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

→ Increases stroke volume due to Starling’s Law

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 it 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 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

Question 31

What is an acute rise in blood pressure offset by?

Answer 31

→ Starling’s law
→ Local positive inotropes
→ Baroreflex

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