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Cardiovascular system > Control of blood flow > Flashcards

Control of blood flow Flashcards

1
Q

What are the three main things that control Total Peripheral Resitance?

A
  1. Poiseuille’s law
  2. Myogenic response
  3. Blood viscosity
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2
Q

Darcy’s law: how is blood flow (Cardiac Output) calculated?

A

Cardiac output = (Arterial pressure - Central venous pressure) / Total periphral resistance

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3
Q

How is conductance calculated?

A

G= 1/TPR (reciprocal of total peripheral resistance)

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4
Q

How is blood flow calculated (in terms of pressure gradient and conductance)

A

Cardiac output = Arterial pressure - (Central venous pressure x conductance)

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5
Q

What is the mean arterial pressure?

A

90 mmHg

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6
Q

How does TPR control blood flow and blood pressure?

A

Increase in resistance results in the need to increase the pressure and keep the same flow

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7
Q

What is hypertension?

A
  • Over constriction of arterioles
  • Higher arterial blood pressure but less capillary flow
  • Increased peripheral resistance: can under perfuse vital organs with reduced blood flow even though the blood pressure is high
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8
Q

How does TPR affect both blood flow and blood pressure?

A

Blood flow (CO) = pressure gradient/TPR

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9
Q

What occurs when there is a pressure drop between arteries and arterioles?

A

Normal blood flow

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10
Q

What occurs when there is decreasd BP in the arteries and decreased TPR in the arterioles?

A

Vasodilation in the arteriole:

  • decreased blood pressure upstream
  • greater flow
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11
Q

What occurs when there is increased BP in the arteries and increased TPR in the arterioles?

A

Vasoconstriction in the arterioles:

  • Increased blood pressure upstream
  • Less flow
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12
Q

What are the changes in blood flow during the Sedentary state?

A
  • Superior mesenteric dilated, leading to increased flow to intestines
  • Common iliac constricted, leading to decreased flow to the legs
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13
Q

What are the changes in blood flow during Exercise?

A
  • Superior mesenteric constricted, leading to decreased flow to the intestines
  • Common iliac dilated, leading to increased flow to the legs
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14
Q

Darcy’s law

A

CO = Pa - CVP x G

CO: Blood flow
Pa: Arterial pressure
CVP: Central venous pressure
G: Conductance

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15
Q

Poiseuille’s Law (with resistance being the subject of the formula)

A

Resistance: (8 x N x L)/ pi x r4

r: radius of vessel
N: blood viscosity
L: vessel length
Blood vessel radius to power of 4 controls TPR

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16
Q

Poiseuille’s Law (with conductance being the subject of the formula)

A

Conductance (G): (pi x r^4) / 8 x N x L

r: radius of vessel
N: blood viscosity
L: vessel length
Blood vessel radius to the power of 4 controls TPR

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17
Q

What is Poiseille’s Law?

A

Describes the parameters that govern TPR

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18
Q

What is the combined Darcy and Poiseuille’s law?

A

CO= Pa - CVP x [ (pi x r^4) / (8 x N x L) ]

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19
Q

Describe the r^4 effect

A

With the same arterial blood pressure, doubling the radius vessel means that the change in r^4 is equal to 2^4 which is 16.

  • Vessel 2 therefore equals 1/16th of the resistance of vessel 1
  • There is 16 times greater flow in vessel 2 (as the flow is proportional to r^4)
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20
Q

What is the pressure drop in arterioles?

A

Arterioles have the largest pressure drop of 40-50 mmHg among vessels

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21
Q

What is the radius of arterioles controlled by?

A

Arteriole radius is tightly controlled by sympathetic nerves providing constant tone: we both dilate and constrict

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22
Q

What are the 3 main parameters that TPR is controlled by?

A
  1. Radius: r^4
  2. Pressure difference across vessels: P1-P2
  3. Length
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23
Q

What vessel is TPR not controlled by?

A

Capillaries

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24
Q

Why does pressure drop influence the capillaries’ inability to control TPR?

A

Less pressure drop across capillaries due to less resistance to blood flow in capillaries

25
Q

Why does Radius influence the capillaries’ inability to control TPR?

A

No sympathetic innervation/smooth muscle in capillaries so cannot alter radius

26
Q

Why does length influence the capillaries’ inability to control TPR?

A

Individual capillaries are short

27
Q

Why does blood viscosity influence the capillaries’ inability to control TPR?

A

Less resistance in capillaries because blood flow reduces viscosity

28
Q

What is the difference in arrangement in capillaries and arterioles and how does this influence resistance?

A

Capillaries are arranged in parallel, so have a low total resistance as Rtotal= 1/R1 + 1/R2.
Whereas, arterioles are arranged in series so the total resistance is greater as Rtotal = R1 + R2

29
Q

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

A

Changes in radius of arterioles supplying a given tissue/organ

30
Q

What is the meaning of intrinsic control mechanisms of arteriole radius?

A

Factors that are entirely within an organ or tissue (allow response to local factors)

31
Q

What is the meaning of extrinsic control mechanisms of arteriole radius?

A

Factors outside the organ or tissue (nervous and hormonal control)

32
Q

List examples of intrinsic control mechanisms of arteriole radius

A
  • Local hormones: e.g. in the event of a bee sting
  • Tissue metabolites
  • Myogenic properties of the muscle
  • Endothelial factors: the endothelium is constantly producing low amounts of Nitrous Oxide which has the tendency to dilate blood vessels
33
Q

List examples of extrinsic control mechanisms of arteriole radius

A
  • Neural: e.g. sympathetic nervous system

- Hormonal: e.g. adrenaline

34
Q

What is muscle tone?

A

Situation in between contraction and relaxation

35
Q

What is the effect of increased distension of a vessel?

A

It constricts

36
Q

What is the effect of decreased distension of a vessel?

A

It dilates

37
Q

Why is the expected linear resistance (for flow vs pressure) different to the true curve?

A

BAYLISS MYOGENIC RESPONSE:

  • Having a linear relationship would entail very large differences in blood flow with differences in pressure
  • Maintains blood flow at the same level during changes in arterial pressure
  • At higher pressures, when the vessel is stretched it contracts to reduce flow (which is not linear)
38
Q

What are the three factors that blood flow depends on?

A
  1. Viscosity of blood
  2. Vessel diameter
  3. Haematocrit
39
Q

What is viscosity a measure of?

A

Internal friction opposing the separation of the lamina

40
Q

What are the clinical implications of:

  • Haematocrit (45%)
  • Typical blood viscosity: 4-5
A
  • Polycythaemia (high blood viscosity), which increases TPR, increases blood pressure and decreases blood flow
  • Anaemia (low blood viscosity): decreases TPR, decreases blood pressure and increases heart rate (as a result of baroreceptor reflex)
41
Q

What is Haematocrit equal to?

A

Red Blood Cell number

42
Q

What are the clinical implications of Tube diameter (Fahraeus-Lindqvuist effect)?

A
  • Blood viscosity falls in narrow tubes (<100 um vessels)

- decrease in resistance and increase in blood flow in microvessels like capillaries

43
Q

What are the clinical implications of Red cell deformability?

A
  • Increase in blood viscosity
  • Decrease in blood flow
  • Sickle cell anaemia crises
44
Q

What are the clinical implications of Velocity of blood?

A

Slow venous flow in immobile legs: increased viscosity due to partial clotting.

45
Q

% of blood volume at rest in pulmonary vessels?

A

12%

46
Q

% of blood volume at rest in heart?

A

8%

47
Q

% of blood volume at rest in systemic arteries and arterioles?

A

15%

48
Q

% of blood volume at rest in systemic capillaries?

A

5%

49
Q

% of blood volume at rest in systemic veins and venules? what is the significance of this?

A

60%

  • the systemic veins and venules function as blood reservoirs
  • blood can be diverted from them in times of need, e.g. exercise, haemorrhage
50
Q

What are the properties of veins that means they form blood reservoirs?

A
  • Thin-walled, collapsible, voluminous vessels
  • Contain 2/3rds of blood volume
  • Contractile: contain smooth muscle
  • Innervated by sympathetic nerves
  • Thinner and more compliant than arterial muscles
    Therefore form blood reservoirs
51
Q

How does contractility influence the volume of blood in veins?

A
  • The contraction of the vessels: expels blood into central veins
  • Increased venous return/CVP/end-diastolic volume
  • Increases stroke volume (Starling’s law)
52
Q

Typical venous pressures in the limb vein, heart level

A

5-10 mmHg

53
Q

Typical venous pressures in the central venous

A

0-7 mmHg

54
Q

Typical venous pressures in the foot vein, while standing

A

90 mmHg

55
Q

What happens to veins under low pressure?

A

Veins collapse

56
Q

What happens to veins under high pressure?

A

Veins distend

57
Q

Describe the pressure-volume curve of veins:

A
  • Stimulation of sympathetic nerves causing vasoconstriction shifts blood centrally
  • Increases venous return, CVP and end-diastolic pressure
  • Increased CVP increases preload and so increases stroke volume (Starling’s law)
58
Q

How does Bernoulli’s law explain blood flow?

A

Mechanical energy of flow is determined by: pressure, kinetic, potential energy, (p= fluid mass)

ENERGY= PRESSURE (PV) + KINETIC (PV^2/2) + POTENTIAL (PGH)

Cardiovascular system (16 decks)

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