CVS: Preload, Afterload Flashcards
What controls stroke volume?
Preload: heart stretches during diastole to increase SV. Starling’s law
Contractility: Strength of contraction due to symp nerves + circulating adrenaline increasing [Ca2+]i
Afterload: Force opposing ejection, reduces SV - Laplaces law.
What is energy of contraction?
Energy of contraction is the amount of work needed to generate stroke volume
It depends on Starling’s Law and Contractility.
This stroke work energy increases chamber pa > aortic pa, so that the aortic valve opens. Once the aortic valve opens blood can be ejected.
What is Starlings Law/Preload?
The greater the stretch of the ventricle in diastole, the greater the energy of contraction
Greater SV thus achieved in systole.
This is an intrinsic property of cardiac muscle (nerves, hormones not involved)
Draw and explain starling’s curve
Describe the molecular basis of preload/starlings law
In an un-stretched fibre, there is overlapping actin/myosin. This means more mechanical inference - less cross-bridge formation available for contraction
In a stretched fibre, there is less overlapping actin/myosin, less mechanical inference. More cross-bridges can form. Increased sensitivity to Ca2+ ions, so more contraction can happen as a result.
What is afterload/laplace’s law?
Afterload opposes ejection. It is determined by Wall Stress (force through the heart wall)
To overcome wall stress and produce ejection, you need more contraction
Laplaces law: s (Wall stress) = P x r / 2w. Pressure (P), Radius (r), wall thickness (w)
So increased s is produced by increasing Pressure and Radius
What is the effect of ventricle radius on afterload?
What is the effect of pressure on afterload?
Laplaces law states that increased arterial BP increases Afterload/Wall Stress – Reduced ejection, more energy needed to maintain SV
Ultimately decreased SV/CO - poor blood flow to end organs
Chronic high blood pressure is bad for the heart!
Use Laplace’s law to explain how changes in radius can cause hypertrophy in heart failure
Increasing radius causes volume-overload heart failure
e.g. MI causes poor SV and so blood volume remains in heart.
The heart thus compensates; increased wall thickness leads to hypertrophy to help maintain ejection
But, this needs more energy and 02 demand (more sarcomeres used). Greater O2 use ultimately decreases contractility - heart failure
Use Laplace’s law to explain how changes in pressure can cause hypertrophy in heart failure
Hypertension causes afterload which heart must work against
Remember : S = P x r / 2w
Increased r or P will increase Wall Stress (afterload), opposing ejection
The heart thus compensates: increased Wall Thickness (w) leads to hypertrophy. Same Wall Stress but now over greater area (more sarcomeres) helps maintain ejection/SV
But, this needs more energy and 02 demand. Greater O2 use ultimately decreases contractility - heart failure
How does Laplaces law oppose Starling’s law at rest?
Preload means more blood returning to the heart, causing a bigger chamber and chamber radius
Laplaces law states that increased radius will increase Afterload, opposing ejection
In healthy heart, Starling’s Law overcomes Laplaces law to maintain good ejection
How does laplaces law facilitate ejection during contraction?
Ventricular contraction decreases the chamber radius. Laplace’s law states this will reduce Afterload/ in ‘emptying’ chamber
Aids ejection during reduced ventricular ejection phase of cardiac cycle- less and less wall stress enables max ejection
What is the effect of preload on left ventricular pressure-volume loop?
Increased preload e.g. during venoconstriction or administration of IV fluids increases end-diastolic volume, Starling’s law, SV
Decreased preload eg loss of blood volume in haemorrhage, dehydration decreases EDV, Starling’s law and SV
What is the effect of afterload on left ventricular pressure-volume loop?
Increased Afterload e.g. chronic hypertension increases isovolumetric contraction.
This is to overcome greater aortic pa and open aortic valves for ejection. Less energy left for ejection, reducing SV
Decrease in Preload e.g. reduced BP, high dynamic exercise (running) reduces isovolumetric contraction. It is thus easier to open aortic valve, more energy for ejection, increased SV
