Flashcards in L18 – Capillary and Lymphatic Function Deck (50):
How is capillaries' walls specialized for its function?
Very thin wall= exchange of substances between blood and tissues via extracellular /
What are the 2 main mechanisms by which substances cross the capillary wall? Are they active or not?
Diffusion and Filtration
What does the Amount of diffusion of a substance depends on?
Size of concentration gradient across capillary wall only
How does available surface area for diffusion differ between water-soluble and lipid-soluble substances?
Water-soluble substances: has to be small -- Diffuse through pores in intercellular material or protein carrier
Lipid-soluble substances can be large – can diffuse through cells in capillary wall or pores in intercellular material
What is Net driving force for filtration?
algebraic sum of hydrostatic AND oncotic forces across capillary wall
What is the equation for fluid movement?
Fluid movement = k [(Pc + πi) – (Pi + πc)]
According to equation for fluid movement, what movement happens if the result of equation is positive/ negative?
+ve means filtration
-ve means absorption
What does (Pc + πi) mean?
Filtration force - push fluid out of capillary
What does (Pi + πc) mean?
Absorption force - push fluid into capillary
What is k in fluid movement equation?
k = filtration constant for the capillary membrane = Permeability (number of pores in capillary wall) x Surface area
What is Pc?
capillary hydrostatic pressure = intravascular pressure = main filtration / driving
force pushing water out of capillary
What does Pc depend on?
Depends primarily on volume of blood inside capillaries, which depends on:
Arterial and venous pressures
Ratio between post: pre capillary resistance (diameter)
How does the pressure gradient vary across a capillary bed?
Pressure gradient: Pa (arteriole) > Pc (capillary) > Pv (vein)
Drops along length of capillary
What is the capillary pressure equation?
Pc= [(Rv/Ra)Pa + Pv ] / [1+(Rv/Ra)]
Pa = arterial pressure
Pv = venous pressure
Ra = precapillary resistance
Rv = postcapillary resistance:
How does arterial and venous pressure compare? What figures for human skin?
arterial pressure (similar for most tissues): 32 mmHg in human skin
venous pressure (relatively low): 15 mmHg in human skin
What happens to blood in capillary and Pc if Pv increases?
Increase Pv > more blood retained in capillaries > increase Pc
What happens to blood in capillary and Pc if Pa increases?
Increase Pa > more blood to capillary > increase Pc
What controls increase and decrease in Pc?
Ra = precapillary resistance: controls entry of blood from artery
Rv = postcapillary resistance: controls exit of blood to vein
If Ra increase, what happens?
more blood in artery, less in capillary = lower Pc
If Rv increase, what happens?
more blood retained in capillaries = increase Pc
Does pre- and post-capillary resistance change in different organs?
e.g. Liver = 6-7 mmHg
Renal glomerulus = 70 mmHg
What pressure opposes filtration?
Pi = interstitial fluid hydrostatic pressure
What does the sign of Interstitial pressure say about the tissue surrounding the organ?
Always very low value (can be positive / negative):
- Brain / kidney: +1~2 mmHg (absorption)
- Subcutaneous tissues / muscle: –3~–7 mmHg (filtration)
+ve value means organ is surrounded by indistendible tissue
What does negative Pi mean?
Negative pressure involves removal of fluid from interstitial space by Lymphatics or Reabsorption into capillaries
What is the other major pressure involved in filtration apart from hydrostatic pressure?
Oncotic pressure = colloid osmotic pressure
What is oncotic pressure similar to?
Similar to osmotic pressure: tend to equal solute concentration across semipermeable membrane
What is the only cause of oncotic pressure?
only non-diffusable (large) molecules contribute to oncotic pressure
Does glucose contribute to oncotic or osmotic pressure?
glucose is small enough to travel through capillary pores > contributes to osmotic pressure but
What is πc ? How about πi?
πc= capillary (plasma) oncotic pressure
πi= interstitial fluid oncotic pressure
What does capillary (plasma) oncotic pressure Arises from?
Arises from plasma proteins (too big to travel through capillary pores)
What does πc/ capillary (plasma) oncotic pressure do?
main reabsorption force trying to draw water back into capillaries
What are the two main plasma proteins contributing to capillary oncotic pressure?
(65% albumin, 15% globulin)
Which of the two oncotic pressures is higher?
πc ~25 mmHg
πi ~0.1-5 mmHg
What causes interstitial oncotic pressure?
Arises because a little plasma albumin leaks into interstitial space (e.g. albumin that can only leak out)
Depends on capillary leakiness
What is Starling's hypothesis on capillary fluid movement?
At arterial end of capillary: Pc is high >filtration occurs (positive sum)
At venous end: Pc is lower > net reabsorption occurs (negative sum)
amount of fluid filtered = amount of fluid reabsorbed
Does Starling/s hypothesis show the formation of interstitial fluid?
Postulated to be the mechanism by which interstitial fluid is formed
Does an idealized balance of filtration and reabsorption apply to all tissues? Name one organ that does, and two that doesnt have this balance.
Balance in human skin
Renal glomerulus: high Pc, only filtration occurs
Lungs: right heart operates at low P > very low Pc
in lungs > only absorption occurs
Which pressure in Starling's hypothesis stay constant?
Colloid osmotic/ oncotic pressure
Assume little to no leakage of plasma protein
Hydrostatic pressure> oncotic pressure = movement out of capillary ... etc
How can Disturbances in fluid pressures change fluid movement?
Any change in Pc or πc = change in balance between filtration and reabsorption
remember Fluid movement = k [(Pc + πi) – (Pi + πc)]
How does congestive heart failurert cause increase capillary filtration?
Congestive heart failure: failing heart pumps out less blood >
Increase residual volume of heart >
Increase right atrial pressure>
DECREASE venous return > Increase venous pressure >
Increase capillary pressure (more blood retained in capillary) >
Increase Capillary filtration
How does Starvation or nephrosis change fluid balance?
Starvation and Nephrosis>
Less synthesis of plasma protein/ Plasma protein lost through urine >
Decrease plasma pretein concentration>
Less capillary oncotic pressure >
Less reabsorption >
Filtration exceeds reabsoprtion >
How does lymphatics relate to interstitial fluid?
Comprise a secondary drainage system for interstitial space
What is the role of lymphatic system?
Clearance of excess fluid, protein, lipid, foreign materials from extracellular space
How does lymphatic flow compare to cardiac output?
Lymph flow = 2-4 L/day
cardiac output = 5.5 L/min
What does obstruction of lymphatics cause?
What can Left or right sided congestive heart failure be distinguished? (which body parts have oedema) Think of where the vein leading to that side of heart is from.
Right = swelling of hands and legs
Left = Pulmonary oedema
Where do lymph vessels start form? (which end of the capillary bed?)
Commence from blind ends in interstitial spaces
No arteriole end, ONLY drain centrally at VENOUS end
What are the features of the lymphatic system what allows movement along the lymph vessels?
Terminal chamber has ‘non-return valve’:
Edges of myoepithelial cells do not join (free edges overlap)
Valve opens when interstitial pressure increases
How does interstitial fluid move along lymph vessels?
When the chamber is not contracting: pressure remains ~0
Presence of fluid in a chamber stretches its wall > causes chamber to contract (one at a time) > pressure rises to ~25 mmHg > propels fluid past the valve into the next chamber along the length of lymphatic vessel
FLUID STRETCH WALLS AND CHAMBER CONTRACTS...etc