Control of cardiac output

Question 1

What is cardiac output

Answer 1

The amount of blood ejected from the heart per minute

Question 2

Equation for Cardiac output

Answer 2

CO = HR x SV

Question 3

What does cardiac output affect

Answer 3

BP/TPR

Question 4

What is preload

Answer 4

Stretching of heart at rest, increase 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

Energy of contraction

Answer 6

Amount of work required to generate stroke volume

Question 7

What are the 2 functions stroke work carries out

Answer 7

1) Contracts until chamber pressure > aortic pressure (isovolumetric contraction)
2) Ejection from ventricle

Question 8

Define starling’s law

Answer 8

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

Question 9

Roles and effects of starling’s law

Answer 9

1) Balance output of right ventricle and left ventricle
2) Responsible for fall in CO during a drop in blood volume
3) Restores CO in response to intravenous fluid transfusions
4) Responsible for fall in CO during orthostasis leading to postural hypotension and dizziness as blood pools in legs
5) Contributes to increased SV and CO during upright exercise

Question 10

Define Laplace’s law

Answer 10

That increased blood pressure (P) will increase wall stress. This will increase afterload and reduce ejection.

Laplace’s law is good ejection with small radius, bad with large radius

Question 11

How is afterload increased

Answer 11

Increasing pressure and radius

Question 12

How is afterload reduced

Answer 12

Increase wall thickness

Question 13

Afterload of a ventricle with a small radius

Answer 13

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

Question 14

Afterload of a large ventricle radius

Answer 14

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

Question 15

Afterload of a huge theoretical radius

Answer 15

Negligible wall curvature

Virtually all stress directed through wall

Question 16

Importance of Laplace’s law

Answer 16

1) Oppose Starling’s law
2) Facilitates ejection during contraction
3) Contributes to a failing heart at rest and during contraction

Question 17

Effects of chronic increase in arterial blood pressure

Answer 17

Increased energy expenditure attempt to maintain stroke volume but ultimately stroke volume will gradually decrease
Decrease in blood pressure would increase efficiency of the heart.

HIGH BP = REDUCE CO

Question 18

Starling’s law and ventricular pressure - volume loop

Answer 18

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

Question 19

Laplace’s law and ventricular pressure - volume loop

Answer 19

With high blood pressure, increased afterload
Longer time spent in isovolumetric contraction to increase pressure in the chamber above that in aorta to open the valve
This uses more energy and lowers the force of contraction reducing stroke volume