1999A08 Explain the significance of plasma oncotic pressure in capillary fluid dynamics.

· 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 = K_f[(P_c – P_i) – σ(π_c-π_i)]
K_f	· Filtration coefficient = surface area x hydraulic conductivity
σ	· Staverman reflection coefficient ∝1 / (permeability to protein) · ~1 at renal glomerulus, ~0 at hepatic sinusoids
P_c	· Capillary hydrostatic pressure (highest pressure, drives transudation)
P_i	· Interstitial hydrostatic pressure (normally very low)
π_c	· Capillary oncotic pressure (note the only factor opposing transudation) · Decreased in cirrhosis, nephrosis, malnutrition
π_i	· Interstitial oncotic pressure (normally very low)

(falsely predicts filtration only at arterial end)

	Afferent arteriolar end (mmHg)	Efferent arteriolar end (mmHg)
P_c	60	58
P_b	15	15
π_c	21	33
π_b	0	0
Net	+24	+10

(predicts filtration across the length of the capillary)