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sem 2 CVS > Control of Cardiac Output > Flashcards

Control of Cardiac Output Flashcards

1
Q

what is cardiac output and what is it proportional to?

A

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)

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2
Q

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

A

yes

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3
Q

the cardiac output changes according to what?

A

demand

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4
Q

what is the cardiac output at rest and during exercise?

A

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

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5
Q

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

A

blood pressure= cardiac output x total peripheral resistance

BP= CO x TPR

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6
Q

what affects resistance to blood flow?

A

width and cross-sectional area

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7
Q

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

A

vasodilating blood vessels reduces the resistance

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8
Q

what are preload and after load important in?

A

preload and after load are important in stroke volume

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9
Q

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

A
  • 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
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10
Q

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

A
  • 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
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11
Q

sympathetic and parasympathetic nerves control what?

explain how strength of contraction is controlled

A
  • they control heart rate

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

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12
Q

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

A

the amount of work done required to generate stroke volume

-dependent on Starling’s Law and Contractility

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13
Q

what 2 functions does stroke work carry out?

A
  1. Increases chamber pressure to make it greater than aortic pressure (isovolumetric contraction)
  2. Ejection from the ventricle
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14
Q

what is stroke volume?

A

Amount of blood pumped by the LV in one contraction.

SV= end diastolic volume-end systolic volume

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15
Q

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?

A

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.

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16
Q

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?

A

EDV= end diastolic volume

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

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17
Q

Explain what happens during isovolumetric contraction and the phases afterwards:

A
  • 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
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18
Q

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

A

amount of work done in a heartbeat

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19
Q

explain Pre-load in further detail:

A

(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
20
Q

what was starlings experiment?

A

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.

21
Q

relationship between Starling’s Law and haemorrhage

A

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.

22
Q

a small change in central venous pressure can cause what?

A

considerable changes in stroke volume

23
Q

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

A

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

24
Q

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

A

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.
25
Q

role of Starling’s law

A
  • 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
26
Q

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

A
  1. vasodilation
  2. haemorrhage
  3. sepsis
27
Q

what happens when you stand up for a long time?

A
  • 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
28
Q

what happens during exercise?

A

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

29
Q

explain after load in further detail:

A

(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
30
Q

3 factors that affect after load and their relationship:

A
  1. Wall Tension (T)
  2. Pressure (P)
  3. Radius (R)

T=PR/2

31
Q

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

A

because a chamber has 2 directions of curvature

32
Q

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

A 
wall stress (S) and wall thickness (W)
T= S x W
33
Q

what factors increase after load?

A
  • increasing pressure and radius

- decreasing wall thickness

34
Q

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

A

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

35
Q

How does radius affect wall stress?

A

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)
36
Q

what is the importance of Laplace’s Law (3 things)?

look at notes for the actual detailed answer!!!!

A
  • 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)
37
Q

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

A

small

38
Q

what does Laplace’s Law state?

A

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

39
Q

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

A

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
40
Q

chronic increase in arterial blood pressure occur

A
  • 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,
41
Q

why does BP need to kept fairly constant during exercise?

A

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

42
Q

Once pressure goes above a certain threshold what happens?

A

Laplace’s Law is dominating and cardiac output decreases

43
Q

What problems does volume overload cause?

A

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.

44
Q

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

A

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)

45
Q

Starlings Law and the ventricular pressure-volume loop

A
  • 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
46
Q

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

A

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

sem 2 CVS (17 decks)

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