The Capillary

Question 1

Blood Fluid Compartments

• Blood concentrations
• To reach the intracellular compartment from plasma

Answer 1

• 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

Question 2

Capillaries

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

Answer 2

• 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

Question 3

Osmolarity

Answer 3

• 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

Question 4

Hydrostatic Pressure

Answer 4

• 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

Question 5

Starling’s Equation

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

Answer 5

• 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)

Question 6

Significance of Starling’s Equation

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

Answer 6

• 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

Question 7

Factors that alter osmotic relationships

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

Answer 7

• 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

Question 8

Lymphatic System

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

Answer 8

• 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

Question 9

Lymphatic System

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

Answer 9

• 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

Question 10

Lymphatic System

• Movement of large molecules
• Forces inducing lymph movement

Answer 10

• 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

Question 11

Edema & Elephantiasis

Answer 11

• 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

Question 12

Ramifications of the actions of the lymphatic system

Answer 12

• 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

Question 13

Changes in plasma osmolarity and cell size

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

Answer 13

• 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

Question 14

Osmotic relationships in lung capillaries

Answer 14

• 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