how CO is governed by filling pressure and inotropic state

Question 1

def of CO

Answer 1

the amount of blood ejected from the heart per minute

Question 2

eqn for CO

Answer 2

HR * SV

Question 3

def of SV

Answer 3

how much blood is ejected per beat

Question 4

CO at rest

Answer 4

4900ml /min (approx 5l)

Question 5

CO in exercise

Answer 5

approx 22l/min

Question 6

CO * TPR =

Answer 6

BP

Question 7

BP / TPR =

Answer 7

blood flow

Question 8

what determines blood pressure and blood flow

Answer 8

cardiac output

Question 9

what is preload

Answer 9

stretch of myocytes at rest

Question 10

what is afterload

Answer 10

wall stress produced due to ejection of blood - pressure OPPOSING ejection

Question 11

how is preload measured

Answer 11

EDV/central venous pressure

Question 12

why can’t preload be measured directly

Answer 12

because it’s related to sarcomere length at end of diastole, which can’t be measured

Question 13

what relates pressure, wall stress, and radius

Answer 13

Laplace’s Law

Question 14

what is contractility

Answer 14

strength of contraction at a given resting stretch

Question 15

what modulates contractility

Answer 15

sympathetic nerves, circulating adrenaline increasing [Ca}i

Question 16

what is energy of contraction

Answer 16

amount of work needed to generate SV

Question 17

what determines energy of contraction

Answer 17

starling’s law and contractility

Question 18

what does stroke work do

Answer 18

increases chamber pressure > aortic pressure; ejection

Question 19

what is starling’s law of the heart

Answer 19

energy of contraction of cardiac muscle is proportional to the muscle fibre length at rest

Question 20

describe the ventricular function curve

Answer 20

y = SV = 70ml: normal filling pressure at rest, 5mmHg CVP
plateau at about SV = 100, from 10-15mmHg CVP
muscle overstretched at >15mmHg CVP

Question 21

what is the molecular basis of Starling’s Law

Answer 21

in a stretched fibre, c.f. an un-stretched one:
less overlapping actin & myosin means there’s less MECHANICAL inference, so there is greater potential for cross bridge formation and therefore increased Ca2+ sensitivity

Question 22

What are the 5 roles of Starling’s Law

Answer 22

1. Balances RV and LV output
2. Responsible for fall in CO during drop in blood vol eg sepsis/haemorrhage
3. Restores CO in IV fluid transfusion
4. Responsible for fall in CO in orthostasis (standing); or else postural hypotension, dizziness
5. increased SV in upright exercise

Question 23

What does Laplace’s law describe

Answer 23

how effectively wall tension (T) is converted into pressure (P) within a chamber (ventricle)

Question 24

Equations for Laplace’s law

Answer 24

P = 2T/r

therefore:

T proportional to r AND P

Question 25

how does a small luminal radius affect ejection

Answer 25

greater wall curvature
more wall tension directed to centre of chamber
more pressure developed
more ejection

Question 26

how does larger luminal radius affect ejection

Answer 26

less wall curvature
less wall tension directed towards centre of chamber
less pressure developed
less ejection

Question 27

how does laplace’s oppose starling law at rest

Answer 27

increased pre load > increased stretch of chamber > increased chamber radius > decreased curvature

Question 28

Which law is dominant in a healthy heart

Answer 28

Starling’s overcomes laplace’s for good ejection

Question 29

how does laplace’s facilitate ejection

Answer 29

in ventricular contraction, radius decreases and curvature is increased

Question 30

how does laplace’s contribute to a failing heart

Answer 30

in heart failure, chambers are often dilated i.e. increased radius - not enough wall tension converted to pressure.

Question 31

what contributes to wall tension (T)

Answer 31

wall stress S * wall thickiness w, so P = 2Sw/r

Question 32

rearrange the eqn for wall tension to find the eqn for wall stress S (or afterload)

Answer 32

S = (P*r)/2w

Question 33

why does afterload oppose contraction

Answer 33

it’s directed within chamber wall NOT directed towards the chamber centre

Question 34

what effect does increased wall thickness have on wall stress

Answer 34

increased wall thickness reduces wall stress