Lecture 19: Microcirculation Flashcards

1
Q

What is transmural pressure?

A

The pressure across a vessel wall

P = Pi-Po

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2
Q

What is transmural pressure important for?

A

Determining the tension of the blood vessel wall

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3
Q

What is wall tension dependant on?

A

Laplace

  • Radius
  • Pressure
  • 1/wall thickness
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4
Q

What is the implication of the laplace law?

A

The larger the vessel radius, the greater the wall tension required to withstand an internal pressure

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5
Q

What is an aneurysm and what does it lead to?

A
  • Thinning of vessel wall
  • Increased lumen radius
    = Wall tension increases
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6
Q

What is compliance?

A

The stretchability at various points along the P/V curve i.e the local change in volume for a given pressure

Reflects the elasticity of the vessel. (veins are more complicated than art.)

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7
Q

What is capacitance of blood vessels?

A

The measure of the volume to pressure relationship over the entire P/V curve. Reflects the storage capacity of the vessels.

i.e Change in volume / change in pressure

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8
Q

Describe the structures of capillary beds;

A

Terminal arteriole
- Branching true capillaries with pre-capillary sphincters

Post-capillary venule

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9
Q

Describe the precapillary spincters

A

Cuffs of smooth muscle (pre-capillary sphincters)

open = blood to true capillarys

closed = blood flows through meta-arteriole throughfare channels

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10
Q

What acts as the precapillary resistance vessels?

A

Arterioles, metarterioles, precapillary sphincters act together as the pre-capillary resistance vessels

i.e parrallel

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11
Q

Whats the width generally of a capillary?

A

1 RBC

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12
Q

What is the permeability of cap like?

A

Varying depending on tissue (Covered elsewhere)

i.e liver - highly permable

muscle, skin, heart - low

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13
Q

What are the intrisinc mechanisms of microcirculation flow control?

A

Metabolic control

  • pH
  • O2
  • CO2
  • K
  • NO

Myogenic control

  • Enothelins
  • Stretch
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14
Q

Describe the metabolic autoregualtion of blood flow:

A

Factors promote dilation include:

  • Reduced tissue oxygen demand
  • Inflammatory molecules i.e histamine
  • NO
  • K or H+ from intersitial fluid
  • Lactic acid or others
  • Increased CO2
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15
Q

What is reactive hyperemia and what is it driven by?

A

Restoration of blood flow after brief occlusion results in flow exceeding pre-occlusion levels for a period of time proportional to the duration of occlusion

Driven by metabolites and entirely localised

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16
Q

What is the definition of autoregulation?

A

Intrinsic capacity to maintain constant blood flow despite changes in perfusion pressure

In many tissues (I.e brain) blood flow is near constant over a wide range of pressures), less important in other tissues i.e skin

17
Q

What is myogenic autoregulation?

A
  • Myogenic mechanisms are intrinsic to the smooth muscle bed

- Stretch b/c inc BP = Stretch sensitive Ca channels opening and depolarisation of cells (vasoconstrictio)

18
Q

What are the three cap. transport mechanisms?

A
  1. Diffusion, through membrane, dependant on [Conc]
  2. Filtration, depnd on pressure gradient
  3. Large molecule movement i.e pinocytosis or via fenestrations
19
Q

What is diffusion dependant on?

A
  1. Concentration gradient (flow dependant) and ability to cross membrane i.e lipid soluble or via endothelial process

Described by Ficks law.

  • Membrane thickness
  • Diffusable area
  • Free diffusion co-eficient
20
Q

Describe filtration:

A

The bulk flow of fluid across membrane
- Pressure dependant (hydrostatic and osmotic)

Starlings law of ultrafiltration

21
Q

Describe starlings law of ultrafiltration:

A

Fluid movement (FM) = K [ (Pc-Pi) - Colloid osmotic]

k = permeability
Pc = Capillary hydrostatic pressure
Pi = Interstitial hydrostatic pressure
Colloid osmotic pressure (some proteins cant leave blood and exert pressure)

22
Q

What influences cap. pressure?

A

Inc. cap pressure = more filtration
- Increased art. or venous pressure increases Pc

Increasing resistance

  • Increasing upstream resistance decreases Cap pressure
  • Increasing downstream resistance increases Cap pressure.
23
Q

How does vasodilation and venous pressure influence filtration?

A

Vasodilation = Increased Cap hydrostatic pressure = more filtration

Increased venous pressure i.e heart failure = Results in an increase in cap hydrostatic pressure at venous end = reduced reabsorption

24
Q

What happens in heart failure at a microcirculation level?

A
Reduced CO
- Poor peripheral perfusion = neural and hormonal adaptations
- Fluid retention
- Increased EDP
- Increased central venous pressure
- Increased Cap resistance
- Increased net filtration
= Oedema
25
Increased oncotic pressure leads to;
Reduced filtration
26
How does increased oncotic pressure decrease filtration?
Decreasing fluid content or increasing large protein content of blood will increase the colloid osmotic (oncotic) pressure within the cap., sucking fluid back into the capilliary. i.e in dehydration
27
What is the function of lymphatic vessels?
1. Return blood components to circulation 2. Absorption from gut 3. Removal of RBC's from tissue 4. Immunological - Removal/isolation of bacteria
28
Describe lymph flow:
Terminal lymphatics = Endothelium + highly permeable basement membrane - Vlaves to prevent backflow - Vasomotion pushes lymph - Fluid flow into primary lymph vessels requires interstitial hydrostatic pressure to exceed the lymph pressure. Localised swelling indicates that rate of lymph flow is at its max.