3 - Microcirculation Flashcards
What following substances move across capillary endothelia using the following:
Diffusion
Bulk Flow
Vesicles
Active Transport
Diffusion - O2, CO2, lipid-soluble substances (lipid bilayer!)
Bulk Flow - H20, Electrolytes, small molecules
Vesicles - Macromolecules
Active Transport - Ions, small molecules
Equation: What law govern’s movement of a substance via diffusion?
What is the equation?
Fick’s First Law of Diffusion (Gases, Lipid-Soluble substances)
J = DA(ΔC/ΔX)
J = Flux (moles/sec)
D = Diffusion constant of specific substance
A = Surface area available for diffusion
ΔC = Concentration gradient
ΔX = Diffusion distance
What are the three types of capillaries?
Continuous
Fenestrated
Discontinuous
Where does bulk flow occur?
What substances use this method?
Intercellular clefts between endothelial cells
Water, Lipid-soluble, electrolytes
Continuous Capillaries
Have very tight endothelium and continuous basement membrane
Reduces bulk flow across capillar
Found in Skeletal muscle, skin, lung, and brain
Fenestrated Capillaries
Have perforations (fenestrae) in the endothelium, resulting in relatively high permeability and bulk flow
Example: Exocrine glands, renal globeruli, intestinal mucosa
Discontinuous Capillaries
Large intercellular gaps, as well is gaps in basement membrane–highest permeability
Liver, Spleen, Bone Marrow
What two types of transport likely play a smaller role in movement of substances across capillaries?
Vesicular Transport (proteins), Active Transport (ions, glucose, amino acids)
What law governs oxygen diffusion?
How can you increase this rate?
Fick’s Law of Diffusion: J = DA (ΔC/ΔX)
You can increase capillary blood pO2 (breathe pure oxygen)
You can decrease tissue pO2
You can increase the surface area available for oxygen diffusion (increasing number of flowing capillaries)
What law governs carbon dioxide diffusion?
How does it compare to oxygen?
What does the removal of CO2 from the tissue primarily depend on?
J = DA(ΔC/ΔX)
Diffusion constant 20x > Oxygen
Removal depends on blood flow
Transcapillary Fluid Exchange:
Fluid Filtration / Reabsorption
Lymph Flow
What does total filtration equal?
A steady state of volume is filtered to interstitium, and returned to vasculature (reabsorbed)
Anything left over is taken up by terminal lymphatics, and transport them to larger lymphatics
Filtration = Reabsorption + Lymph Flow
What physical mechanisms govern fluid exchange?
What are the Starling forces?
Hydrostatic and Oncotic Pressure
- Capillary Pressure (Pc) - tends to force fluid outward
- Interstitial Fluid Pressure (Pi) - Tends to force fluid in through cap. membrane
- Capillary Plasma Colloid Osmotic (oncotic) Pressure (πc) - Cause osmosis of fluid inward through cap membrane
- Interstitial fluid colloid osmotic (oncotic) pressure (πi) - Tends to cause osmosis of fluid outward through cap membrane
Summary:
Pc/πi = Out
Pi/πc = In
What is the reflextion coefficient?
Factor to reprsent the permeability of a capillary to the proteins respnsible for generating oncotic pressure
1 = Impermeable
0 = Freely permeable
0 > a > 1 ; Somewhat permeable
Equation: What is method to calculate net fluid flux?
J = Kf x A (NDF)
NDF = (Pc-Pi) - a(πc-πi)
J = Net flux
Kf (constant) = Capillary filtration constant (permeability)
A = Surface area for exchange
NDF = driving force (4 components + constant)
How does filtration constant vary among the different types of capillaries?
What effect does histamine hace on KF?
Discontinuous > Fenestrated > Continuous
Increase
What determines capillary hydrostatic pressure? (Pc)
How do changes in this effect capillary hydrostatic pressure?
Arterial/Venous Pressure, Precapillary (arterioles) and Postcapillary (venules) Pressure
Increase in Arterial/Venous Pressure = Increase Capillary Hydrostatic Pressure
Increase in Arteriolar Resistance reduces capillary pressure; Increase in Venule Resistance increases capillary pressure
(like a hose, blocking the begininng lowers pressure, blocking the end builds pressure)
Provide The Effect on Capillary Pressure of the Following, and provide examples:
Arteriole Dilation
Arteriole Constriction
Venous Dilation
Venous Constriction
Increased Arterial Pressure
Decreased Arterial Pressure
Increased Venous Pressure
Decreased Venous Pressure
Arteriole Dilation - Increase - Decreased sympathetics (parasympathetics increase)
Arteriole Constriction - Decrease - Increased Sympathetics
Venous Dilation - Decrease - Increased metabolism of tissue
Venous Constriction - Increase - Physical compression
Increased Arterial Pressure - Increase - Increase C.O.
Decreased Arterial Pressure - Decrease - Hemorrhage/dehydration
Increased Venous Pressure - Increase - Congestive heart failure (blood backing up, not being moved from venous circulation)
Decreased Venous Pressure - Decrease - Hemorrhage/dehydration
(Equation) What is interstitial compliance?
What are examples of high vs low compliance?
High = Subcutaneous Tissue
Low = Brain (your brain can’t swell!)
C = ΔV / ΔP
Or–slope of these two plotted
ΔPi = ΔVi / C
ΔP = Change in interstitial fluid pressure
ΔV = Change in interstitial fluid volume
C = Interstitial Compliance
(C = ΔV / ΔP )
What determines capillary plasma oncotic pressure? (piC)
What determines tissue (interstitial) fluid oncotic pressure? (pi-i)
Presence of proteins within the capillary
Albumin generates majority
Interstitial protein concentration
What type of substances can enter lymphatic capillaries easily?
High MW, like proteins
“Saloon-Door” one way valves allow this to occur
What factors can increase lymph flow?
What slows this down?
Anything that increases interstitial fluid pressure
- Elevated capillary pressure
- Decreased plasma colloid osmotic pressure
- Increased interstitial fluid colloid osmotic pressure
- Increased permeability of the capillaries
- -
Increased compression from more fluid impedes lymph flow
What is the Lymphatic Pump?
Lymph vessels beyond the terminal lyphatic capillaries surrounded by smooth muscle.
When stretched, the smooth muscle automatically contracts
Major intrinsic for propelling lymph
How can external intermittent compression effect lymphatics?
Pumping
Contraction of muscles, movement, pulsations of arteries, compression of tissue from outside
very active during exercise
Clinical: Edema Formation
When fluid in interstitial compartment increases beyond filtration/reabsorption rate
Can result in tissue death, impair gas exchange
What are four major categories which cause edema?
- Increased Capillary Pressure (heart failure, increased blood volume, thrombosis, gravity, hypertension, vasodilation)
- Increased Capillary Permeability (vascular damage, burns, trauma)
- Decreased Plasma Oncotic Pressure (Malnutrition)
- Lymphatic Blockage (Lymphedema) (Tissue injury, parasites)
Clinical: Elephantiasis
Parasites block lymph collection
