Control Of Cardiac Output Flashcards
What is afterload?
The load the heart must eject blood against (roughly equivalent
to aortic pressure)
If pressure in aorta is high, heart must work harder to eject eh blood out.
What is the definition of preload?
Amount the ventricles are stretched (filled) in diastole.
It is related to the end diastolic volume or central venous pressure.
What is the Total peripheral resistance?
It is sometimes referred to as systemic vascular
resistance – resistance to blood flow offered by all the systemic vasculature.
Resistance can be increased or decreased
The majority of the resistance occurs at the arterioles
What happens to pressure of fluid in a tube as it encounters resistance
The pressure that the blood exerts drops downstream as
it flows through ‘a resistance’.
The pressure will increase before the resistance and decrease after the resistance.
Since the arterioles feed blood into the capillaries,venues and veins, this causes a decrease in pressure in those three vessels, while serving the pressure in the arterioles (before the resistance) is greater.
The arterioles offer the greatest resistance
Constriction of the arterioles increases the resistance.
This will cause pressure in the
capillaries and on the venous side to fall but will cause pressure on the arterial side to rise.
Why does the majority of the resistance occur at the arterioles?
They have very thick muscular walls
What are the effects of changing total peripheral resistance (TPR) when the cardiac output is unchanged?
If TPR falls and CO is unchanged
– Arterial pressure will fall (as the dilation must increase if TPR falls)
– Venous pressure will increase
If TPR increases and CO is
unchanged
– Arterial pressure will increase
– Venous pressure will fall
What are the effects of changing cardiac output?
If CO increases and TPR is
unchanged
– Arterial pressure will increase (heart pumping out a lot)
– Venous pressure will fall
If CO decreases and TPR is
unchanged
– Arterial pressure will fall
– Venous pressure will rise (heart is not pumping out as much so venous pressure increases)
WHy and how does the heart have to change in demand for blood?
The heart must meet changes in demand for blood
If the tissues need more blood the arterioles and precapillary sphincters will dilate
What happens if the tissues need more blood?
The arterioles and precapillary sphincters will dilate
Therefore peripheral resistance falls
The heart needs to pump more so that arterial pressure does not fall and
venous pressure doesn’t rise
The heart ‘sees’ changes in this demand as changes in arterial blood
pressure (aBP) and central venous pressure (CVP)
The heart responds to changes in CVP and aBP by INTRINSIC and
EXTRINSIC mechanisms
This is controlled by autonomic nervous system
What is the cardiac output?
Cardiac Output = Stroke Volume x Heart Rate
The volume of blood pumped out of the heart per min (5l for 70kg man)
What is the stroke volume?
Stroke Volume = end diastolic volume – end
systolic volume
The amount of blood that is pumped out of the heart per beat (70ml for 70kg man)
This is about 67% of normal EDV (ventricles do not empty out completely)
Can increase SV by increasing EDV or decreasing ESV
What happens during ventricular filling?
As the ventricles fill, the ventricular pressure increases.
In a normal ventricle it is 120ml volume (systole) at approx 10mm Hg (diastole)
As the ventricle fills it gets harder to fill more so the roes sure goes up steeply not linearly
The ventricle fills until the walls stretch
enough to produces an intraventricular
pressure equal to the venous pressure
The higher the venous pressure the more the heart fills
Relationship is the Ventricular Compliance Curve
Compliance can be increased or decreased in diseased states. What is the consequence of that?
In decreased compliance, eg caused by LV hypertrophy, it makes it harder to fill the ventricles and so the pressure is higher.
In increased compliance, it is easier to fill but the pressure is lower.
Explain the Frank – Starling Law of The Heart
Like skeletal muscle – if you stretch the fibres of the heart before contracting, it will contract harder.
The more the heart fills, the harder it contracts (up to a limit)
The harder the heart contracts, the bigger the stroke volume
An increase in venous pressure will fill the heart more
– How much the ventricles fill depends on the compliance
What is the relationship between the left ventricular end-diastolic pressure and the stroke volume?
Increasing venous return leads to increased left ventricular end-diastolic pressure (LVEDP) and volume (‘increased preload’).
This causes an increase in stroke volume, so that the extra blood is pumped out of the ventricle.
The ‘normal’ operating point at rest is with an LVEDP around 8-10mm Hg and stroke volume of ~70 ml
Volume that is ejected depends on how much the heart is stretched.
What happens if the sarcomere length is too short?
If sarcomere length is too short filament overlap interferes with contraction
In cardiac muscle also get an increase in calcium sensitivity as the muscle fibres are stretched so the graph is steeper than skeletal muscle. Not sure why though.
Why is starlings law of the heart important?
It ensures both sides are balanced. These two sides are in series with each other.
The increased stroke volume with
increased filling of the heart is and
INTRINSIC control mechanism
It ensures that both sides of the heart
pump maintain the same output
The pulmonary and systemic circulations operate in series
– The same volume of blood pumped to the body must also be pumped to the lungs
What is contractility?
The force of contraction for a given fibre length
How do you change the slope of the Starling curve?
Change the contractility
An increase in contractility will increase the force of contraction for a given left EDP
EXTRINSIC factors such as sympathetic stimulation and circulating adrenaline can increase contractility and thus affect the slope
Reducing sympathetic stimulation will
reduce contractility and thus affect the slope
What is the effect of increasing arterial pressure on stroke volume?
Afterload is the pressure that the heart has to pump against
This is the pressure in the aorta (aortic impedance)
The aorta is impeding blood flow
Arterial (aortic) pressure is increased when the peripheral resistance is
increased
– This makes it harder for the heart to pump out
Increased TPR also reduces venous pressure and therefore reduces filling of
the heart
Over time you can get an inappropriate increase in arterial pressure. The
heart will have to work harder (see hypertension lecture)
What are the factors that determine cardiac output?
How much the ventricle empties (end systolic volume) depends on
- How hard it contracts
determined by the end diastolic volume (how much the heart fills) and contractility (increased by sympathetic drive) - How hard it is to eject blood
determined by aortic impedance (roughly arterial pressure - if pressure is too high, harder to get blood out)
Cardiac Output = Stroke Volume x Heart Rate
Contractility and heart rate are controlled by the autonomic nervous system
A decrease in arterial BP will reduce parasympathetic NS activity and
stimulate sympathetic NS increase heart rate and increase contractility
What will happen to the TPR, arterial pressure and venous pressure if the metabolism of the body increases?
• If the metabolism of the body increases, then TPR will fall to supply
more blood
• This will result in a fall in arterial
pressure and an increase in venous
pressure
• The heart will respond by pumping
more
How does the CVS respond after you eat a meal?
Local vasodilation in the gut
Decrease in TPR
Thus decreased in arterial and increased venous pressure which leads to an increase in stroke volume and cardiac output
The decrease in arterial pressure leads to an increase in heart rate. This increase in heart rate combined with the the increase in stroke volume leads to an increase in cardiac output.
This later leads to an increase in arterial pressure and a decrease in venous pressure to put the system back into the a stable pressure.
How does the CVS respond to standing up?
Standing up causes ‘pooling’ of blood in legs due to effect of gravity on a column of liquid
The venous pressure decreases, the cardiac output decreases and the arterial pressure decreases.
Now both arterial and venous pressure have changed in the same direction
This is not very good as the arteriole pressure needs to be high in order to perfuse vital organs
Cannot adjust by intrinsic mechanisms
Baroreceptor reflex and autonomic nervous system increase heart rate AND increase TPR
If reflexes don’t work you get postural hypotension
