Control of cardiac output and responses of whole system Flashcards
What will happen to arterial and venous pressure if total peripheral resistance falls, but cardiac output says the same?
Arterial pressure will fall
Venous pressure will rise
Both pressures change in most circumstances
If the body needs less blood, (at a constant cardiac output) what happens to total peripheral resistance, arterial pressure and venous pressure?
Total peripheral resistance will rise
Arterial pressure will rise
Venous pressure will fall
If cardiac output rises and total peripheral resistance stays the same, what will happen to arterial and venous pressures?
Arterial pressure will rise
Venous pressure will fall
At a constant total peripheral resistance, if cardiac output falls, what will happen to arterial and venous pressures?
Arterial pressure will fall
Venous pressure will rise (due to less blood being pumped by the heart and less blood being removed from veins)
What happens during changes in demand for blood in the systemic system?
- Total peripheral resistance is inversely proportional to the body’s need for blood
- If metabolism changes, total peripheral resistance will change
- And generate ‘signals’ in the form of changes in arterial and venous pressure
Give an example of demand-led pumping.
- If we eat a meal the gut needs more blood
- Local vasodilators dilate arterioles (less resistance in blood flow to gut)
- Total peripheral resistance falls (caused by an increase in blood flow to gut)
- If cardiac output does not change, arterial pressure will fall and venous pressure will rise
How is demand-led pumping driven?
Driven by circulation
Heart responds to signals of arterial and venous
pressure
What is demand-led pumping?
• If the body needs more blood, the heart needs to pump more to meet the ‘demand’
• Demand is expressed as changes in arterial and venous pressure
• If the heart responds to falls in arterial pressure and rises in venous pressure by pumping more blood, then it will meet the demand and bring arterial and venous pressures
back to normal
Which pressures affect cardiac output (stroke volume and heart rate)?
Arterial and venous pressures affect both.
How is stroke volume controlled?
• Stroke volume is the difference between
– end diastolic volume
– end systolic volume
• To increase stroke volume- increase diastolic volume or decrease end systolic volume (fills more in diastole or beats harder in systole).
• End diastolic volume is normally less than the maximum diastolic volume, and end systolic volume is normally larger that the maximum systolic volume (hence why they can be increased/decreased)
How does diastole work?
- In diastole the ventricle is isolated from the arteries, and connected to the veins
- The ventricle fills until the walls stretch enough (passively) to produce an intra-ventricular pressure equal to venous pressure
- Pressure in veins determines atrial pressure, which determines how much the heart fills
- The higher the venous pressure, the more the ventricle fills
- The more the ventricle fills, the higher the pressure in the ventricle
What is the relationship between venous pressure and ventricular volume known as?
Ventricular compliance curve
What is end diastolic volume?
The extent to which the ventricle is filled at the end of diastole.
What is the relationship between end diastolic volume and force of contraction? What is this relationship known as?
The higher the end diastolic volume, the greater the force of contraction (ejecting more blood), due to the stretch and elastic properties of the ventricular muscle (same as other muscle).
This is known as Starling’s law of the heart.
What is Starling’s law?
- An intrinsic property of the heart, and the principal means by which CVS is controlled
- The more the heart fills the harder it contracts (up to a limit)
- The harder it contracts the bigger the stroke volume
- i.e. Rises in venous pressure automatically lead to rises in stroke volume
What is the Starling curve?
- Relates stroke volume to venous pressure
- The slope is known as the contractility of the ventricle
- There is a simple (almost linear) relationship between venous pressure and stroke volume, however after a point, contractility of the heart is limited.
What is contractility?
Contractility is not the force of contraction! It is the nature of the relationship between force of contraction and venous pressure (slope- not point on curve)
What limits contractility?
Heart contained by pericardium- after a certain point, it won’t fill any more, and past that point, it causes it to fill less.
What does an increase in contractility mean?
A rise in venous pressure will lead to a greater rise in stroke volume.
What is the end systolic volume, and what does it depend on?
• How much the ventricle empties
• Depends on:
– how hard it contracts (determined by end diastolic volume - starling’s law)
– how hard it is to eject blood
What factors affect force of contraction?
• Determined by
– end diastolic volume (Starlings Law)
– contractility (steeper relationship, more blood will come out at a given venous pressure)
• Contractility increased by sympathetic activity
(increased NA- acts directly on cardiac myocytes, the steeper the relationship between venous pressure and stroke volume)
What is aortic impedence? What does it depend on?
Stretch of arteries.
Depends on total peripheral resistance (indirectly, TPR can affect end systolic volume (relatively small effect))
The harder it is to eject blood the higher the pressure rises in the arteries
What is the relationship between arterial pressure and end systolic volume?
- The easier it is to eject blood, the more comes out in systole
- So, if arterial pressure falls, end systolic volume will fall and stroke volume will rise
Describe the direct effects of arterial and venous pressures on stroke volume.
- If venous pressure rises, stroke volume will rise
* If arterial pressure falls, stroke volume will rise
How is heart rate controlled?
- Autonomic outflow to the heart is controlled by signals from baroreceptors (stretch receptors) in the carotid sinus, which sense arterial pressure
- Send nervous signals to medulla to neurones that detect pressure changes, which controls the autonomic input to the heart
- Works as a reflex system- not an intrinsic property of the heart
What is the carotid sinus?
A swelling in artery between common and internal carotid arteries in which baroreceptors are located.
What happens during a fall in arterial pressure?
• Heart rate increased by
– reducing parasympathetic activity- vagal outflow (parasympathetic tone at rest)
– increasing sympathetic activity
• Contractility increased by
– increasing sympathetic activity (amplifies changes in venous pressure)
• NB. If heart rate is increased beyond a certain point, diastole is shortened, the less it’s filled and the less it’s able to empty, so CO falls (above about 150bpm).
How does a rise in venous pressure contribute to increasing cardiac output?
- Rise in venous pressure sensed in right atrium
- Leads to reduced parasympathetic activity
- Leads to a rise in heart rate
- This is called the ‘Bainbridge reflex’
- Not very important (only a small contribution)
What are the ‘rules’ of the cardiovascular system?
• Total peripheral resistance is inversely proportional to the
need for blood
• At a constant cardiac output – falls in TPR increase venous pressure and falls in TPR decrease arterial pressure
• At a constant TPR – increases in CO decrease venous
pressure and increases in CO increase arterial pressure
• Increases in venous pressure increase cardiac output
• Decreases in arterial pressure increase cardiac output
• Falls in arterial pressure increase flow resistance to certain tissues (skin, gut)
• Falls in arterial pressure lead to venoconstriction
How does the cardiovascular system respond when eating a meal?
- Increased activity of the gut leads to local vasodilatation
- Total peripheral resistance falls
- Venous pressure rises, causing a rise in cardiac output
- Arterial pressure falls, causing a rise in heart rate (and therefore cardiac output)
- Venous pressure reduced by extra pumping of heart
- Arterial pressure also raised
- Demand met – system stable
How does the cardiovascular system respond to a change in heart rate alone?
- If heart rate increases with no other change
- Initially cardiac output will tend to rise, but total peripheral resistance the same
- Rise in cardiac output reduces venous pressure
- So stroke volume falls and cardiac output back to original value
- The heart cannot drive the circulation, it is driven by it
How does the cardiovascular system respond to exercise?
- In exercise there is an enormous increase in demand and ‘muscle pumping’ which 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 great increase in venous pressure is the main problem as it tends to overfill the heart, pushing the ventricles onto the flat part of the Starling curve.
- So there is a risk of pulmonary oedema, because the outputs of the right and left ventricles cannot be matched
- Overfilling of the ventricles is prevented by a rise in heart rate, which occurs as exercise begins
- This is driven by the brain
- So when the venous pressure starts to rise, heart rate is already high, keeping stroke volume down
How is the output of the right and left ventricles matched?
- Both sides beat at the same rate, so can only match by matching stroke volume
- This relies on a Starling curve
- If the right heart pumps more, the left fills more, and so pumps more
- If on the top of the Starling curve, however, the left heart cannot respond to the right and blood accumulates in the lungs
How does the cardiovascular system respond to standing up?
- On standing, blood ‘pools’ in the superficial veins of the legs because of gravity, so central venous pressure falls
- By Starlings law, cardiac output falls, so arterial pressure falls
- Now both arterial and venous pressure are changing in the same direction
- This cannot be corrected by normal mechanisms
- Baroreceptors detect fall in arterial pressure and raise heart rate, but venous pressure still low
- TPR increased in the skin and the gut to defend arterial pressure
- Sometimes reflexes don’t work- ‘postural hypotension’
How does the cardiovascular system respond to haemorrhage?
• Reduced blood volume lowers venous pressure
• So cardiac output falls - Starlings law
• Arterial pressure falls
• Baroreceptors detect fall in arterial pressure
• Heart rate rises
• Total peripheral resistance increased
• Rise in heart rate lowers venous pressure further, making problem worse, not better
• Heart rate can become very high
• Rise in total peripheral resistance helps
arterial pressure, but lowers venous pressure, so does not
solve original problem
• Need to increase venous pressure by venoconstriction and auto-transfusion
• Need to eventually replace blood volume lost
What is autotransfusion?
The collection of blood from an active bleeding site and reinfusion of that blood into the same patient for the maintenance of blood volume.
When might the cardiovascular system find it hard to cope?
If arterial and venous pressure change in the same direction
How does the cardiovascular system respond to an increase in blood volume?
- The kidney controls blood volume
- If blood volume increases for days, venous pressure increases
- Cardiac output increases, so arterial pressure rises, forcing more blood through tissues
- This autoregulates and increases total peripheral resistance
- So arterial pressure rises further, and stays up
How is blood volume related to blood pressure?
- In the long term blood volume control mechanisms control mean blood pressure
- Very important in hypertension
- Hypertension often treated with diuretics
