Session 6 Flashcards
Describe the effects of changes in cardiac output, at a given total peripheral resistance, on arterial and venous pressures
If cardiac output falls, arterial pressure will fall and venous pressure will rise.
If cardiac output rises, arterial pressure will rise and venous pressure will fall.
Describe the effects of changes in total peripheral resistance, at a given cardiac output on arterial and venous pressure
If TPR falls, arterial pressure will fall and venous pressure will rise.
If TPR 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.
TPR changes in response to metabolic demand, altering arterial and venous pressure. CO changes in response to this.
Define the terms ‘stroke volume’, ‘end diastolic volume’ and ‘end systolic volume’
Stroke volume - the difference between end diastolic and systolic volume
End diastolic volume - volume of blood in the ventricles at the end of diastole
End systolic volume - volume of blood in the ventricles at the end of systole
Describe the factors 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
The amount the ventricle fills depends on venous pressure. The ventricle fills until the walls stretch and it has filled with enough blood to equal venous pressure.
Graph - the higher the venous pressure, the more the heart fills in diastole (ventricular compliance curve)
Define the terms ‘pre-load’ and ‘after-load’ on the ventricular myocardium
Pre-load - end diastolic stretch of the myocardium, determined by venous pressure
After-load - force necessary to expel blood into the arteries
Describe how changes in end diastolic volume affect the force of contraction of myocardium during the following systole
The more the heart fills, the harder it contracts (up to a limit). The harder it contracts the bigger the stroke volume.
Starling’s law of the heart - more in, more out.
Be able to draw a graph of the relationship between venous pressure and stroke volume at a constant after load (Starling Curve)
The more the heart fills due to increased venous pressure, the harder it contracts (greater stroke volume)
Explain why at the top of the Starling Curve stroke volume falls with increasing venous pressure
The heart is prevented from expanding too much by the fibrous pericardium. The heart starts to put pressure on the veins so stroke volume decreases as less blood enters the heart. Muscle fibres reach a critical length and are unable to contract efficiently.
Define the term ‘contractility’ and describe, in principle, how the Starling Curve is changed by factors which increase the contractibility of the ventricular myocardium
Contractibility - the gradient (the stroke volume you get for a given venous pressure)
Contractibility is increased by sympathetic activity.
Autonomic outflow is controlled by signals from baroreceptors (in the arch of aorta and carotid sinus) which sense arterial pressure.
Discuss the response of the cardiovascular system to eating a meal
Increased activity of the gut —> release of metabolites and local vasodilation. TPR falls, arterial pressure falls, venous pressure rises.
Rise in venous pressure –> rise in cardiac output
Fall in arterial pressure –> rise in heart rate and cardiac output
Extra pumping of the heart –> reduces venous pressure, raises arterial pressure
Discuss the response of the cardiovascular system to exercise
Enormous increase in demand, muscle pumping forces blood back to heart.
Overfilling of ventricles prevented by a rise in heart rate. Venous pressure rises but heart rate is already high. Stroke volume kept down but cardiac output increases.
(Risk of pulmonary oedema if left heart cannot respond to right heart)
Discuss the response of the cardiovascular system to standing up and postural hypotension
Standing up - blood pools in superficial veins of legs (great/short saphenous veins) due to gravity.
Central venous pressure falls –> cardiac output falls –> arterial pressure falls.
Baroreceptors detect fall in arterial pressure –> raise heart rate.
If the baroreceptor reflex does not work, postural hypotension occurs.
Discuss the response of the cardiovascular system to haemorrhage
Reduced blood flow lowers venous pressure –> cardiac output falls –> arterial pressure falls.
Baroreceptors detect fall in arterial pressure –> raise heart rate and TPR –> venous pressure falls.
Heart rate can become very high. Venous pressure increases by venoconstriction.
Discuss the response of the cardiovascular system to a long term increase in blood volume
Increase in blood volume for days –> venous pressure rises –> cardiac output rises –> arterial pressure rises.
More blood perfuses tissues, TPR rises.
Arterial pressure rises further and stays up.
