Physio - Microcirculation Lecture Flashcards

1
Q

What is Microvasculature? What is the function?

A

Basics:

  • Arterioles
  • Capillaries
  • Venules

Function:

  1. Preserve arterial blood pressure
    • Arterioles are normally partially constricted
      • Provide MOST vascular resistance
  2. Allow sufficient perfusion of tissue
    • cap & venules exchange nutrients/water/gas/hormones/waste

Notes:

  • Virtually all cells in body are close to a microvessel
  • End-to-end, they reach about 3x around Earth
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2
Q

What is the role of Capillaries?

A

Basics:

  • Primary site for water & solute exchange
    • venules also permeable
  • Contribute 10-15% of TPR
    • diameters do not change significantly

​Components:

  • Made of epithelial cells that role –> tube
    • surrounded by basement membrane (3)
  • Pericyte (Rouget cells = 1,2)
    • primitive form of vascular smooth muscle
    • add structural integrity to capillaries

Notes:

  • Cap diameters can be SMALLER than RBCs
  • Systemic h_emodynamic laws dont apply_
    • viscosity = decreased when blood flows thru a small diameter vessel
      • Lindqvist Effect
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3
Q

Exchange of substances across the capillary wall

A

Basics:

  • Exchange mechanisms = highly variable, but primarily due to structural differences
    • NOT due to neuronal or hormonal reg

Methods of Exchange:

  1. Lipid-soluble molecules
    • pass via simple diffusion
    • ex: O2 & CO2
  2. Small water-soluble molecules (3-6nm)
    • diffuse through water-filled pores/clefts w/in tight junctions
    • ex: glucose & AAs
  3. Some large water-soluble substances
    • ​pass via few large pores, or pinocytosis
    • ex: complement can
  4. Many large water-soluble substances
    • do not pass/are retained in blood
    • ex: albumin & Abs

Notes:

  • Size & number of pores vary
    • few & small = brain & spinal cord
      • blood-brain barrier
    • many & large = liver/GI/kidney
      • fenestrated epithelia or sinusoids
  • Pores may contain fibers = act as filters
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4
Q

What is the Starling Equation?

How does it help us predict the exchange of fluid across the capillary wall?

A

Starling Equation:

  • Jv = Kf [(Pc-Pi) - (πc - πi)]
    • Jv = net fluid flux
    • Kf = filtration coefficient
    • Pc = cap hydrostatic pressure
    • Pi = interstitial hydrostatic pressure
    • πc = cap oncotic pressure
    • πi = interstitial oncotic pressure
  • basically… (hydrostatics - oncotics)
    • (+) = moves out
    • (-) = moves in

Note:

  • Pc & πc = primary forces that determine cap fluid filtration/absorption
    • ​Pi & πi = minor forces
  • Oncotic pressure = “moves in”
    • determined by proteins (ie. albumen)
  • Hydrostatic pressure = “moves out”
    • determined by fluid volume?
  • 1/10 of fluid enters lymphatic vessels
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5
Q

Do physiological changes in blood pressure effect filtration?

A
  • Physiological changes in BP have LITTLE effect on filtration
    • due to compensatory mechanism
    • Kf does not change much
  • Path-physiological changes in BP can have FAST & devastating effect for cap fluid exchange
    • in hypoxia (& more extreme in burns), Kf ↑
      • pores open up & liquid leaves

Example:

  • When arterial pressure falls after standing
    • ↓ cap blood flow (local hypoxia, ↑ metabolites)
    • ↓ precap resistance
    • ↑ Pc
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6
Q

What is the importance of Lymphatic Vasculature?

A

Basics:

  • Lymph vessels have blind-ended lymphatic bulbs
    • movement into bulbs = passive
  • When fluid outside cap pushes against overlapping cells, the swing slighly inward
  • Compression/relaxation cycles create very l_ow intrabulbular pressure_
    • lymph is “sucked up”
      • flow from high pressure –> low pressure

Function:

  1. Collects ~3L of lymph each day –> returns it to blood
  2. Critical to clear large proteins from interstitial space
  3. Carry fat/digestion products to the liver

Apparent Flow of Intralymphatic pressure from low –> high

  • Explained by:
    1. Contraction of intrinsic smooth muscle
      • as part of larger lymphatic vessels
    2. Contracting of extrinsic skeletal muscle
      • compress the lymph vessels
    3. Lymph valves = help avoid back flow
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7
Q

Explain Pulmonary Edema Formation

A

Basics:

  • Back-up of fluid in the pulmonary vein —>
  • Increased pressure in the L.A.

Result:

  • Pulmonary cap hydrostatic pressure > Pulmonary cap oncotic pressure
    • leads to pulmonary edema
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8
Q

Explain Ankle Edema in Pregnancy

A

Basics:

  • Elevated venous pressure –> increased filtration beyond capacity of lymph system to remove it

Results:

  • Ankle edema due to external pressure
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9
Q

Explain ascites as a result of Alcoholism

A

Basics:

  • Increase in hepatic vascular resistance (Cirrhosis) –> raises portal venous pressure & cap pressure in splanchnic circulation

Results:

  • Ascites (due to fluid moving out)
    • accumulation of fluid in the peritoneal cavity
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10
Q

Explain Edema in child with Malnourishment

A

Basics:

  • Severely decreased plasma protein concentration
    • ie: low albumen
  • Cap hydrostatic pressure = unopposed
    • (super low oncotic pressure)

Results:

  • Edema
    • fluid moving out
  • Kwashiorkor
    • extended belly
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11
Q

Explain Edema in (Congenital) Nephrotic syndrome

A

Basics:

  • Proteins lost in urine –> decreased plasma protein concentration
  • Cap hydrostatic pressure = unopposed
    • (super low oncotic pressure)

Results:

  • Edema
    • low oncotic pressure cannot hold liq in blood vessels
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