Physio - Microcirculation Lecture

Question 1

What is Microvasculature? What is the function?

Answer 1

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

Question 2

What is the role of Capillaries?

Answer 2

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

Question 3

Exchange of substances across the capillary wall

Answer 3

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

Question 4

What is the Starling Equation?

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

Answer 4

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

Question 5

Do physiological changes in blood pressure effect filtration?

Answer 5

• 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

Question 6

What is the importance of Lymphatic Vasculature?

Answer 6

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

Question 7

Explain Pulmonary Edema Formation

Answer 7

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

Question 8

Explain Ankle Edema in Pregnancy

Answer 8

Basics:

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

Results:

• Ankle edema due to external pressure

Question 9

Explain ascites as a result of Alcoholism

Answer 9

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

Question 10

Explain Edema in child with Malnourishment

Answer 10

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

Question 11

Explain Edema in (Congenital) Nephrotic syndrome

Answer 11

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