Control of cardiac output

Question 1

Describe cardiac output

Answer 1

The amount of blood ejected from the heart per minute

Question 2

Cardiac output equation

Answer 2

Cardiac output (CO) = Heart rate (HR) x Stroke volume (SV)

HR: how often the heart beats per minute
SV: how much blood (ml) is ejected per beat

Question 3

Blood flow (CO) equation

Answer 3

Blood flow (CO)= BP / TPR

BP: blood pressure
TPR: total peripheral resistance

Question 4

Blood pressure equation

Answer 4

BP: CO x TPR

Question 5

Cardiac output at rest

Answer 5

70 bpm x 70 ml = approx 5 litres/min

Question 6

Cardiac output during exercise

Answer 6

180 bpm x 120 ml = 22 litres/min

Question 7

Define Preload and the Law governing it

Answer 7

STRETCHING OF LEFT VENTRICLE ON FILLING:

• The end diastolic volume that stretches the right or left ventricle of the heart to its greatest dimensions under variable physiologic demands
• Stretching at rest
• Increases Stroke Volume
• Governed by Starling’s law

Question 8

Define Afterload and the Law governing it

Answer 8

RESISTANCE TO EJECTION

• The pressure against which the heart must work to eject blood during systole
• The end load against which the heart contracts to eject blood
• Governed by Laplace’s law

Question 9

What controls heart rate?

Answer 9

• SA node pacemaker

- Sympathetic and Parasympathetic nerves

Question 10

What is contractility?

Answer 10

Strength of contraction due to sympathetic nerves and circulating adrenaline increasing intracellular calcium

Question 11

What is meant by energy of contraction?

Answer 11

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

Question 12

What two factors does energy of contraction depends on?

Answer 12

1. Starling’s law

2. Contractility

Question 13

What two functions does stoke work carry out?

Answer 13

1. Contracts until chamber pressure is greater than aortic pressure (isovolumetric contraction)
2. Ejection from ventricle

Question 14

Relationship between preload and stroke volume

Answer 14

Preload increases the stroke volume

Question 15

Relationships between afterload and stroke volume

Answer 15

Afterload opposes the stroke volume

Question 16

What does starling’s law state?

Answer 16

• “energy of contraction of cardiac muscle is relative to the length of the muscle fibre 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 (blood leaving)

Question 17

Intrinsic property of cardiac muscle

Answer 17

More blood in = more blood out

Question 18

Describe the graph that represents Starling’s law (Stroke Volume against Central Venous Pressure)

Answer 18

• Throughout the graph, the CVP increases, the SV increases
• At the end of the graph, with an increasing CVP, the SV starts falling
• Indicates the point at which Laplace’s law takes over, so the heart pumps less blood out

Question 19

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

Answer 19

• With an unstretched fibre, there is overlapping actin and myosin, not much room for Ca2+
• 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
• 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 and more space for Ca2+ to interact
• Therefore contracts harder when stretched beforehand

Question 20

What are some of the roles and effects of Starling’s law in cardiac physiology?

Answer 20

• Balances outputs of the right and left ventricle
• Responsible for fall in Cardiac output during a drop in blood volume/vasodilation (e.g. sepsis)
• Restores Cardiac output in response to intravenous fluid transfusions
• Responsible for fall in Cardiac output during orthostasis (standing for a long time): leading to postural hypotension and dizziness as blood pools in legs
• Contributes to increased stroke volume and cardiac output during upright exercise

Question 21

Define Laplace’s law

Answer 21

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

Question 22

What are some of the implications of Laplace’s law?

Answer 22

• Stress through the wall of the heart prevents muscle contraction
• More energy of contraction is needed to overcome wall stress: producing cell shortening and ejection

Question 23

Wall tension equation (in terms of pressure and radius)

Answer 23

T = P x r / 2

T= Wall tension 
P= Pressure 
r= radius in a chamber (ventricle)

Question 24

Why is the wall tension calculated by diving (P x r) by two?

Answer 24

Divided by two because the chamber has two directions of curvature

Question 25

Wall tension equation (in terms of stress and thickness)

Answer 25

T= S x w ``` T= wall tension S= wall stress w= wall thickness ```

Question 26

Wall stress equation

Answer 26

S= (P x r)/2w ``` S= wall stress P= pressure r= radius w= wall thickness ```

Question 27

How is afterload increased?

Answer 27

Increasing pressure and radius

Question 28

How is afterload reduced?

Answer 28

Increasing wall thickness

Question 29

how does a small radius effect wall stress/afterload?

Answer 29

- Small ventricle radius - Greater wall curvature - More wall stress is directed towards centre of chamber - Less afterload - Better ejection

Question 30

how does a big radius effect wall stress/afterload?

Answer 30

- Larger ventricle radius - Less wall curvature - More wall stress directed through heart wall - More afterload - Less ejection

Question 31

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

Answer 31

Increased preload gives an increased stretch of the chamber | This increased chamber radius decreases curvature: therefore increased afterload

Question 32

How does Laplace's law facilitate ejection during contraction?

Answer 32

Contraction reduces chamber radius so less afterload in 'emptying' chamber. This helps expulsion and increases stroke volume

Question 33

How does Laplace's law contribute to a failing heart at rest and during contraction?

Answer 33

In a failing heart the chambers are often dilated: so increased afterload opposing ejection

Question 34

When does Laplace's law result in good ejection?

Answer 34

with a small radius

Question 35

When does Laplace's law result in bad ejection?

Answer 35

with a large radius

Question 36

Relationship between Laplace's law and afterload

Answer 36

Laplace's law states that increased blood pressure (P) will increase wall stress. This will increase afterload and reduce ejection.

Question 37

What does increased radius result in (in terms of Laplace's law )

Answer 37

Heart failure, where the heart does not contract properly (MI, cardiomyopathies, mitral valve re-gurgitation). Blood left in the ventricle leads to eventual volume overload

Question 38

What does increased pressure result in (in terms of Laplace's law)

Answer 38

Pressure-overload heart failure due to increased pressure/afterload in chamber (hypertension, aortic stenosis)

Question 39

Relationship between Laplace's law and hypertrophy in heart failure

Answer 39

- Increase in radius or pressure will increase wall stress (afterload) which opposes ejection - Heart compensates with ventricular hypertrophy (greater myocyte size and more sarcomeres) increasing wall thickness. This decreases wall stress per sarcomere and therefore afterload (SV and CO is maintained) - This requires more energy (more sarcomeres used): greater O2. The amount of energy required increases and so contractility will decrease and produce more heart failure

Question 40

Describe relationship between Starling's law and ventricular pressure-volume loop

Answer 40

- During exercise, increased venous return leads to an increase in EDV: increased preload and more stretch - This causes shorter isovolumetric contraction phase, and increase in SV due to Starling's law - More blood back to the heart and more blood ejected from the heart

Question 41

Describe the relationship between Laplace's law and ventricular pressure-volume loop

Answer 41

- High blood pressure leads to increased afterload - Longer time spent in isovolumetric contraction to increase the pressure in the chamber above that in the aorta: opens the valve - This uses more energy and lowers the force of contraction, reducing Stroke Volume - Therefore end systolic volume increases