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Flashcards in The Capillary Deck (14):
1

Blood Fluid Compartments

  • Blood concentrations
  • To reach the intracellular compartment from plasma

  • Blood concentrations
    • Intracellular fluid > interstitial fluid > plasma
  • To reach the intracellular compartment from plasma
    • Molecules must diffuse across the capillary plasma membrane into the interstitial compartment
    • Molecules then diffuse across the cell membrane into the intracellular fluid

2

Capillaries

  • Composition
  • Pericytes
  • Diffusion
  • Aquaporins
  • Transcytosis
  • Fenestrated capillaries

  • Capilalry composition
    • Single layer of endothelial cells
    • Basement membrane
  • Pericytes
    • Small intercellular clefts that separate endothelial cells
    • Elongated, highly branched cells tha tform a meshlike layer b/n endothelium & interstitial fluid
    • Contribute to restricting capillary permeability
  • Exchange of materials through capillaries occurs through diffusion
    • Small, uncharged, lipid-soluble molecules (O2, CO2) pass easily through the capillayr wall
    • Large, charged molecules pass through intercellular clefts or via vesicular transport
  • Aquaporins
    • Water crosses the capillary through intercellular channels & through these specialized water channels in the endothelial cell membrane
  • Trancytosis
    • Large moleucles have a difficult time escaping from capillaries
    • Some capillaries have receptors for particular proteins
    • Once the protein binds, it's carried across the membrane via this process
  • Fenestrated capillaries
    • In some organs, large proteins need to enter or leave circulation
    • Fenestrations (large pores) facilitate this exchange
    • Ex. intestine
      • Contains fenestrated capillaries so that large molecules can be moved from teh GI tract to the bloodstream
    • Ex. blood marrow & spleen
      • Endothelial cells are discontinuous to permit RBCs to enter the circulation

3

Osmolarity

  • Measure of the absolute concentration of osmotically active particles
  • A solution of 1 mole/L of non-dissociable solute = 1 osmole/L (1 Osm)
    • Normal osmolarity of body fluids: 300 mOsms
    • Osmotic pressure = 5107 mmHg (large)
  • Protein: only molecular species for which there's a significant concentration difference b/n the plasma & interstitial fluid
    • All other solutes (Na+, Cl-, HCO3-) are present in ~ equivalent concentrations on both sides of the membrane
    • Differences in protein concentrations = only osmotic driving force at the capillary level
    • Oncotic pressure: osmotic pressure generated by protein = 25-28 mmHg
      • This difference b/n capillary plams & interstitial fluid draws fluid INTO the capillary

4

Hydrostatic Pressure

  • Pressure in the blood imparted by the contraction of the ventricle
  • Higher in the capillary than in the interstitial fluid
    • Forces fluid OUT of the capillary
  • Balance b/n oncotic pressure & hydrostatic pressure across the capillary
    • Determines whether there's a net gain or loss of fluid across the vessel
    • Expressed quantitatively via Starling's equation

5

Starling's Equation

  • Equation
  • Jv
  • Kf
  • σ
  • Other factors
  • Restatement of equation

  • Jv = Kf * ( [Pc - Pi] - σ [πc - πi] )
  • Jv = net fluid movement b/n compartments
    • Filtration: Jv is positive, fluid leaves the capillary
    • Absorption: Jv is negative, fluid enters the capillary
  • Kf = filtration coefficient
    • High: high capillary permeability
    • Low: low capillary permeability
    • Product of capillary surface area & capillary hydraulic conductance
    • Usually constant
    • In the kidney
      • Number of aquaporin channesl in the nephron is dependent on vasopressin levels
      • Low vasopresson --> few squaporins --> low Kf
  • σ = reflection coefficient
    • Correction factor
      • Corrects for ineffectiveness of the oncotic pressure gradient
    • Difference in oncotic pressures contributes to the net driving force
      • Capillaries are fairly impermeable to the large molecular weight proteins
      • Smaller proteins can leak across the membrane through intercellular clefts --> decreases driving force
    • Usually constant from 0 to 1
      • Non-fenestrated vessels: 1
      • Fenestrated capillaries: lower
  • Forces that contribute to the net driving force
    • Pc = capillary hydrostatic pressure
    • Pi = interstitial hydrostatic pressure
    • πc = capillary oncotic pressure
    • πi = interstitial oncotic pressure
  • Jv = (Pc - Pi) - (πc - πi)

6

Significance of Starling's Equation

  • Arterial vs. venule end
  • Net filtration
  • Effect of hematocrit on plasma oncotic pressure

  • Arterial end of a capillary
    • Jv = +8mmHg --> fluid will leave the capillary --> filtration
  • As blood passes along a capillary, it loses hydrostatic pressure due to friction
  • Venule end of a capillary
    • Jv = -7mmHg --> fluid will enter the capillary --> absorption
  • Net filtration = 1mmHg
    • Lose 2-4 L of fluid per day into interstitial space due to the filtration-absorption imbalance
    • Excess fluid lost into the interstitial space is collected by the lymphatic system
  • Changing hematocrit doesn't affect plasma oncotic pressure
    • Blood cells have plasma membranes that place their contents in a separate osmotic compartment from the plasma

7

Factors that alter osmotic relationships

  • Increased venous pressure
  • Hypoproteinemia
  • Increased capillary permeability
  • Decreased arterial pressure

  • Increased venous pressure
    • Increase venous (or arterial) pressure --> increase capillary pressure
      • Greater impact from increased venous pressure b/c resistance b/n arteries & capillaries > resistance b/n veins & capillaries
    • Facilitates movement of fluid out of capillaries (filtration)
    • Ex. stand for a long time
      • Increase venous pressure in feet
      • Additional blood is present in capillary
      • Raises capillary pressure
      • Increases filtration
      • Accumulation of fluid in interstitial space --> edema
  • Hypoproteinemia
    • Starvation, liver disease (protein metabolism deficits), & kidney disease --> protein lost in urine
    • Decreased plasma & interstitial protein --> decreased πc & πi
    • Increased filtration --> edema
  • Increased capillary permeability
    • Releae of histamine from mast cells --> expand gaps b/n endothelial cells in capillary wall
    • Increase Kf --> increase net fluid movement
    • Fluid accumulates in interstitial space --> edema
  • Decreased arterial pressure
    • Hemorrhage --> decreased blood volume --> decreased arterial pressure
    • Baroreceptor reflex --> vasoconstriction to return BP to normal
    • Decreased pressure in downstream capillaries
    • Absorption of fluid from interstitial space
    • Over time, increases blood volume --> increases BP
    • Diluted concentration of RBCs --> decreased hematocrit

8

Lymphatic System

  • Functions
  • Lymph capillaries
  • Throacic duct
  • Where lymphatic system makes connections w/ the CV system

  • Functions
    • Filtration > absorption in capillaries --> net loss of fluid into interstitial space
      • Lymphatic system collects this fluid & returns it to circulation
    • Picks up materials in the liver & intestine
    • Filter: captures & destorys foreign pathogens
  • Lymph capillaries
    • Close to real capillaries in all tissues except CNS & kidney
    • Thin walls of lymph capillaries are held open by attachments to surrounding cells
      • Adjacent cells overlap in lymph capillaires --> provide valves that allow materials to enter (but not leave) interstitial space
    • Coalesce to form larger collecting lymphatics
  • Thoracic duct
    • Largest collecting lymphatic
    • Empties fluid from entire lower body back into the cardiovascular system
  • Where lymphatic system makes connections w/ the CV system
    • Near collarbones
    • Near junction b/n subclavian veins & internal jugular veins

9

Lymphatic System

  • Major force driving fluid to enter lymph capillaries
  • Factors that increase fluid flow into lymph capillaries

  • Major force driving fluid to enter lymph capillaries
    • Interstitial fluid pressure
      • Lymph: name for pressure once fluid is inside the lymphatic system
    • Increase insterstitial fluid pressure --> fluid enters lymph capillaries --> assures all fluid leaving the CV system is returned
  • Factors that increase fluid flow into lymph capillaires (increase interstitial fluid pressure)
    • Increase capillary pressure --> increase filtration
    • Decrease plasma oncotic pressure --> decrease absorption
    • Increaes interstitial fluid protein --> decrease absorption
    • Increase capillary permeability

10

Lymphatic System

  • Movement of large molecules
  • Forces inducing lymph movement

  • Large molecules easily enter lymph capillaries in the liver & digestive system to allow large molecules to enter circulation
    • Pathogens in interstitial fluid also enter the lymphatic system
    • Lymph passes through immunologically-active lymph nodes
  • Forces inducing lymph movement
    • Compression of lymph vessels during muscular contraction
    • Valves in large lymph vessels permit only unidirectional fluid
    • Largest lymph vessels have smooth muscle in the walls that contract automatically when stretched

11

Edema & Elephantiasis

  • Edema
    • When interstitial fluid builds up
    • Often happens after injury
      • Breakage of blood vessels --> increased protein in interstitial space --> pulls fluid from CV system
      • Damaged lymph capillaries --> increased interstitial pressure doesn't lead to increased lymphatic drainage
  • Elephantiasis
    • Lymph vessels blocked by parasites --> compromised fluid drainage
    • Increased protein concentration --> decreased oncotic pressure gradient --> decreased absorption

12

Ramifications of the actions of the lymphatic system

  • Hydrostatic pressure in the interstitial space is slightly negative
    • Due to the presence of the lymphatic system & the movement of fluid into this system
    • As long as the interstitial pressure is negative, no edema will occur
  • Lymphatic system is a conduit for protein to leave the interstitial space
    • W/o the lymphatic system, the oncotic gradient b/n the plasma & interstitial space would equalize
    • Would contribute to the develpoment of edema

13

Changes in plasma osmolarity and cell size

  • Normal capillary permeability
  • Effect of intravenous hypertonic saline solution
  • Effect of intravenous hypotonic saline solution

  • Most capillaries are highly permeable to small ions (Na+, Cl-, water)
    • Normally selective about the ions that enter
    • Maintain low [Na]i through the Na+/K+ ATPase pump
    • Freely permeable to water
  • Effect of intravenous hypertonic saline solution
    • Add ions to intravascular space
    • Increase osmolarity of plasma relative to interstitial space
    • Na+ & Cl- diffuse into the interstitial space while water diffuses into capillaries to balance osmolarity
      • Lose water (by osmosis) from the interstitial space
    • Na+ & Cl- diffuse into the intracellular space while water diffuses into the interstitial space to balance osmolarity
      • Lose water (by osmosis) from the intracellular space
    • Intracellular volume decreases --> cells shrink
      • Given to patients to combat edema to lose fluid from the interstitial & intracellular spaces
  • Effect of intravenous hypotonic saline solution
    • Causes plasma osmolarity < interstitial osmolarity
    • Na+ & Cl- diffuse into capillaries while water diffuses into the interstitial space
      • Water moves into the interstitial space
    • Na+ & Cl- diffuse into the interstitial space while water diffuses into the intracellular space
      • Water moves into the intracellular space
    • Intracellular volume increases --> cells expand/swell
    • Problem: RBCs can swell & burst when plasma osmolarity is suddenly decreased via a hypotonic solution

14

Osmotic relationships in lung capillaries

  • Pressure in lung capillaries < systemic capillaries
    • Interstitial hydrostatic pressure = 0
    • Plasma protein content doesn't change b/n systemic & pulmonary circulation, so πc is the same
  • Differences
    • Interstitial plasma concentration in the lung (πi) is high
    • Rate of water movement through the pulmonary interstitial space is low
  • Pulmonary Jv = 4 mmHg
    • Fluid is lost in the lung into the interstitial space
    • Little fluid remains in the interstitial space due to the lymphatic system
    • Lympahtic system can clear the fluid from the interstitial space as long as capillary hydrostatic pressure remains less than 25 mmHg