25A12: Exam Report

  1. Outline the structural features of the capillary membrane that facilitate the movement of water (15% marks).

  2. Outline the mechanisms by which water moves across the capillary membrane (10% marks).

  3. Describe the forces that influence fluid movement across the capillary membrane (75% marks).

34% of candidates passed this question.

Parts (a) and (b) required a description of the general capillary structure (single cell layer, thin etc) with specific unique features relevant to capillary membranes (fenestrated, non-fenestrated and sinusoidal), and a discussion of water movement across capillary membranes via ultrafiltration primarily.

Majority of marks were allocated for part (c), and the following headings provide a useful structure: hydrostatic pressure, oncotic pressure, net-filtration pressure and the filtration coefficient.

The contribution of the Gibb’s Donnan effect to plasma oncotic pressure attracted a small amount of additional marks, as did the glycocalyx and its influence on fluid movement. The effect of arterial and venous tone on the capillary hydrostatic pressure was a common omission.

G4i / 25A12: Outline the structural features of the capillary membrane that facilitate the movement of water

a) Outline the structural features of the capillary membrane that facilitate the movement of water

  • Capillaries are part of the microcirculation
  • Features of the capillary are favourable for movement of molecules:
    • Physical structure:
      • High surface area to volume ratio
      • Thin walled (single endothelial cell with only a thin BM)
    • Large number
    • Enormous SA for exchange
    • High capillary density

b) Outline the mechanisms by which water moves across the capillary membrane

  • Water moves across the capillary primarily via bulk flow
  • Via intercellular clefts between endothelial cells
    • Flow depends on:
      • Pressure gradients (colloid and osmotic)
      • Pore size
        • Increase size = increase flow (analogous to vessel radius in the Poiseuille equation).
      • Number of pores
  • Different physical structures of capillaries affect exchange via bulk flow:
      • Continuous capillaries
        • Tight endothelium and continuous basement membrane (skin, lung, brain)
        • Lowest bulk flow across capillary wall
      • Fenestrated capillaries
        • Fenestrae in endothelium (renal glomeruli, exocrine glands)
        • High permeability and bulk flow
      • Discontinuous capillaries
        • AKA sinusoidal
        • Large intercellular gaps AND gaps in the basement membrane (spleen, liver, bone marrow).
        • Highest permeability and greatest bulk flow
Diagram of vesicular transport across cells.

c) Describe the forces that influence fluid movement across the capillary membrane

  • Movement of fluid across the capillary is dependent on Starling Forces as given by the Starling equation.
  • It depends on hydrostatic pressure, oncotic pressure and the filtration coefficient
  • J = KF A[(Pc-Pi) – s(pc–pi)]

J

  • Flux
  • of molecules of H2O per unit time that moves across a capillary

Kf

  • Filtration constant
  • The permeability of the capillary to H2O
  • Determined by:
    • Size and number of pores
    • Thickness of the capillary
  • Kf increases with HA, bradykinin and leukotriene

A

  • Surface area
  • Related to length, diameter and number of capillaries available for exchange
  • It is dynamic
    • e. skeletal muscle in exercise  number of perfused capillaries + SA increases.
  • Kf x A = Filtration coefficient

NDF

  • NDF = [(Pc-Pi) – s(pc–pi)
  • Net Driving Force
  • Dependent on Starling Forces (below)
  • NDF > 0, filtration occurs
  • NDF < 0, reabsorption occurs
  • NDF = 0, nil net movement of fluid
Diagram of Starling's forces in capillary exchange.

Capillary Hydrostatic Pressure (Pc)

  • Promotes filtration out of capillary
  • Highest at arteriolar end (30mmHg) lowest at venular end (15mmHg)
  • Filtration is thus favoured at the arteriolar end
  • Determinants:
    • Arterial Pressure
    • Venous Pressure
    • Post capillary resistance
    • Pre-capillary resistance
    • \(P_c = \frac{0.2 , P_A + P_V}{1.2}\)
  • Mean capillary hydrostatic pressure is more influenced by changes in venous pressure than arterial pressure
    • Due to higher precapillary resistance c.f postcapillary
      • High precapillary resistance blunts effect of increased arterial pressure on the downstream capillaries
  • Ie. Increased venous pressure (RHF, venous thrombosis) → significantly increases capillary hydrostatic pressure.

Interstitial Hydrostatic Pressure (Pi)

  • Pressure in the tissue interstitium opposing capillary hydrostatic pressure
  • Promotes fluid reabsorption into capillary.
  • \(\Delta P_i = \frac{\Delta V_i}{C}\)
  • Depends on interstitial compliance (V/P)
  • Ie. Brain has low interstitial compliance
    • Small increases in volume (bleed, oedema) can lead to a large increase in interstitial pressure
  • Subcutaneous tissue has high interstitial compliance
    • Large increases in volume can occur with relatively small increases in interstitial pressure
  • Net hydrostatic pressure favours fluid filtration (as Pc >Pi)

Reflection coefficient (σ)

  • Permeability of capillary to protein (“leakiness” to protein).
  • If σ= 1, impermeable to protein
    • Brain has σ > 0.9
  • If σ = 0, capillary is freely permeable to protein
    • e. liver and spleen have a low σ

Capillary Plasma Oncotic Pressure (πc)

  • Determined by plasma proteins (albumin – 80% of oncotic pressure. Rest by globulins and fibrinogen)
  • Opposes filtration, promotes reabsorption
  • Generally, 25-30mmHg along length of capillary
    • Increases near venular end as filtered fluid leaves behind protein  increased [protein]

Interstitial Oncotic Pressure (πi)

  • Promotes fluid filtration out of capillary
  • Determined by interstitial proteins
  • These are found in the glycocalyx
    • Lines the endothelium luminal surface
  • Around 5mmHg

  • Net oncotic pressure favours fluid reabsorption (as πc > πi)

Capillary Exchange Model

  • NDF falls from +10 to -5mmHg from arteriolar to venular end of capillary
  • Due to falling capillary hydrostatic pressure
    • Pc falls from 30mmhg to 15mmhg at end of capillary
    • Due to filtration
  • Thus, filtration occurs along capillary where NDF >0
  • Reabsorption occurs along capillary where NDF <0
  • Assumes Pi = 1mmHg, πc = 25mmHg, πi = 6mmHg and σ = 1
Capillary filtration and reabsorption diagram with graphs.

Vessel Tone

↑ Arterial Tone

  • Increases upstream resistance
  • Reduces capillary inflow
  • Pc falls (less hydrostatic pressure transmitted to the capillary).
  • → Promotes absorption (less filtration).

↓ Arterial Tone

  • Decreases upstream resistance
  • Increases capillary inflow
  • Pc rises (more hydrostatic pressure transmitted)
  • → Promotes filtration

↑ Venous Tone

  • Raises venous pressure (Pv).
  • Because Pc is downstream‐dependent, an increase in Pv is transmitted back into the capillary.
  • Pc rises (hydrostatic “back pressure”).
  • → Strongly promotes filtration and edema risk.

↓ Venous Tone

  • Lowers Pv
  • Pc falls
  • → Promotes absorption (fluid movement into capillary)

Pc is generally more sensitive to changes in venous pressure than to arterial pressure ~75% of Pc is determined by venous pressure.

Sources:

  • Cardiovascular Physiology Concepts (Klabunde, 3rd edition) – Not a formal prescribed text but an amazing resource. I didn’t use Pappano.

Author: Alex Fagarasan