Physio - Microcirculation Lecture Flashcards
What is Microvasculature? What is the function?
Basics:
- Arterioles
- Capillaries
- Venules
Function:
-
Preserve arterial blood pressure
- Arterioles are normally partially constricted
- Provide MOST vascular resistance
- Arterioles are normally partially constricted
-
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
What is the role of Capillaries?
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
- viscosity = decreased when blood flows thru a small diameter vessel
Exchange of substances across the capillary wall
Basics:
- Exchange mechanisms = highly variable, but primarily due to structural differences
- NOT due to neuronal or hormonal reg
Methods of Exchange:
-
Lipid-soluble molecules
- pass via simple diffusion
- ex: O2 & CO2
-
Small water-soluble molecules (3-6nm)
- diffuse through water-filled pores/clefts w/in tight junctions
- ex: glucose & AAs
-
Some large water-soluble substances
- pass via few large pores, or pinocytosis
- ex: complement can
-
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
- few & small = brain & spinal cord
- Pores may contain fibers = act as filters
What is the Starling Equation?
How does it help us predict the exchange of fluid across the capillary wall?
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
Do physiological changes in blood pressure effect filtration?
-
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
- in hypoxia (& more extreme in burns), Kf ↑
Example:
-
When arterial pressure falls after standing
- ↓ cap blood flow (local hypoxia, ↑ metabolites)
- ↓ precap resistance
- ↑ Pc
What is the importance of Lymphatic Vasculature?
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
- lymph is “sucked up”
Function:
- Collects ~3L of lymph each day –> returns it to blood
- Critical to clear large proteins from interstitial space
- Carry fat/digestion products to the liver
Apparent Flow of Intralymphatic pressure from low –> high
- Explained by:
- Contraction of intrinsic smooth muscle
- as part of larger lymphatic vessels
- Contracting of extrinsic skeletal muscle
- compress the lymph vessels
- Lymph valves = help avoid back flow
- Contraction of intrinsic smooth muscle
Explain Pulmonary Edema Formation
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
Explain Ankle Edema in Pregnancy
Basics:
- Elevated venous pressure –> increased filtration beyond capacity of lymph system to remove it
Results:
- Ankle edema due to external pressure
Explain ascites as a result of Alcoholism
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
Explain Edema in child with Malnourishment
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
Explain Edema in (Congenital) Nephrotic syndrome
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