W7 - Microcirculation, lymph (2.10) Flashcards Preview

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Flashcards in W7 - Microcirculation, lymph (2.10) Deck (45):

Describe the general structure of a capillary bed.

How is the blood flow in such a capillary bed regulated?

arterioles → capillaries → venules

smooth m. precapillary sphincter (btw arteriole + capillary) + arteriole regulate blood flow by contraction and relaxation


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What are the 2 roles of microcirculation?

  • nutritive: metarterioles
  • non-nutritive: shunt (e.g. in glomeruli of kidney, blood flow through skin for T regulation, etc.)


What is a thoroughfare channel?

channel btw arteriole and venule, shortcut through capillary network (e.g. metarteriole)

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Differentiate btw the 3 types of capillaries.

Give one example for each type.

  • continuous capillaries: most common type, interendothelial junctions 10 - 15nm, e.g. in skeletal m.
  • fenestrated capillaries: perforated endothelium (50 - 80nm holes), e.g. in small intestine
  • discontinuous capillaries (= sinusoids): fenestrations + large gaps (100 - 1000nm) in btw cells, e.g. liver sinusoids

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Which structures form tight junctions btw endothelial cells?

Where can they characteristically be found?

claudins 1, 3, 5 + occludin

e.g. in CNS → blood-brain barrier
(esp. CLDN 5 + occludin)



What are the 4 routes of transport across a capillary?


  • transcellular route
  • small-pore paracellular route
  • large-pore paracellular route
  • transcytosis


What is the mechanism of transcellular transport?

Which molecules undergo this kind of transport?

by simple diffusion

  • gases
  • lipid soluble molecules (anesthetics)
  • water (via aquaporin-1)


Describe the model of August Krogh's tissue cylinder.

single capillary supplies cylinder-shaped volume of a tissue, many cylinders adjacent to each other resemble capillary bed in tissue

⇒ predicts how [O2] within capillary lumen falls along the length as O2 exits
→ highest at arterial end, lowest at venous end

describes transcellular transport of gases in capillaries

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Which factors determine the [O2] along the length of a capillary?

  1. concentration of free O2 in the arteriolar blood + O2 content of the blood (usually 20%)
  2. radial diffusion coefficient (how fast O2 diffuses out of capillary lumen)
  3. dimension of capillary (r, l) and tissue cylinder (r)
  4. capillary blood flow
  5. O2 consumption

BUT: capillary flow + metabolic demand are the most important ones


Describe how [O2] changes in response to metabolic demand and capillary flow.

  • ↑ flow → more O2 → smaller fraction of [O2] needed to satisfy tissue's needs
  • ↑ metabolic demand → tissue extracts more O2


What does the radius of Krogh's tissue cylinder determine?

Physiological consequence?

capillary density
the larger the cylinder, the more distant the next capillary

⇒ during increased O2 consumption (e.g. exercise) arterioles + precapillary sphincters dilate to open all capillaries → reduce radius of Krogh cylinder

NOTE: in resting state only 20% of all capillaries open

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What is the mechanism of small-pore paracellular transport?

Which molecules undergo this kind of transport?

by diffusion through intercellular clefts, gaps, fenestrae

  • water soluble molecules
  • small polar molecules (less than albumin size)

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Which law describes the flux of paracellular transport?


Fick's law of diffusion
solute flux that crosses a distinct capillary surface is proportional to the conc. difference across the wall + the permeability for this solute

J = -PS (Coutside - Cinside)

  • J = solute flux
  • = capillary permeability
  • = capillary surface
  • Coutside - Cinside = concentration difference


What is the difference btw flow- and diffusion limited transport?

  • flow-limited transport: only blood flow restricts rate of diffusion due to rapid equilibration of small molecules across the capillary
  • diffusion-limited transport: diffusion is restricted if no equilibrium across the capillary (?)


What is the mechanism of large-pore paracellular transport?

Which molecules undergo this kind of transport?

very slow diffusion through large pores

  • protein-sized molecules


What is the mechanism of transcytosis?

Which molecules undergo this kind of transport?

vesicular transport

  • translocation of macromolecules


What is convection?

Another name.

also: solvent drag

water can cause diffusion of dissolved solute together with own bulk movement through aquaporin 1



What are the 2 driving forces of convection?

  • hydrostatic pressure difference (ΔPc-if):
    difference btw intra- and extravascular pressure
  • oncotic pressure difference (Δπc-if):
    difference btw intra- and extravascular osmotic pressure caused by plasma proteins 

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Which formula is used to determine whether there is a net fluid movement into or out of the capillary?

What does the plus or minus sign indicate?


Starling equation
if Jv > 0 → filtration (out of cap.)
if Jv <  0 → absorption (into cap.)

Jv = Kf [(Pc - Pi) - σ(πc - πi)]

  • Jvfluid flow
  • Kf = filtration coefficient
  • Pc = capillary hydrostatic pressure
  • Pi = interstitial hydrostatic pressure
  • σ = reflection coefficient
  • πc = capillary oncotic pressure
  • πi = interstitial oncotic pressure

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What is the unit of the filtration coefficient?

What does it describe?

Another name.

also: hydraulic conductance

describes water permeability of capillary wall by ensemble of AQP1 channels and paracellular pathway

in [ml/min * mmHg]


What does the reflection coefficient describe?


how a semipermeable barrier reflects solute X as water moves across the barrier, btw 0 and 1

  • 0 = solute does not cause osmotic pressure, moving perfectly with water across the membrane
  • 1 = solute cannot cross the membrane, causes osmotic pressure



What does Pc describe?


What is its effect?


capillary hydrostatic pressure
35 mmHg arteriolar end → 15 mmHg venous end



How can it Pc changed?

What is its effect?

↑Part, ↑Pven, arteriolar dilation, venous constriction
→ ↑ Pc

⇒ the greater Pc, the greater filtration
(bc fluid is forced into interstitium)

NOTE: ↑ Pven increases Pc more than ↑ Part does


What does Pi describe?



interstitial hydrostatic pressure
0 mmHg

  • usually slightly negative bc of fluid removal by lymph
  • in encapsulated organs slightly positive due to expansion of high-P vessels pushing interstitial fluid agains capsule


How does Pi have to be changed in order to increase filtration?

⇒ the smaller Pi, the greater filtration
(bc effect of fluid forced into interstitium is increased)


What does πc describe?


capillary oncotic pressure
∽ 25 mmHg in normal plasma with standard mixed protein concentration of 7 g/dl

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How can πc be changed?

What is its effect?

↓ [protein] → ↓ πc

⇒ the smaller πc, the greater filtration
(bc less water is retained by proteins in capillary)

NOTE: small solutes do not contribute to πc


What would be causes for an increase or decrease in πc?

  • increase: dehydration (rel. [protein] in blood is incr., due to decr. volume)
  • decrease: nephrotic syndrome, protein malnutrition, liver failure (rel. [protein] in blood is decr.)


What does πi describe?


interstitial oncotic pressure
same as lymph, varies from tissue to tissue, 0 - 10mmHg

BUT: increases along axis of capillary, lowest near arteriolar end (bc only protein-free filtration)



How can πbe changed?

How does πi have to be changed in order to increase filtration?

inadequate lymphatic function → ↑ π

⇒ the larger πi , the greater filtration
(bc more fluid attracted by accumulating proteins into interstitium)


Illustrate how the individual and net pressures change along a capillary.

  • Pc
  • Pi = constant
  • πc = constant
  • πi ↑

⇒ filtration at arteriolar end, absorption at venous end

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Describe the features of the revised model for fluid exchange across continuous endothelia w/ interendothelial junctions.


  • primary barrier for oncotic pressure is not capillary, but only glycocalyx overlying paracellular clefts
  • glycocalyx not in direct contact w/ interstitium, but subglycocalyx fluid (2nd compartment)
  • intercellular cleft (3rd compartment), interstitium (4th compartment)

BUT: similar to Starling model when 0 net fluid movement


Describe ultrafiltration w/r/t the revised model of fluid exchange.

Psubglyco > Pcleft and Pi

→ J = positive = ultrafiltration at arteriolar end of capillary into interstitium

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Describe absorption w/r/t the revised model of fluid exchange.

How do you call the observable mechanism?

J = negative = absorption
H20 + small solutes from subglycocalyx space to cap. lumen

concentrating protein in subglycocalyx space
→ incr. πsg opposes further absorption + quickly brings it to a halt

⇒ osmotic assymetry/rectification

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What is the function of lymph?

normally filtration > absorption

excess filtered fluid + leaked proteins returned to circulation via the lymph 



At which rate is lymph produced?

How much of the total filtrate becomes lymph?

120 ml/h → 2-4 l/d

1/10 of filtration → lymph


Describe the structure of the interstitium.

2 phases:

  • proteoglycan mesh traps H20 molecules → tissue gel / gel phase
  • small free fluid vesicles (sol phase

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Explain the general structure of lymphatics.

thin-walled endothelial channels similar to capillaries, endothelial cells overlap to form primary lymph valves

anchoring filaments tether lymphatics to interstitium


How does the interstitium respond to an increased filtration load?

↑ filtration → ↑ Pi

  • part of fluid taken up by gel → expansion - atmospheric P level (then primary lymph valves open + fluid leaves into lymphatics)
  • larger part accumulates as free fluid vesicles → rapid V incr. > atmospheric P level 

NOTE: if too accumulation > lymph flow → edema

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Describe the lymph flow through the body, listing the different types of lymph vessels.

  1. interstitium → initial lymphatics w/ primary lymph valves
  2. fuse to collecting lymphatics w/ secondary lymph valves 
  3. eventually drain into v. subclava dex./sin.

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How does the pressure in initial lymphatics differ from that in collecting lymphatics?

  • initial lymphatics: -1 - 1mmHg
  • collecting lymphatics: 1 - 10 mmHg


Lymph flow is unidirectional.


primary lymph valves permit interstitial fluid to enter, secondary lymph valves prevent retrograde lymph flow, then pushed forward by sk. muscle contraction


Lymph flow depends on... ?

depends on Pi
↑Pi → ↑lymph flow


What is an edema?


occurs when volume of interstitial fluid exceeds capacity of lymphatics to return it to the circulation

due to excess filtration/blocked lymphatics (e.g. by tumors, parasites)


What are the effects of histamine?

  • arteriolar dilation
  • venous constriction

⇒ large ↑ Pc + local edema