Control of Cardiac Output

Question 1

What is cardiac output?

Answer 1

The amount of blood ejected from the heart per minute

Question 2

How do you calculate cardiac output?

Answer 2

Heart Rate x Stroke Volume

Question 3

How do you calculate blood pressure?

Answer 3

Cardiac output x Total peripheral resistance

Question 4

What is ‘preload’ ?

Answer 4

• Stretching of heart at rest

- Increases stroke volume, due to Starling’s law

Question 5

What is ‘afterload’ ?

Answer 5

• Opposes ejection

- Reduces stroke volume, due to Laplace’s law.

Question 6

What is Starling’s law of the heart ?

Answer 6

Energy of contraction of cardiac muscle is relative to the muscle fibre length at rest.

Question 7

Describe the effects of ‘preload’

Answer 7

- Greater stretch of ventricle in diastole 
(blood entering).
- Then greater energy of  contraction.
- And greater stroke volume achieved 
in systole.

• more blood in = more blood out
  Intrinsic property of cardiac muscle
  (nerves, hormones etc. not involved)

Question 8

Describe the difference between unstretched heart muscle fibre to stretched heart muscle fibre

Answer 8

Un-stretched fibre

• Overlapping actin/myosin
• Mechanical inference
• Less cross-bridge formation available for contraction#

Stretched fibre

• Less overlapping actin/myosin
• Less mechanical inference -
• Potential for more cross-bridge formation
• Increased sensitivity to Ca2+ ions

*Stretching increases
energy of contraction

Question 9

List the roles and effects of preload and Starling’s Law

Answer 9

• Balances outputs of the right ventricle and
  left ventricle
• Responsible for fall in cardiac output during
  a drop in blood volume or vasodilation (eg.
  haemorrhage, 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 & dizziness as blood
  pools in legs.
• Contributes to increased stroke volume &
  cardiac output during upright exercise.

Question 10

Describe ‘Afterload’

Answer 10

• Afterload opposes the contraction that ejects blood from the heart and is determined by wall stress directed through the heart wall.
• Stress through the wall of the heart prevents muscle contraction.

Question 11

What is ‘Laplace’s Law’ ?

Answer 11

Laplace’s law describes parameters that determine afterload:
Wall tension (T), pressure (P), and radius (r) in a chamber (ventricle)
TaPr

Wall stress (S) is made up of tension (T) over wall thickness (w)

Question 12

How is afterload increased and reduced ?

Answer 12

Afterload is increased by increasing pressure & radius

and reduced by increasing wall thickness

Question 13

How does a small ventricle radius affect wall stress/afterload?

Answer 13

• Greater wall curvature
• More wall stress directed towards centre of chamber
• Less afterload
• Better ejection

Question 14

How does a larger ventricle radius affect wall stress/afterload?

Answer 14

• Less wall curvature
• More wall stress directed through heart wall
• More afterload
• Less ejection

Question 15

List the importance of Laplace’s Law

Answer 15

Opposes Starling’s law at rest

• Increased preload gives increased stretch of chamber (Starling’s law)
• This increases chamber radius (decreases curvature) – increasing afterload
• In a healthy heart, Starling’s Law overcomes Laplace’s so ejection is OK.

Facilitates ejection during contraction

• Contraction reduces chamber radius so less afterload as the chamber empties.
• This aids expulsion of last portion of blood and increases stroke volume.

Contributes to a failing heart at rest and during contraction
- In a failing heart the chambers are often dilated and radius is large - so increased
afterload opposing ejection.