The Peritoneal Membrane

The contract between this chapter and the reader.

1. 1. Describe the anatomy of the peritoneal membrane and identify the capillary endothelium as the principal transport barrier.
2. 2. Explain the three-pore model and attribute each solute and water movement to the correct pore.
3. 3. Differentiate diffusion, convection, and osmosis, and link each to a clinical correlate.
4. 4. Interpret a peritoneal equilibration test and classify the patient's transport status.
5. 5. Predict how transport status shapes the ultrafiltration profile across a dwell.
6. 6. Diagnose ultrafiltration failure and classify its mechanism using sodium sieving and transport data.
7. 7. Relate chronic glucose exposure to membrane injury and the long-term rise in transport.
8. 8. Apply transport physiology to a first-pass prescription direction.

A sixty-second reading. Each bullet stands alone.

• The peritoneum is a mesothelial sheet over an interstitium and a capillary bed; the capillary endothelium, not the mesothelium, is the main barrier.
• Effective transport depends on the perfused vascular surface area in contact with dialysate, not the anatomic area.
• The three-pore model: large pores carry proteins by convection; small pores carry small solutes and water; ultrasmall aquaporin-1 pores carry water only.
• Small solutes (urea, creatinine) move by diffusion down their gradient; solutes also ride convective water flow.
• Glucose drives crystalloid osmosis: ultrafiltration is highest early and fades as glucose is absorbed and the gradient collapses.
• Aquaporin-1 mediates roughly 4 to 5 in every 10 units of ultrafiltration during a hypertonic dwell and produces sodium sieving — an early dip in dialysate sodium.
• Icodextrin uses colloid osmosis through small pores for sustained ultrafiltration across the long dwell, with little sodium sieving.
• Net ultrafiltration equals transcapillary ultrafiltration minus fluid (lymphatic and tissue) absorption.
• The peritoneal equilibration test grades transport by the 4-hour dialysate-to-plasma creatinine ratio.
• Fast transporters absorb glucose quickly and ultrafilter poorly on long dwells; they suit short, frequent exchanges and icodextrin.
• Slow transporters clear solute slowly but ultrafilter well; they need longer dwells and larger volumes.
• Ultrafiltration failure is net ultrafiltration below 400 mL after a 4-hour 4.25% glucose dwell; classify the mechanism rather than calling it generic membrane failure.
• Chronic glucose and glucose-degradation-product exposure injures the membrane, raising transport over years and risking encapsulating peritoneal sclerosis.

The medical core. An expert should agree membrane physiology is fully covered here.

Why it matters at the bedside

Every PD prescription is an argument with the peritoneal membrane. How fast a patient clears solute, how much fluid they remove, and why their ultrafiltration fails are all decided by the membrane's transport behaviour. Read the physiology and the prescription follows; ignore it and the dwell fights the patient.

The anatomy that does the work

• The peritoneal membrane has three layers in series: a mesothelial monolayer, a connective-tissue interstitium, and the peritoneal capillary network. Solutes and water cross all three, but the capillary endothelium is the rate-limiting barrier.
• Visceral peritoneum covers the organs and contributes the largest anatomic area; the parietal peritoneum lining the abdominal wall is smaller but well perfused and carries a disproportionate share of transport.
• What counts is the effective peritoneal surface area — the perfused capillary surface actually bathed by dialysate — not the geometric area. Vasoactive states and inflammation change it.

Why transport happens — the three-pore model

• Large pores (radius ~250 Å). Few in number; they let macromolecules such as albumin cross by convection, which is why PD steadily loses protein into the effluent.
• Small pores (radius ~40–50 Å). Abundant; the main route for urea, creatinine, sodium, potassium, glucose, and water, moving by diffusion and convection.
• Ultrasmall pores — aquaporin-1 (radius ~2–4 Å). Water-only channels in the endothelium; they carry free water during a hypertonic dwell and account for sodium sieving.

How solutes and water actually move

• Diffusion. Solutes move down their concentration gradient — waste out, glucose and lactate in. Its efficiency is summarised by the mass-transfer-area coefficient.
• Convection. Ultrafiltered water drags solutes with it through small and large pores, adding to removal whenever fluid is being pulled.
• Osmosis. Glucose creates a crystalloid osmotic gradient that pulls water from blood to dialysate; icodextrin creates a colloid gradient that does the same more slowly and durably.

Ultrafiltration across a dwell

• With glucose, ultrafiltration is fastest at the start of the dwell, when the osmotic gradient is largest, and slows as glucose is absorbed and the gradient dissipates. Late in a long glucose dwell, net fluid can even be reabsorbed.
• Aquaporin-1 carries water without sodium early in a hypertonic dwell, transiently lowering dialysate sodium — the sodium-sieving dip. Its presence confirms working free-water transport.
• Icodextrin, a glucose polymer, sustains ultrafiltration through the long dwell by colloid osmosis and produces little sodium sieving, which is why it suits the long exchange.
• Net ultrafiltration is transcapillary ultrafiltration minus fluid absorption by lymphatics and tissue, which runs continuously and opposes fluid removal.

What else could explain poor drainage

• Before calling it membrane ultrafiltration failure, exclude mechanical causes — catheter malposition, kinking, constipation, leak — and confirm with a standardized test, because measured ultrafiltration depends entirely on glucose strength and dwell time.

Which tests grade the membrane

• The peritoneal equilibration test uses a standardized dwell and reports the 4-hour dialysate-to-plasma creatinine ratio, which sorts patients into high, high-average, low-average, and low transporters.
• Ultrafiltration failure is defined as net ultrafiltration below 400 mL after a 4-hour dwell of 4.25% glucose; the sodium-sieving dip and the transport category then point to the mechanism.

Why the membrane changes over time

• Continuous exposure to glucose, glucose-degradation products, lactate buffer, and low pH injures the mesothelium, promotes new vessel growth, and lays down submesothelial fibrosis. Over years this raises transport and erodes ultrafiltration, and at the extreme underlies encapsulating peritoneal sclerosis.
• Neutral-pH, low-glucose-degradation-product solutions are designed to slow this injury, though the outcome evidence is mixed.

Five fully-built tables.

Table A — The three pores

Table B — Transport mechanisms

Table C — Transport categories (peritoneal equilibration test)

Table D — Ultrafiltration failure

Table E — Osmotic agents

Visualise & Map

Causal chains, each ending in a named action.

Chain 1 — Glucose ultrafiltration kinetics

Glucose creates an osmotic gradient → water crosses into dialysate → glucose is absorbed and the gradient collapses → ultrafiltration falls (then reverses) over the dwell → ACTION: shorten the dwell or use icodextrin for the long exchange.

Chain 2 — Aquaporin and sodium sieving

Aquaporin-1 carries free water early → sodium is left behind → dialysate sodium dips → the dip indexes free-water transport → ACTION: use the early sodium dip to test aquaporin function during an ultrafiltration-failure workup.

Chain 3 — Fast transport and poor ultrafiltration

A large perfused surface drives rapid equilibration → glucose is absorbed quickly → the osmotic drive is lost early → long-dwell ultrafiltration is poor → ACTION: prescribe short, frequent dwells on automated PD.

Chain 4 — Membrane injury over time

Chronic glucose and glucose-degradation-product exposure → mesothelial injury and new vessel growth → submesothelial fibrosis and rising effective surface area → acquired fast transport and ultrafiltration failure → ACTION: limit glucose load and consider biocompatible solutions.

Chain 5 — Net versus transcapillary ultrafiltration

Transcapillary ultrafiltration removes fluid → lymphatic and tissue absorption returns it → net ultrafiltration is the difference → high absorption can mean poor drainage despite normal transport → ACTION: count high fluid absorption as a distinct ultrafiltration-failure mechanism.

Interpretation rules. These numbers are the cross-reference handle for the cases and flowcharts.

Clinical Reasoning

Four cases. Each stops you at a decision before it answers it.

Every mechanism from Level 3 earns a bedside consequence and an action.

Exhaustive. Every threshold and rule in the chapter is here.

Safety & Evidence

Panel A — Red flags

Panel B — NEVER DO

Anti-patterns clinicians fall into. Each becomes a Level 22 distractor.

The grade reflects strength of evidence, not importance.

Apply & Test

Pre-rounds compression. Numbers and rules only.

Active recall. At least one card per objective.

Micro-teaching for rounds. Two scenarios, five steps each.

Eight items, each anchored in this chapter's own physiology. At least one per objective.