Control of Cardiac Output

Question 1

what is cardiac output and what is it proportional to?

Answer 1

The amount of blood ejected from the heart per minute

-proportional to how often the heart beats per minute (HR) and how much blood is ejected per beat (SR)

Question 2

is the cardiac output from the right side (via PA) and left side (via aorta) the same?

Answer 2

yes

Question 3

the cardiac output changes according to what?

Answer 3

demand

Question 4

what is the cardiac output at rest and during exercise?

Answer 4

rest: 70bpm x 70ml = 4.9 litres/min
exercise: 180bpm x 120 ml= 22 litres/min

Question 5

what is the equation linking cardiac output, blood pressure and total peripheral resistance?

Answer 5

blood pressure= cardiac output x total peripheral resistance

BP= CO x TPR

Question 6

what affects resistance to blood flow?

Answer 6

width and cross-sectional area

Question 7

what effect does vasodilating blood vessels, have on the resistance?

Answer 7

vasodilating blood vessels reduces the resistance

Question 8

what are preload and after load important in?

Answer 8

preload and after load are important in stroke volume

Question 9

explain what is meant by the term ‘pre-load’

Answer 9

• Pre-load describes the stretching of the heart at rest, where stroke volume increases due to Starling’s Law.
• When the heart is filled more, the muscle responds to be being stretched by contracting harder
• increases the stroke volume

Question 10

explain what is meant by the term ‘after-load’

Answer 10

• After-load describes opposing ejection and stroke volume due to Laplace’s Law
• A lot of tension on the muscle, so it won’t contract as hard

Question 11

sympathetic and parasympathetic nerves control what?

explain how strength of contraction is controlled

Answer 11

• they control heart rate

- strength of contraction is due sympathetic nerves and circulating adrenaline, increasing intracellular calcium

Question 12

define the energy of contraction- what is it dependent on?

Answer 12

the amount of work done required to generate stroke volume

-dependent on Starling’s Law and Contractility

Question 13

what 2 functions does stroke work carry out?

Answer 13

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

Question 14

what is stroke volume?

Answer 14

Amount of blood pumped by the LV in one contraction.

SV= end diastolic volume-end systolic volume

Question 15

when looking at a diagram of the LV pressure-volume loop, there is an area called ESV- what does this stand for and what does it mean?

Answer 15

ESV= end systolic volume

When the heart is empty, the pressure and volume are low, ventricle is almost empty and ready to fill. Just after isovolumetric relaxation.

Question 16

on a diagram of the LV pressure-volume loop, there is another area called EDV- what does this stand for and what does it mean?

Answer 16

EDV= end diastolic volume

As the ventricle fills volume increases and mitral valve closes, ventricles start to contract. Just before isovolumetric contraction.

Question 17

Explain what happens during isovolumetric contraction and the phases afterwards:

Answer 17

• Higher energy used reduces energy for ejection
• The more we contract, the more the isovolumetric contraction. More energy gets stored up and the easier the ejection.
• Mitral and aortic valves are closed and the ventricle is full of blood, heart starts to squeeze, increasing the pressure because the volume stays the same. Heart is using a lot of energy. Aortic valve opens when the pressure in the ventricles is higher than the pressure in the aorta.
• Rapid ejection, pressure rises and volume decreases. Pressure decreases towards the end of the ejection period, and aortic valve closes when pressure in aorta is higher than ventricles, so the heart relaxes
• this causes a period of isovolumetric relaxation

Question 18

what does the inside of the LV pressure-volume loop represent?

Answer 18

amount of work done in a heartbeat

Question 19

explain Pre-load in further detail:

Answer 19

(stretching of the heart at rest, where stroke volume increases due to Starling’s Law)

• the greater the stretch of the ventricle in diastole, the greater the energy of contraction and a greater stroke volume is achieved in systole
  ie. more blood coming back to the heart means more blood ejected automatically- an intrinsic property of cardiac muscle

Question 20

what was starlings experiment?

Answer 20

addition of a bolus of fluid to the circulation: -increased amount of blood in venous return

• increased end diastolic volume of blood returning to the heart
• increases strength of contraction
• increases stroke volume

removing the fluid decreased the venous return, decreasing the end diastolic volume, strength of contraction and stroke volume.

Question 21

relationship between Starling’s Law and haemorrhage

Answer 21

During a haemorrhage, you start losing blood and Starling’s law makes this worse because you have decreased venous return, decreasing the: end diastolic volume, strength of contraction and stroke volume.

Question 22

a small change in central venous pressure can cause what?

Answer 22

considerable changes in stroke volume

Question 23

excess fluid will have what kind of effect when it comes to starlings law?

Answer 23

the opposite effect to a bolus of fluid, as starlings law has a limit

Question 24

how does stretching increase the energy contraction? (molecular basis of Starling’s law)

Answer 24

With a stretched fibre there is:

• z bands are where myosin and actin overlap
• less overlapping actin/myosin
• less mechanical interference
• potential for more cross-bridge formation
• increases sensitivity to Ca2+ ions which is a signal for contraction, so better forces of contraction.

Question 25

role of Starling's law

Answer 25

- balances outputs of RV and LV - responsible for fall in cardiac output during a drop in blood volume - 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 and dizziness s the blood pools in the leg - contributes to increased stroke volume and cardiac output during upright exercise

Question 26

Starling's is responsible for fall in cardiac output in which 3 situations?

Answer 26

1. vasodilation 2. haemorrhage 3. sepsis

Question 27

what happens when you stand up for a long time?

Answer 27

- less blood going back to the heart, starlings law means decreased contraction and stroke volume - can't get enough oxygen to the brain, so you faint

Question 28

what happens during exercise?

Answer 28

more blood is mobilised from the legs so the heart pumps harder as there is an increase in venous return.

Question 29

explain after load in further detail:

Answer 29

(opposing ejection and reducing stroke volume due to Laplace's Law) - afterload opposes ejection of the blood from the heart and is determined by heart wall stress, as this stress prevents muscle contraction - more energy of contraction is needed to overcome this wall stress to produce cell shortening and ejection

Question 30

3 factors that affect after load and their relationship:

Answer 30

1. Wall Tension (T) 2. Pressure (P) 3. Radius (R) T=PR/2

Question 31

why is the value divided by 2 for Laplace's Law?

Answer 31

because a chamber has 2 directions of curvature

Question 32

what is wall tension (T) made up of? (equation)

Answer 32

``` wall stress (S) and wall thickness (W) T= S x W ```

Question 33

what factors increase after load?

Answer 33

- increasing pressure and radius | - decreasing wall thickness

Question 34

Why is there less stress on a wider/thicker heart?

Answer 34

there is less stress on a wider/thicker heart as the same tension is spread over a bigger area

Question 35

How does radius affect wall stress?

Answer 35

large radius: - less wall curvature - more wall stress is directed through the heart wall - more after-load and less ejection small radius: - greater wall curvature - more wall stress is directed towards centre of chamber, towards the centre/lumen of the heart - less after-load and better ejection (less resistance to contraction)

Question 36

what is the importance of Laplace's Law (3 things)? look at notes for the actual detailed answer!!!!

Answer 36

- Opposes Starling's Law at rest - Facilitates ejection during contraction (because ventricular contraction reduces the chamber radius and increases curvature) - Contributes to a failing heart at rest and during contraction (in a failing heart the chambers are often dilated, increased radius, so increased afterload opposing ejection)

Question 37

Laplace's Law has a good ejection with a small or large radius?

Answer 37

small

Question 38

what does Laplace's Law state?

Answer 38

increased BP will increase wall stress, increasing after-load and reducing ejection

Question 39

what are acute rises in blood pressure counteracted/offset by?

Answer 39

Starling's law- increased stretch gives increased contraction and increased SV There is an intrinsic increase in contractility - due to Anrep effect or noradrenaline - Baroreflex, decreases blood pressure

Question 40

chronic increase in arterial blood pressure occur

Answer 40

- theres a constant high blood pressure, after load increases, Starling's Law can't cope so Laplace's Law takes over - laplaces law starts to dominate and starlings law isn't as efficient - high volume of blood left after systole, start to get volume overload - increased energy expenditure to maintain and ultimately decrease SV - decreasing BP will increase the efficiency of the heart,

Question 41

why does BP need to kept fairly constant during exercise?

Answer 41

because a high BP will reduce cardiac output, and decreasing BP will increase efficiency of the heart

Question 42

Once pressure goes above a certain threshold what happens?

Answer 42

Laplace's Law is dominating and cardiac output decreases

Question 43

What problems does volume overload cause?

Answer 43

Volume overload leads to an increased radius, so too much blood going into the heart with a big radius- his causes problems with Laplace's law overcoming Starling's Law.

Question 44

What happens when Laplace's Law struggles to overcome Starling's Law?

Answer 44

Heart doesn't contract properly, increase pressure -heart tries to compensate with ventricular hypertrophy, which is good as it reduces stress on the wall BUT increases the amount of work that we need to do, need more 02 and glucose to the heart. 02 demand can't be met, heart pumps harder, hypertrophy so needs more 02 (vicious cycle)

Question 45

Starlings Law and the ventricular pressure-volume loop

Answer 45

- healthy heart during exercise, increased venous return, leads to increase in EDV, causing an increase in preload and more stretch - shorter isovolumetric contraction phase and increase in SV due to Starling's law - more blood back to the heart so more blood ejected from the heart

Question 46

Starlings Law and the ventricular pressure-volume loop in a high BP situation

Answer 46

heart is not as efficient, longer time spent in isovolumetric contraction, takes longer to get to a pressure where blood is releases - uses more energy, lowers force of contraction and SV - ESV increases