Control of Cardiac Output Flashcards
What happens to arterial and venous pressure if total peripheral resistance falls and cardiac output remains constant?
Arterial pressure will fall, Venous pressure will rise
Why will arterial pressure fall if total peripheral resistance falls?
Because the same volume into lower resistance allows pressure to fall
Why will venous pressure rise if total peripheral reistance falls?
Because there is more blood in the venous system than before, as we are not taking any more blood out of the system form the venous side, so in a snapshot of time, the rate that blood is going into the venous system increases
What happens to arterial and venous pressure if cardiac output rises and total peripheral resistance remains constant?
Arterial pressure will rise
Venous pressure will fall
Why does arterial pressure fall if cardiac output falls?
Not as much volume is pumped around
Why does venous pressure rise if cardiac output falls?
Because the blood stays in the venous system for longer, as less is pumped from the heart, and so the pressure rises
How are total peripheral resistance and the body’s need for blood related?
TPR is inversely proportional to need
What can cause a change in total peripheral resistance?
Changes in metabolism
Why does metabolism change total peripheral resistance?
It generates ‘signals’ in the form of changes in arterial and venous pressure. The pressure change is noticed by the body so it can change to respond
What is meant by demand led pumping?
If the body needs more blood, the heart needs to pump more to meet demand
How is demand expressed?
As changes in arterial and venous pressure
To what does the heart respond to by pumping more blood?
Falls in arterial pressure and rises in venous pressure
What is cardiac output the produce of?
Stroke volume
Heart rate
How is cardiac output calculated?
CO = SV x HR
What affects stroke volume and heart rate?
Arterial and venous pressures
How is stroke volume calculated?
It is the difference between end diastolic volume and end systolic volume
What is end diastolic volume?
The volume in ventricles after relaxed
What is the end systolic volume?
The volume in ventricles after contraction
Is end systolic volume minimum systolic volume?
Not under normal conditions
Is end diastolic volume maximum diastolic volume?
Not under normal conditions
Why are end systolic and diastolic volumes not at their maximum?
Otherwise the heart would have nowhere to go when responding to demands
What is the ventricle connected to during filling?
It is isolated from the arteries, and connected to the veins
When do the ventricles fill until?
Until the walls stretch enough to produce an intraventricular pressure equal to venous pressure
What does a higher venous pressure result in?
The heart fills more during diastole
What is the relationship between venous pressure and ventricular volume known as?
The ventricular compliance curve
What happens if ventricular muscle is stretched before contracting?
It contracts harder
What is the ventricle contracting harder when stretched known as?
Starlings Law of the Heart
What does Starlings Law of the Heart state?
The more the heart fills, the harder it contracts, up to a limit
What limits the filling of the heart?
The pericardial sac
What does a harder contraction lead to?
A bigger stroke volume
What do rises in venous pressure automatically lead to?
Rises in stroke volume
What does the Starling curve relate?
Stroke volume to venous pressure
What is the slope of Starlings curve known as?
The contractility of the ventricle
What is contractility saying?
That for a given amount of stretch, if efficiency is increased, you get a higher stroke volume
How is force of contraction different from contractility?
Force of contraction is one myocyte being pulled
Contractility is the efficacy of the whole heart
Force of contraction is variable within contractility
What is an increase in efficiency known as in cardiac terms?
Inotropy
What does positive inotropy do?
Increases the slope of the Starling curve, and therefore increases stroke volume
What does how much the ventricle empties depend on?
How hard it contracts
How hard it is to eject blood
What is the result of it being easier to eject blood?
More comes out in systole
What happens if atrial pressure falls?
End systolic volume will fall, so stroke volume will rise
What is force of contraction determined by?
End diastolic volume
Contractility
Why is the force of contraction determined by end diastolic volume?
Due to Starlings Law of the Heart
What increases contractility?
Sympathetic activity
What is difficulty of ejecting blood known as?
Aortic impedance
What does aortic impedance depend on?
Mainly, total peripheral resistance
What is the result of increased aortic impedance?
The pressure rises in the artries
What is preload?
The volume of blood in the ventricles at the end of diastole
What is preload related to?
End diastolic pressure
What is preload increased in?
Hypervolemia
Regurgitation of cardiac valves
Heart failure
What is afterload?
The resistance that the left ventricle must overcome to circulate blood
When is afterload increased?
Hypertension Vasoconstriction
What does an increased afterload lead to?
An increased cardiac workload
What does heart rate need to control it?
A sensory system that’s not intrinsic
What is autonomic outflow to the heart controlled by?
Signals from baroreceptors
What senses arterial pressure?
The carotid sinus
Why can the carotid sinus sense arterial pressure?
It has a thinner wall, and so can bulge out or contract depending on pressures in the artery
Where does the carotid sinus send signals to?
The medulla
What happens if arterial pressure falls?
Heart rate and contractility are increased
How is heart rate increased?
Reducing parasympathetic activity
Increasing symphathetic activity
How is contractility increased?
Increasing the sympathetic activity
What happens to diastole if there is an increased heart rate?
Diastole time shortens
What is the problem with diastole time shortening?
There is less time to fill the chambers, which is counterproductive in the long term
What is given to slow down heart rate?
Beta blockers
What is the purpose of beta blockers?
To slow down the heart rate so there is more time for filling of the ventricles
Where is a rise in venous pressure sensed?
Right atrium
What does a rise in venous pressure lead to?
Reduced parasympathetic activity, and so a rise in heart rate
What is the rise in venous pressure causing a rise in heart rate known as?
Bainbridge reflex
