac output rises:
- arterial pressure will rise
- venous pressure will fall
How is the cardiovascular system stable when cardiac output can be increased/decreased by changes in arterial and venous pressure?
The system is demand-led
The total peripheral resistance changes in response to metabolic demand, altering arterial/venous pressure and the cardiac output changes in response to this.
Define:
Stroke volume
End diastolic volume
End systolic volume
Stroke volume is the difference between end diastolic and end systolic volume
End diastolic volume is the volume of blood in the ventricle at the end of diastole
End systolic volume is the volume of blood in the ventricle at the end of systole
What factors determine how much the ventricles fill during diastole?
In diastole the ventricle is isolated from the arteries but connected to the veins therefore how much the ventricles fill depends on venous pressure
Higher venous pressure = more filling during diastole
This can be plotted as a ventricular compliance curve
Define: Pre-load & After-load (on the ventricles)
Describe how end diastolic volume affects after-load
Pre-load is the end diastolic stretch of the myocardium (determined by venous pressure)
After-load is the force necessary to expel blood into the arteries
The more the heart fills, the harder it contracts
The harder it contracts, the bigger the stroke volume
(Starlings law)
Define contractility
and describe the factors which change contractility
The gradient of the Starling curve is the contractility of the heart
Contractility is NOT force of contraction, it is the stroke volume you get for a given venous pressure
Sympathetic activity increases contractility, therefore making the heart more susceptible to venous pressure
What is the effect of increases/decreases in venous return (and therefore venous pressure) on cardiac output?
Increase in venous pressure - Cardiac output increases due to increased pre-load (and visa versa)
Increase in arterial pressure - Cardiac output decreases due to increased after-load (and visa versa)
What is postural hypotension?
On standing blood pools into the veins of the legs due to gravity
Central venous pressure falls so cardiac output falls
Arterial pressure falls (detected by baroreceptors)
HR increases and TPR increased to defend arterial pressure
If the baroreceptor reflexes don’t work this results in postural hypotension
How do baroreceptors detect changes in pressure?
Carotid sinus senses arterial pressure (thinner wall than the rest of the artery so more sensitive to pressure changes and bulges out)
This sends a signal to the medulla to control the heart
How does a fall in arterial pressure affect heart rate and contractility?
Heart rate is increased by reducing parasympathetic activity then increasing sympathetic activity
Contractility is increased by increasing sympathetic activity (increases efficiency of ventricular myocytes)
How does the cardiovascular system change in response to exercise?
Huge increase in demand - muscle contractions force extra blood back to the heart
The CVS has to change otherwise there would be a huge increase in venous pressure and huge decrease in arterial pressure.
Overfilling of ventricles prevented by rise in HR
Stroke volume kept down by cardiac output increased
