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sem 2 CVS > Control of blood flow > Flashcards

Control of blood flow Flashcards

1
Q

what is blood flow?

A

blood flow is the difference in pressure (pressure gradient) divided by resistance (TPR)

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2
Q

what does TPR control?

A

blood flow and blood pressure

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3
Q

if you increase resistance and keep pressure the same, what happens?

A

flow decreases

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4
Q

what 3 things control TPR?

A

o Poiseuille’s law
o Myogenic responses
o Blood viscosity

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5
Q

which vessel affects TPR?

A

arterioles

-they can control blood flow to different parts of the body

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6
Q

the vasodilation of arterioles leads to what?

A
  • decrease in TPR
  • decreased blood pressure upstream
  • greater flow downstream
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7
Q

vasoconstriction of arterioles leads to what?

A
  • increase in TPR
  • increased blood pressure upstream
  • less flow downstream
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8
Q

Hypertension of arterioles leads to what?

A
  • over constriction of arterioles
  • higher arterial BP
  • less capillary flow – under perfusion
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9
Q

why are changes in the arteriole blood flow a good thing? give examples

A

you can control where the blood goes

e. g. postprandial (when you’ve just eaten), you want blood flowing around the intestines, helping to remove the products of digestion and help us absorb nutrients from the meal we’ve eaten. Don’t need blood going to the legs, SO:
- Dilate superior mesenteric artery and increase blood flow to digestive canal and intestine
- Constrict the common iliac as won’t need blood to legs – more in digestion than muscle.

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10
Q

what does Poiseuille’s Law describe?

A

parameters that govern TPR

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11
Q

what factors affect TPR according to poiseuilles law?

A
  1. Viscosity – more viscous the more resistance
  2. Length of vessel – longer has more resistance
  3. Radius – a bigger radius means less resistance
  4. Density- a high density means more resistance
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12
Q

explain the r^4 effect?

A

If you double the radius, it will be 16x more conductive

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13
Q

how does a small change in radius affect TPR?

A

a small change in radius has huge effect on TPR

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14
Q

how do vasoconstricters/vasodilators work?

A

They produce small changes in vessel radius by affecting smooth muscle. This has large effects on blood flow.

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15
Q

what are the main vessels involved in TPR?

A

arterioles

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16
Q

what are changes in ventricular pressure moderated by?

A

compliance of aorta and major arteries

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17
Q

when ventricular pressure drops what do arterioles do?

A

the arterioles contract and spring back to keep blood flow even

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18
Q

so how do arterioles change their size?

A

there are circular layers of smooth muscle around the arterioles which can contract and relax

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19
Q

arteriole radius is tightly controlled by what?

A

sympathetic nerves which provide constant tone

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20
Q

TPR is controlled by 3 parameters- name them:

A
  1. Radius - r4 effect
  2. Pressure difference across vessels
    (P1-P2)
  3. Length - arterioles are long vessels
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21
Q

why can’t capillaries change their radius size like arterioles?

A

there is no smooth muscle/sympathetic innervation in capillaries

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22
Q

what arrangement are capillaries in and how does this affect their resistance?

A

capillaries are arranged in parallel, so they have a low total resistance

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23
Q

what arrangement are arterioles in and how does this affect their resistance?

A

arterioles are arranged in series, so their total resistance is greater

24
Q

where is the biggest pressure drop

A

arterioles have largest pressure drop of 40-50 mmHg amongst vessels

25
Q

what is the consequence of capillaries not being able to change their radius size?

A

r^4 has no effect

26
Q

local blood flow through individual organs/tissues is mainly controlled by what?

A

changes in the radius of arterioles supplying a given organ/tissue

27
Q

the control mechanisms of arteriole radius can be split into which 2 categories?

A

intrinsic and extrinsic

28
Q

what do we mean by ‘intrinsic factors’? name some

A
  • factors entirely within an organ or tissue
  • allow response to local factors
  • affect tissue or vessel locally

E.G.

  1. local hormones
  2. tissue metabolites
  3. myogenic properties of muscle
  4. endothelial factors - production of NO from endothelium causes vasodilation
29
Q

what do we mean by ‘extrinsic factors’? name some

A
  • factors outside the organ or tissue
  • neural control or hormonal control from distant glands

E.G.

  1. Neural - sympathetic nervous system
  2. Hormonal - adrenaline
30
Q

what is Bayliss myogenic response?

A

a vessels response to stretch, looking at pressure and flow

31
Q

when a vessel is stretched what does it do at higher pressures?

A

it contracts to reduce flow

32
Q

if there was a vessel with a fixed diameter, what would happen when pressure was increased?

A
  • flow would increase in a linear way

- having a linear relationship would mean a small increase in pressure would cause a very large increase in flow.

33
Q

do blood vessels follow this linear relationship?

A

no

34
Q

how do blood vessels act if they don’t follow this linear relationship?

A

they have a natural tendency when stretched to squeeze against it

35
Q

so what is the myogenic response?

A

when the pressure gets up to certain level and the vessels contract and reduce the flow automatically

36
Q

so how does this vessel contraction occur?

A

stretching of the muscles in vessels causes ion channels to open, which then depolarise, leading to muscle contraction

37
Q

why is this myogenic response important? (name some organs and example)

A

protects organs from extreme high pressures

  1. Brain – sudden high rise in BP, arterioles will start to sequence to maintain flow but not let it go too high
  2. Kidney – don’t want to damage glomerulus
38
Q

if there is a sudden drop in BP how does the body ensure the tissues aren’t starved of blood?

A

As BP drops the vessels will dilate to allow more flow

So, intrinsically the nature of vessels maintains flow

39
Q

relationship between distention and dilation of blood vessels?

A

Increased distension of vessel makes it constrict

Decreased distension of vessel makes it dilate

40
Q

how does viscosity affect blood flow?

A

there is friction with the wall, and if the blood is more viscous these interactions are stronger

41
Q

when laminar flow becomes more viscous how does this affect resistance?

A

Laminar flow has more resistance with more viscosity

42
Q

what does a haematocrit measure?

A

how many RBC’s there are in the blood

43
Q

is dehydrated blood more or less viscous?

A

dehydrated blood is more viscous

44
Q

therefore, in anaemia does the blood have a high or low viscosity?

A

low viscosity, not enough RBC’s

45
Q

how does tube diameter affect blood viscosity?

Fahraeus – lindquvist effect

A
  • blood viscosity falls in narrow tubes, RBC’s pushed towards the centre and there is a reduction in frictional forces
  • decreased resistance, increased blood flow in microvessels
46
Q

explain the effect that red cell deformability has on viscosity:

A

red cell deformability causes an increase is viscosity and a decrease in blood flow

47
Q

what happens with sickle cell and why does it caused an increase in viscosity?

A

In sickle cell you get stiff and crystallised Hb and so capillaries get blocked, so a decrease in BF – ischaemia

48
Q

explain how the velocity of blood affects viscosity:

A

slow venous flow in immobile legs (ie. low velocity) can cause partial clotting which will increase the viscosity
-thrombolysis and thrombosis balance in body all the time

49
Q

so name the factors that affect blood viscosity:

A
  • haemocrit
  • blood velocity
  • tube diameter
  • red cell deformability
50
Q

at rest, where is 60% of the blood located?

A

systemic veins and venues (capacitance vessels)

51
Q

what do these systemic veins and venules act as?

A

reservoirs

-blood can be diverted from them when necessary, eg. during exercise or haemorrhage

52
Q

are veins or arterioles more contractile? explain why

A

arterioles

Veins are contractile – but much less contractile than arteriole (as done to increase cardiac output)

53
Q

explain how sending a nervous signal to the veins to constrict them will bring more volume back to the heart (Starling’s Law):

A

-Stimulation of sympathetic nerves causing vasoconstriction sifts blood centrally
-increase venous return and CVP
increase end-diastolic pressure
-increase preload
-increases SV according to starling’s law

54
Q

what is Bernoulli’s law and what does it explain?

A

blood flow
-mechanical energy of flow is determined by pressure, kinetic, potential energies
energy = pressure (PV) + kinetic + potential (pgh)

55
Q

using Bernoulli’s law what is involved when it comes to returning blood to the heart?

A

-no pressure gradient BUT there is kinetic energy

56
Q

what is greater at the heart according to Bernoulli’s law and what does this mean?

A
  • Ejected blood has greater kinetic energy at heart than feet (more velocity, V)
  • Also, greater potential energy than at heart than feet (more height, h)
  • Greater kinetic/potential energy overcome pressure gradient to maintain flow

This is enough to overcome the venous pressure.

57
Q

is the energy of the blood leaving the heart or the feet higher?

A

the energy of blood leaving the heart is higher than the blood leaving feet.

sem 2 CVS (17 decks)

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