· Intro
· Aetiology
· Starling forces
· Normal values: systemic and renal corpuscle
|
Definition |
· Osmotic pressure resulting from large membrane-impermeable particles |
|
Purpose |
· It’s the only Starling force that opposes transudation o Maintain intravascular volume o Minimize oedema · 4L/day transudation occurs; returned to circulation by lymphatics |
|
Normal value |
· 20-30mmHg |
|
Plasma proteins (creates 10-15mmHg) |
· Albumin 40g/L · Globulins 25g/L · Fibrinogen 3g/L · Small amount of others |
|
Gibbs-Donnan effect (amplifies ~2x) |
· Anionic proteins attract cationic sodium · Hence plasma water [Na+] 146mM while interstitial [Na+] 140mM · (total plasma [Na+] is 140mM) · But Cl- shift from plasma to ECF mostly offsets this effect · Hence increase only 0.4mOsmol/L = 12.5mmHg, not 6mOsmol/L = 112.5mmHg (see Kerry Brandis’ explanation) |
|
Equation |
Q = Kf[(Pc – Pi) – σ(πc-πi)] |
|
Kf |
· Filtration coefficient = surface area x hydraulic conductivity |
|
σ |
· Staverman reflection coefficient ∝1 / (permeability to protein) · ~1 at renal glomerulus, ~0 at hepatic sinusoids |
|
Pc |
· Capillary hydrostatic pressure (highest pressure, drives transudation) |
|
Pi |
· Interstitial hydrostatic pressure (normally very low) |
|
πc |
· Capillary oncotic pressure (note the only factor opposing transudation) · Decreased in cirrhosis, nephrosis, malnutrition |
|
πi |
· Interstitial oncotic pressure (normally very low) |
|
|
Arterial end (mmHg) |
Venous end (mmHg) |
|
Pc |
35 |
15 |
|
Pi |
0 |
0 |
|
πc |
28 |
28 |
|
πi |
3 |
3 |
|
Net |
+10 |
-10mmHg |
(falsely predicts filtration only at arterial end)
|
|
Afferent arteriolar end (mmHg) |
Efferent arteriolar end (mmHg) |
|
Pc |
60 |
58 |
|
Pb |
15 |
15 |
|
πc |
21 |
33 |
|
πb |
0 |
0 |
|
Net |
+24 |
+10 |
(predicts filtration across the length of the capillary)