Control Of Cardiac Output

Question 1

As a recap, describe cardiac output, including the two equations involving it.

Answer 1

Cardiac Output (CO) is the amount of blood ejected from the heart per minute.
It’s proportional to how often the heart beats per minute (heart rate, HR) and how much blood is ejected per beat (stroke volume, SV).

CO = HR x SV

BP = CO x TPR (total peripheral resistance)

Question 2

Define preload and afterload, including the laws governing them.

Answer 2

PRELOAD: the end diastolic volume that stretches the right or left ventricle of the heart to its greatest dimensions under variable physiologic demand. It is governed by Starling’s Law. Stretching of heart at rest - increases stroke volume

AFTERLOAD: the pressure against which the heart must work to eject blood during systole. In other words, it is the end load against which the heart contracts to eject blood. It is governed by Laplace’s Law. - opposes ejection, reduces stroke volume

Question 3

What does Starling’s Law state?

Answer 3

It states that the “energy of contraction of cardiac muscle is relative to the muscle fibre length at rest”.

This means that the greater the stretch of the ventricles in diastole (blood entering), the greater the energy of contraction and the greater the stroke volume achieved in systole.

This is not a control system, it is just a property of the heart muscle.

Question 4

Starling’s Law can be represented by a graph showing SV against CVP (central venous pressure).
Describe what the graph presents, and what the last part of the graph means.

Answer 4

Throughout the graph, as the CVP increases, the SV increases. However, at the end of the graph, with an increasing CVP, the SV starts falling. This indicates the point at which the Laplace’s Law takes over, so the heart pumps less blood out.

This emphasises the importance of care with fluid replacement, as giving too much may have an opposing effect.

Question 5

Why does Starling’s Law work (in the sense of contractile fibres)?

Answer 5

With an unstretched fibre, there is overlapping actin and myosin. This means that there is mechanical interference, so there is less cross-bridge formation available for contraction.

With a stretched fibre, there is less overlapping actin and myosin. This means that there is less mechanical interference, so there is potential for more cross-bridge formation. There is also an increased sensitivity to Ca2+ ions.

Question 6

List some roles of Starling’s Law in cardiac physiology.

Answer 6

• it balances outputs of the right and left ventricle (which is important = isovolumetric)
• it restores CO in response to intravenous fluid transfusions
• it is responsible for the fall in CO during a drop in blood volume or vasodilation (eg. haemorrhage, sepsis)
• it is also responsible for the fall in CO during orthostasis (standing), leading to postural hypotension and dizziness
• it contributes to increased SV and CO during upright exercise

Question 7

Define and describe the implications of Laplace’s Law.

Answer 7

Afterload opposes the ejection of blood from the heart and is determined by wall stress directed through the heart wall.

Stress through the heart wall prevents muscle contraction. More energy of contraction is needed to overcome this wall stress to produce cell shortening and blood ejection.

Laplace’s Law describes the parameters that determine afterload.

Question 8

What is the equation associated with wall stress?

Answer 8

This describes the relationship between, wall stress (S), pressure (P), radius (r), wall thickness (w) and wall tension (T).

P = 2T/r combined with T = Sw gives us:

S = Pr / 2w

Question 9

Why does the radius determine wall stress/afterload?

Answer 9

With a SMALLER ventricle RADIUS, there is greater wall curvature. This means that more wall stress is directed to the centre of the chamber. This, in turn, means that there is less afterload, so there is BETTER EJECTION.

With a LARGER ventricular RADIUS, there is less wall curvature. This means that more wall stress is directed to the heart wall. This, in turn, means that there is more afterload, so there is LESS EJECTION.

With Huge Theoretical Radius, there is negligible wall curvature meaning virtually all stress directed through the walls and no stress to the centre

Bigger the Radius - More stress in the walls

Question 10

What is Strength of contraction due to?

Answer 10

Due to Sympathetic nerves and circulating adrenaline

increases Intracellular calcium levels

Question 11

What does stretching of the heart mean?

Answer 11

Contract and eject harder

Question 12

What is Energy of contraction? what does it depend on?

Answer 12

Energy of contraction is the amount of work required to generate stroke volume

depends on:

• Starlings Law
• Contractility

Question 13

What are the two functions that Stroke volume carries out?

Answer 13

1. Contracts until … chamber pressure > aortic pressure
   (isovolumetric contraction )
2. Ejection from Ventricles

Question 14

During Preload greater stretch of ventricles in diastole means…

Answer 14

Greater energy of contraction

Greater stroke volume achieved in systole

Question 15

How does stretching increase energy of contraction ?

Answer 15

1. Actin and Myosin bands where contraction occur
2. Contraction is triggered by the binding of Ca2+
3. when muscle fibres are stretched there are more sites for Ca to bind to
4. therefore more spaces to form cross bridges

Question 16

Name the 3 importance’s of Laplace’s Law

Answer 16

1. Opposes Starling’s Law at rest
2. Facilitates ejection during contraction
3. Contributes to a failing heart at rest and during contraction

Question 17

How does Laplace’s law oppose Starling’s law at rest ?

Answer 17

• increased preload gives increases stretch of chamber
• increases chamber radius and increase afterload

in healthy heart - starlings law overcome Laplace and ejection occurs

Question 18

How does Laplace’s Law facilitate ejections during contraction

Answer 18

• contraction reduces chamber radius so less afterload in emptying chamber
• aids expulsion of last portion of blood and increase stroke volume

Question 19

How does Laplace’s Law contribute to a failing heart ?

Answer 19

• in failing heart the chambers often dilates and radius is large
• so increased afterload opposing ejection

Question 20

Laplace’s law is good at ejecting with what feature ?

Answer 20

Good ejection with small radius and bad with large radius

Question 21

What does Laplace’s law state of an increased blood pressure?

Answer 21

Increased blood pressure will increase wall stress
this will increase afterload
Reduce ejection

Question 22

What causes acute rises in blodd pressure?

Answer 22

1. Starlings Law - increased stretch gives increased contractions and increased SV
2. Local positive inotropes -nor adrenaline -increases CO
3. Baroreflex - decreased sympathetic tone decreased BP

Question 23

What are results of Chronic increase in arterial blood pressure ?

Answer 23

1. Increase Energy expenditure attempts - maintain SV but ultimately SV will decrease
2. Decrease in BP - decrease efficiency of heart
3. HIGH BP - reduce CO

Question 24

What does an increased radius do in hypertrophy heart failure according to Laplace’s law?

Answer 24

Heart failure where heart does not contract properly means blood will be left in ventricle leading to eventual volume overload

Question 25

What does an increased pressure do in hypertrophy heart failure according to Laplace’s law?

Answer 25

Pressure overload heart failure will occur due to increased afterlaod in chamber

Question 26

What will also increase due to an increase in radius or pressure

Answer 26

wall stress - which opposes ejection

Question 27

How does the heart compensate increase in wall stress?

Answer 27

with Ventircular Hypertorphy

1. increase wall thickness
2. decrease wall stress per sarcomere
3. decrease afterload
4. maintain SV and CO
5. require more energy - greater O2

Question 28

During exercise with an increased venous return what occurs?

Answer 28

Leads to increased preload 
more stretch 
shorter isovolumetric contraction phase 
increase SV due to Starling's Law 
more blood back to the heart and more blood ejected