Control of Cardiac Output

Question 1

what is the definition of cardiac output?

Answer 1

→amount of blood ejected from the heart per minute

Question 2

what is the equation for cardiac output?

Answer 2

→CO = HR x SV

Question 3

what is the equation for blood pressure?

Answer 3

→BP = CO x TPR (total peripheral resistance)

Question 4

what does cardiac output determine?

Answer 4

→blood pressure

→blood flow

Question 5

what is cardiac output proportional to?

Answer 5

how often the heart beats PM

→ how much blood is ejected per beat

Question 6

what controls stroke volume? (list)

Answer 6

→ preload
→ heart rate
→ contractility
→ afterload

Question 7

what is the equation for blood flow (CO)?

Answer 7

CO = BP/TPR

Question 8

what is preload?

Answer 8

→ The stretching of the heart at rest. Heart is stretched by the ventricles
→ Increases stroke volume due to Starling’s Law.
Increases energy of contraction

Question 9

what is afterload?

Answer 9

→ Opposes ejection

→Reduces stroke volume due to Laplace’s law

Question 10

what controls heart rate?

Answer 10

→ sympathetic and parasympathetic nerves control heart rate

Question 11

What is energy of contraction and what does it depend on?

Answer 11

→ amount of work done required to generate stroke volume

→ depends on Starling’s Law and contractility

Question 12

What two functions does stroke work carry out?

Answer 12

→ Increases chamber pressure to make it greater than aortic pressure (isovolumetric contraction)
→ Ejection from the ventricle

Question 13

what do afterload and preload do to the stroke volume?

Answer 13

→ Preload increases the stroke volume and afterload opposes the stroke volume

Question 14

State Starling’s law

Answer 14

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

→the greater the stretch of the ventricle in diastole (blood entering)
→the greater the energy of contraction
→ a greater stroke volume is achieved in systole

Question 15

what is the equation for stroke volume?

Answer 15

→ SV = end diastolic volume - end systolic volume

Question 16

Describe an unstretched fibre

Answer 16

→ overlapping actin/myosin
→ mechanical interference
→ Less cross-bridge formation available for contraction

Question 17

Describe a stretched fibre

Answer 17

Less overlapping actin/myosin
→ Less mechanical interference
→ potential for more cross-bridge formation
→ Increases sensitivity to Ca2+ ions

Question 18

What are the roles of Starling’s law during preload?

Answer 18

Balances the output of the right and left ventricle which is very important
→ responsible for the fall in cardiac output during a drop in blood volume
→ Restores cardiac output in response to IV fluid transfusions
→ Responsible for fall in cardiac output during orthostasis leading to postural hypotension + dizziness
→ contributes to increased stroke volume and cardiac output during upright exercise

Question 19

What is afterload determined by?

Answer 19

it is determined by wall stress directed through the heart wall

→stress through the wall of the heart prevents muscle contraction

Question 20

What is the equation for Laplace’s law?

Answer 20

→ P = 2t/r
T = wall tension
P = pressure
R = radius

Question 21

what is the equation for wall tension?

Answer 21

→ T = S (wall stress) x ( wall thickness) W

Question 22

How is afterload increased and how is it reduced ?

Answer 22

→ by increasing pressure + radius and reduced by increasing wall thickness

Question 23

what happens if there is a small radius (in terms of afterload)?

Answer 23

→ greater wall curvature
→ more wall stress directed towards the center of the chamber
→ less afterload
→ better ejection

Question 24

what happens if there is a big radius (in terms of afterload)?

Answer 24

→ less wall curvature
→ more wall stress directed through heart wall
more tension in the wall
→more afterload
→ less ejection

Question 25

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

Answer 25

Increased preload gives increased stretch of chamber
→increases chamber radius and decreases curvature
→ increases afterload
→ opposes ejection of blood from a full chamber

Question 26

What law takes precedence in a healthy heart?

Answer 26

→Starling’s law overcomes Laplace’s

Question 27

How does Laplace’s law facilitate ejection during contraction?

Answer 27

→ Ventricular contraction reduces the chamber radius and increases curvature

→Laplace’s law states there will be less afterload in the emptying chamber

→ This aids expulsion and increases stroke volume

Question 28

what do the chambers look like in a failing heart?

Answer 28

→ In a failing heart the chambers are often dilates - increased radius

Question 29

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

Answer 29

→ In a failing heart the chambers are often dilated

→increased afterload opposing ejection

Question 30

What does Laplace’s law state about blood pressure and wall stress?

Answer 30

→ Increased blood pressure will increase wall stress which increases afterload and reduce ejection

Question 31

What is an acute rise in blood pressure offset by?

Answer 31

→ Starling’s law-increased stretch give increased contraction and increased stroke volume
→ Local positive inotropes- noradrenaline
→ Baroreflex– decreased sympathetic tone which decreases blood pressure.

Question 32

What is the baroreflex?

Answer 32

→ decreased sympathetic tone

→ decreasing blood pressure

Question 33

What is chronic increase in arterial blood pressure caused by?

Answer 33

→ increased energy expenditure to maintain stroke volume

→ ultimately decrease in stroke volume

Question 34

what does decreasing blood pressure do to the heart?

Answer 34

→ increases the efficiency of the heart

Question 35

What happens if there is an increased radius in the heart?

Answer 35

→Heart failure where the heart does not contract properly (MI, cardiomyopathies, mitral valve re-gurgitation)
→ blood is left in the ventricle leading to eventual volume overload

Question 36

What happens if there is increased pressure in the heart?

Answer 36

→ pressure overload heart failure due to increased pressure

Question 37

What happens with increase in radius and pressure?

Answer 37

→ wall stress increases which opposes ejection

Question 38

how does the heart compensate for an increase in radius + pressure?

Answer 38

→ the heart compensates with ventricular hypertrophy (greater myocyte size and more sarcomeres)
→ Increasing wall thickness
→ decreases wall stress per sarcomere and therefore afterload so maintains SV and CO

Question 39

why does ventricular hypertrophy eventually cause heart failure?

Answer 39

→ the more sarcomeres used the more O2 is used
→ amount of energy required continues to increase
→ contractility decreases and produces more heart failure

Question 40

describe Laplace’s law and the ventricular pressure-volume loop with high blood pressure

Answer 40

→ increased afterload
→ a longer time is spent in isovolumetric contraction to increase pressure in the chamber to be > aorta to open the valve
→ this uses more energy and lowers the force of contraction and SV
→end systolic volume increases

Question 41

describe Starling’s law and the ventricular pressure-volume loop during exercise

Answer 41

→ increased venous return leads to an increase in EDV
→increased preload
→more stretch
→shorter isovolumetric contraction phase
→ increase in SV due to Starlings law.
→More blood back to the heart
→ more blood ejected from the heart

Question 42

Describe the importance of Laplace law

Answer 42

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.
2.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.
3.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.

Question 43

What does a high blood pressure mean for cardiac output?

Answer 43

a high blood pressure will reduce cardiac output.

Question 44

What does higher energy used in isovolumetric contraction mean for ejection?

Answer 44

Higher energy used in isovolumetric contraction reduces energy available for ejection

Question 45

What happens to afterload in hypertension?

Answer 45

– increased afterload
There is a longer time spent in isovolumetric contraction to increase pressure in the chamber above that in the aorta to open the valve.

This uses more energy and lowers the force of contraction reducing stroke volume and an increasing end-systolic volume (ESV).
More energy required

Question 46

What are the functions of the stroke volume?

Answer 46

Contracts until chamber pressure > aortic pressure (isovolumetric contraction).

Ejection from ventricle.

Question 47

What does energy of contraction depend on?

Answer 47

Depends on Starling’s Law and contractility