Control of Cardiac Output -- 6.1 Flashcards
At rest, what are the relative venous and arterial pressures?
Venous pressure = lower
Arterial pressure = higher
What happens when TPR falls?
arterial pressure will fall
venous pressure will rise
What happens when TPR rises?
arterial pressure will rise
venous pressure will fall
What happens when cardiac output rises?
arterial pressure will rise
venous pressure will fall
What happens when cardiac output falls?
arterial pressure will fall
venous pressure will rise
What is demand-led pumping?
When the body needs more blood, it causes the heart to beat more in order to facilitate the need
What is stroke volume?
The difference between the end diastolic volume and the end systolic volume
– the amount of blood ejected by the heart
Describe ventricular filling
In diastole, the ventricles are connected to the veins so blood can enter.
The ventricle fills until the walls stretch enough to produce an intra-ventricular pressure equal to venous pressure, due to the stretchiness of the walls.
What does the degree of ventricular filling depend on?
Venous Pressure (higher venous pressure, the more the heart fills in diastole)
Stretchiness of the walls
Intraventricular pressure
What is the ventricular compliance curve?
The relationship between venous pressure and ventricular volume
What does Starlings Law of the Heart state?
If the myocardium is stretched before contracting it contracts harder
- the more the heart fills up, the more it contracts
- the harder the heart contracts the bigger the stroke volume
Increased venous pressure causes an increase in the stroke volume
What is “pre-load”?
End diastolic stretch
What is “after-load”?
The force necessary to expel blood into the arteries
Define contractility
The slope of the curve showing the relationship between venous pressure and stroke volume
What is the force on contraction determined by?
End diastolic volume
Contractility
What can have an effect on contractility?
Sympathetic activity
Neurotransmitters
Hormones
Drugs which act on the myocardium
What affect does increasing the venous pressure have on the stroke volume?
Increases stroke volume
– more blood is entering the ventricles
What happens when it is easy to eject blood from the heart?
The volume output increases and there will be a small pressure increase in the arteries
What happens when it is difficult to eject blood from the heart?
The volume output decreases and the arterial pressure will increase
What will a fall in the TPR do to the stroke volume?
It will increase the stroke volume
How is the heart rate controlled?
Autonomic outflow is determined by signals from baroreceptors
Where are baroreceptors situated in the heart and what do they do?
Carotid sinus at the bifurcation of the common carotid artery and in the walls of the aorta.
They detect changes in the arterial pressure
What section of the brain controls the heart?
Medulla oblongata
What is the effect on sympathetic and parasympathetic activity when the arterial pressure falls?
Increases heart rate by: -- reducing parasympathetic activity -- increasing sympathetic activity Increases contractility by: -- increasing sympathetic activity
What does a rise in venous pressure do?
CARDIAC OUTPUT RISES
- increase stroke volume (Starlings Law)
- increase heart rate
What does a fall in arterial pressure do?
CARDIAC OUTPUT RISES
- increase stroke volume (sympathetic activity and direct effect)
- increase heart rate
What do baroreceptors ensure?
If arterial pressure falls, heart rate and stroke volume will both rise
How does the ANS affect cardiac output?
By changing heart rate
By changing contractility
What is the mean filling pressure?
The mean pressure in the arteries and veins when the heart stops
What happens to the arterial and venous pressure when the heart stops?
Arterial pressure falls
Venous pressure rises
Why at the top end of a starling curve, does the stroke volume fall with increasing pressure?
The elastic fibres in teh myocardium have been stretched past a certain point where they do not return to their original shape
What factors/illnesses increase the central venous pressure?
hypervolaemia, heart failure, tension pneumothorax, cardiac tamponade
What factors/illnesses decrease the central venous pressure?
hypovolaemia, shock
