Control of the CVS Flashcards
Describe the changes in total peripheral resistance, at a given cardiac output, on arterial and venous pressures
- TPR falls, arterial pressure will fall, and venous pressure will rise.
- TPR rises, arterial pressure will rise, and venous pressure will fall.
Describe the changes in cardiac output, at a given total peripheral resistance, on arterial and venous pressures
- CO falls, arterial pressure falls and venous pressure will rise.
- CO rises, arterial pressure will rise and venous pressure will fall.
Explain how the cardiovascular system will be stable if the cardiac output is increased by rises in venous pressure and falls in arterial pressure and vice versa
The system is demand led and stable, the TPR changes in response to metabolic demand, altering arterial and venous pressure and the CO changes in response to this.
Define the terms stroke volume, end diastolic volume and end systolic volume
End Diastolic Volume
The volume of blood in the ventricle at the end of diastole
End Systolic Volume
The volume of blood in the ventricle at the end of Systole
Stroke Volume
The difference between end diastolic/systolic volume
Describe the factor’s which determine how much the ventricles fill during diastole, and draw a graph of the relationship between end-diastolic volume in the left ventricle and venous pressure
In diastole, the ventricle is isolated from the arteries, and connected to the veins. Therefore the amount the ventricle fills depends on the venous pressure. The ventricle fills until the walls stretch until it has filled with enough blood to equal the venous pressure.
The higher the venous pressure, the more the heart fills in diastole. This relationship can be plotted on a graph known as the ventricular compliance curve.
Define the terms ‘pre-load’ and ‘after-load’ on the ventricular myocardium
Pre-Load
Pre-Load is the end diastolic stretch of the myocardium. It is determined by venous pressure (see above).
After-Load
After-load is the force necessary to expel blood into the arteries.
Draw a graph of the relationship between venous pressure and stroke volume at a constant after-load (The Starling Curve)
The more the heart fills due to increased venous pressure, the harder it contracts (at a constant after load). There is however a limit, when the heart becomes over-filled and the myocardium is overstretched (alignment of actin and myosin - crossover)
Define the term ‘contractility’ and describe, in principle, how the Starling curve is changed by factors which increase the contractility of the ventricular myocardium
The gradient is the contractility of the heart. Contractility is not the force of contraction of the heart, rather the stroke volume you get for a given venous pressure. Contractility is increased by sympathetic activity. Sympathetic activity therefore makes the heart more susceptible to venous pressure.
Autonomic outflow to the heart is controlled by signals from the baroreceptors, which are located in the arch of the aorta and the carotid sinus. The baroreceptors sense arterial pressure and send signals to the medulla, which controls the heart.
State in words the effect of increases and decreases in venous return (and therefore venous pressure) on cardiac output
If venous pressure rises, cardiac output rises (increased pre-load). If venous pressure falls, cardiac output falls (decreased pre-load).
If arterial pressure rises, cardiac output falls (increased afterload). If arterial pressure falls, cardiac output rises (decreased afterload).
Discuss the response of the cardiovascular system to eating a meal
o Increased activity of the gut leads to the release of metabolites and local vasodilation. The total peripheral resistance falls, causing the arterial pressure to fall and the venous pressure to rise.
The rise in venous pressure causes a rise in cardiac output.
The fall in arterial pressure triggers a rise in heart rate and cardiac output.
The extra pumping of the heart reduces venous pressure
The extra pumping of the heart raises arterial pressure
Demand met – System Stable
Discuss the response of the cardiovascular system to exercise
o Enormous increase in demand
o ‘Muscle pumping’ forces extra blood back to the heart
With no other changes, venous pressure would rise greatly, and arterial pressure would fall greatly. These changes may be too big to cope with. The large increase in venous pressure pushes starling’s curve onto the flat part
Overfilling of the ventricles is prevented by a rise in heart rate
When venous pressure starts to rise, heart rate is already high
Stroke volume kept down, but CO increased
Demand met – System Stable
Discuss the response of the cardiovascular system to Standing Up, and postural hypotension
On standing, blood pools in the superficial veins of the legs (Great/Short Saphenous veins) due to gravity
Central venous pressure falls
Cardiac output falls due to the fall in venous pressure (starling’s law)
Arterial pressure falls
Both arterial and venous pressure falling
Baroreceptors detect fall in arterial pressure
Raise HR, but venous pressure is still low
TPR increased to defend arterial pressure (skin, gut)
Discuss the response of the cardiovascular system to Haemorrhage
Reduced blood volume lowers venous pressure
So cardiac output falls (Starlings law)
Arterial pressure falls
Baroreceptors detect fall in arterial pressure, HR rises, TPR increased
Rise in HR lowers venous pressure further Problem is worse, not better
HR can become very high
Venous pressure needs to be increased to solve original problem
Veno-constriction
Blood transfusion to replace lost volume
Discuss the response of the cardiovascular system to a long term increase in blood volume
Blood volume is under the control of the kidney
If blood volume increases for days Increase in venous pressure
Cardiac output rises
Arterial pressure rises
More blood peruses tissues, which auto-regulate and increase TPR
Arterial pressure rises further, and stays up
