Changing cardiac output Flashcards
What is preload?
Amount that ventricles are filled and stretched in diastole
What are the factors of preload?
Central venous pressure (CVP)
LV-EDV
Left ventricular end diastolic pressure (LV-EDP)
Ventricular compliance
What is afterload?
The load that the LV must eject blood against
What is afterload aprroximately equivalent to?
Aortic pressure
What effect does resistance have on blood pressure?
Causes blood pressure to decrease
How does blood pressure change in the arteries? Why?
Increases and decreases
due to systole and diastole
How does blood pressure change in the arterioles? Why?
Decrease in blood pressure
due to arterioles offering resistance
What is the relative blood pressure in capillaries, venules, veins etc.? Why?
Low blood pressure
because come after arterioles, after drop in blood pressure
How does blood pressure change in capillaries, venules, veins etc.?
Slowly decreases
If cardiac output remains the same, and TPR drops, what happens to arterial pressure? Why?
Pressure in arteries decreases
more blood flow through arterioles, less blood in arteries
If cardiac output remains the same, and TPR drops, what happens to venous pressure? Why?
Pressure in veins increases
more blood flow through arterioles, more blood entering veins
If cardiac output remains the same and TPR increases, what happens to arterial pressure? Why?
Pressure in arteries increases
less blood flow through arterioles, backs up in arteries
If cardiac output remains the same and TPR increases, what happens to venous pressure? Why?
Pressure in veins decreases
less blood flow through arterioles, less blood in veins
If TPR remains the same and cardiac output increases, what happens to arterial pressure? Why?
Arterial pressure increases
heart pumps more blood into arteries
If TPR remains the same and cardiac output increases, what happens to venous pressure? Why?
Venous pressure decreases
blood removed from veins by heart and pumped into arteries
If TPR remains the same and cardiac output decreases, what happens to arterial pressure? Why?
Arterial pressure decreases
less blood entering arteries
If TPR remains the same and cardiac output decreases, what happens to venous pressure? Why?
Venous pressure increases
blood backs up in veins
How does the heart prevent changes in arterial and venous blood pressure due to changes in TPR?
Changes its cardiac output
What is the equation for cardiac output?
Cardiac output = heart rate x stroke volume
What are the factors of cardiac output?
Heart rate
Stroke volume
Afterload
What is stroke volume?
The volume of blood ejected from the LV per heartbeat
What is the equation for stroke volume?
Stroke volume = EDV - ESV
What is the average stroke volume?
What percentage is this of the average EDV?
70ml
67%
What are the factors of stroke volume?
Preload
Contractility
What are the types of mechanisms by which the heart changes its cardiac output? What do they each mean?
Intrinsic mechanisms - within the heart itself
Extrinsic mechanisms - outside of the heart
What is the intrinsic mechanism of changing the heart’s cardiac output?
Frank-starling mechanism
What is the Frank-Starling mechanism?
Increased preload
increased force of contraction
increased stroke volume
up to a point
How is preload increased?
Increase CVP
bigger difference in pressure between veins and LV, more blood flow from veins to LV
increased LV-EDV, LV-EDP
Increase ventricular compliance
How are CVP and LV-EDP related to each other? Why?
They are approximately equal
because then there is no pressure difference between the veins and LV, so no blood flow from veins to LV
What is ventricular compliance?
The ability of the ventricle to distend to hold a higher volume of blood
without significant increase in pressure
What are the axes of the ventricular compliance curve? And their units?
x axis = LV blood volume (ml)
y axis = LV pressure (mmHg)
What is normal LV pressure with LV blood volume of 120ml?
8mmHg
What is decreased ventricular compliance?
Higher pressure with same blood volume
What is an example of a cause of decreased ventricular compliance?
LV hypertrophy
How does decreased ventricular compliance affect the gradient of the ventricular compliance curve?
Steeper gradient
What is an example of a cause of increased ventricular compliance?
Dilation of LV
What is increased ventricular compliance?
Higher blood volume with same pressure
How does increased ventricular compliance affect the gradient of the ventricular compliance curve?
Flatter gradient
What are the axes of the Frank-Starling curve?
x axis = CVP/LV-EDV/LV-EDP
y axis = stroke volume
Why does the Frank-starling mechanism work?
Sarcomeres in cardiac muscle are stretched
decreased actin-myosin overlap
increased sensitivity to calcium ions
increased force of contraction
What are the axes of the length tension curve for cardiac muscle?
x axis is sarcomere length
y axis is force of contraction
What is the extrinsic mechanism of changing the heart’s cardiac output?
Autonomic nervous system affecting
- heart rate
- contractility
What is heart rate?
Number of heart beats per minute
What is contractility?
Force of contraction for a given fibre length
How can contractility be increased?
Sympathetic nervous system innervating the LV is activated more
How does an increase in contractility affect the cardiac output?
Increased force of contraction
increases stroke volume
increased cardiac output
How does an increase in contractility affect the Frank-Starling curve?
Larger stroke volume for same CVP/LV-EDV/LV-EDP
Steeper gradient
How does a decrease in contracility affect the Frank-Starling curve?
Smaller stroke volume for same CVP/LV-EDV/LV-EDP
Flatter gradient
How does the Frank-starling mechanism affect the cardiac output of the right side of the heart compared to the left side?
Ensures their cardiac outputs are equal
e.g. if one side of the heart has a higher cardiac output, then the other side of the heart has a higher preload, has a higher stroke volume, giving a higher cardiac output
How does standing up affect blood in the legs? Why?
Causes pooling of blood in the legs
due to effect of gravity
How does blood pooling in the leg affect arterial and venous pressure?
Both decrease
as blood collects in the veins of the leg
Which mechanism is responsible for changing the heart’s cardiac output when both arterial and venous pressure decrease? Why?
Extrinsic mechanism
Not intrinsic because venous pressure has decreased, smaller pressure difference between veins and LV, less blood flow from veins to LV, decreased preload