S2 Control of Cardiac Output Flashcards
What is afterload?
The load the heart must eject blood against e.g. the aortic pressure
What is the preload?
The amount the ventricles are stretched/filled in diastole (linked to EDV and central venous pressure (CVP))
What is the total peripheral resistance?
Resistance to blood flow from all the systemic vasculature
Also known as systemic vascular resistance
What happens to pressure of fluid in a tube as it encounters resistance?
the pressure exerted drops as it flows through resistance
Which blood vessels give the largest resistance? What does this increased resistance mean for capillaries, arterial side and the venous side?
Constriction of arterioles
Causes pressure decrease in capillaries and venous side and the pressure to increase in the arterial side - pressure builds up prior to resistance and less gets through past resistance
If total peripheral resistance decreases and cardiac output stays the same, what happens to the arterial and venous pressures?
- arterial pressure decreases - as cardiac output not increased
- venous pressure increases
What happens to arterial pressure and venous pressure if total peripheral resistance increases and cardiac output stays the same?
- arterial pressure increases
* venous pressure decreases - blood can’t flow to venous side as easily
If cardiac output is increased and total peripheral resistance stays the same, what happens to the arterial and venous pressures?
- arterial pressure increases - heart is pumping out more
* venous pressure decreases
If cardiac output decreases and total peripheral resistance stays the same, what happens to the arterial and venous pressures?
- arterial pressure decreases
* venous pressure increases (only a small increase)
How do you calculate cardiac output?
Stroke volume x heart rate
How do you calculate stroke volume?
End diastolic volume - end systolic volume (SV = EDV - ESV)
How can you increase the stroke volume?
By increasing EDV or decreasing ESV
What is the stroke volume of a average 70kg man at rest?
About 70ml
When does the ventricle stop filling? What are the consequences for how much the heart fills and how high the pressure is?
What is this relationship called?
When the walls stretch enough to produce a intraventricular pressure equal to the venous pressure
The higher the venous pressure, the more the heart fills
And the more the heart fills, the higher the ventricular pressure
Ventricular compliance curve
When can compliance be increased or decreased?
In diseased states
What is the Frank-Starling Law of the heart?
(Like skeletal muscle) if you stretch the fibres of the heart before contracting, the contraction produced will be harder
The heart is stretched by increasing the amount the heart is filled
An increased heart contraction means what for the stroke volume (increase of decrease)?
Increased stroke volume
How does how much the ventricles fill and compliance relate?
How much the ventricles fills depends on the compliance
What does increasing the venous return do to the left ventricular end diastolic/filling pressure? And ‘preload’ volume?
Increases the pressure
Increases the volume
What is the normal sarcomere length?
What is the optimum sarcomere length?
- 9 micrometers
2. 2 micrometers
What does a short sarcomere length mean for heart contraction?
If the sarcomere length is too shirt, filament overlap interferes with the contraction
What ion do you get increased sensitivity to when muscle fibres are stretched?
Calcium ions
What is an intrinsic control mechanism of the heart? Why is this a control mechanism?
Increasing stroke volume with increase filling of heart - it ensures both sides of heart maintain the same output
What are the extrinsic control mechanism of the heart? Why are these mechanisms?
Sympathetic stimulation and circulating hormones e.g. adrenaline
They increase contractility
What happens to the aortic/arterial pressure when peripheral resistance (TPR) is increased? What does this mean for the pumping?
What happens to the venous pressure in the same TPR situation?
Arterial/aortic pressure increases so harder to pump out blood due to reduced pressure gradient
Venous pressure decreases and so decrease in filling of the heart
What two factors determine how much the ventricles empty (ESV)? And what are they determined by?
- How hard it contracts - determined by EDV and contractility
- How hard it is to eject blood - determined by aortic impedance/aortic pressure
What is demand led pumping?
If metabolism increases, total peripheral resistance decreases so more blood can be supplied to tissues - decrease in arterial pressure, increase in venous pressure
The heart responds by pumping more
What are contractility and heart rate controlled by?
The autonomic nervous system
A decrease in blood pressure stimulates which part of the ANS?
The sympathetic nervous system
How does the CVS respond after eating a meal?
- Reduced total peripheral resistance
- Decreased arterial pressure, increased venous pressure
- Increased heart rate and stroke volume
- Increased CO
- Increased arterial pressure, decreased venous pressure
A negative feedback loop
How does the CVS respond to standing up?
- Reduced venous pressure
- Reduced CO
- Reduced arterial pressure
- Baroreceptor reflex and ANS increase heart rate and TPR
Why do we need a CVS response to standing up?
Standing up causes ‘pooling’ of blood in legs due to gravity, if your reflexes don’t work, you get postural hypotension - feel dizzy
What do baroreceptors detect? Where are they?
In the aortic arch
Detect changes in pressure
How does the CVS respond to exercise?
- Increased venous pressure, heart rate and contractility (decreased TPR)
- Increased CO
Why is there a fall in stroke volume as pressure increases on the Frank Starling curve?
Contraction has a limit
The sarcomere lengthens due to stretching and so actin and myosin filaments can no longer interact
