1. Anatomy and Physiology of Peritoneal Dialysis

2. Peritoneal Membrane Anatomy Key Points
Serosal membrane with area equivalent to body surface area, I.e. 1 to 2 metres2
80% is visceral peritoneum and gets its vascular supply via the mesenteric arteries and portal veins
20% is parietal peritoneum and gets its vascular supply via arteries and veins of abdominal wall
Lymphatic drainage of peritoneal cavity is mainly via diaphragmatic stomata

3. Peritoneal Membrane Anatomy Key Points
Peritoneal cavity is lined by a mesothelial monolayer which produces a lubricating fluid
Under the mesothelium is a gel-like interstitium containing connective tissue fibres, capillaries and lymphatics
The effective surface area is critical for dialysis and depends on the vascularity of the peritoneum as well as its surface area

4. The Normal Peritoneal Membrane
Mesothelial cell monolayer
Interstitium
Peritoneal vasculature

5. The Normal Peritoneal Membrane

6. Pathways for Peritoneal Transport
Capillaries
Macro molecules
Water
Small solutes
Endothelium
Glucose
Crystalloid osmosis
Colloid osmosis
Interstitium
Mesothelium
Peritoneal tissue layer
Dialysate

7. Peritoneal transport Two clinical end-points
Clearance of solutes (by diffusion and convection)
Fluid removal (transcapillary UF – fluid absorption)

8. Peritoneal Transport Three Distinct Processes
Diffusion
Ultrafiltration
Fluid Absorption

9. What Happens with Solute Removal During a CAPD Dwell?
Diffusion is at a maximum, and urea and creatinine equilibration are fastest, in the first hour but become slower as the gradient lessons with time
By 4 hours, urea is >90% and creatine > 65% equilibrated in most patients
Dialysate to plasma (D/P) ratios measure degree of equilibration at a given dwell time (e.g. D/P Urea, D/P Creatine)

10. Peritoneal Equilibration Test

11. Diffusion How to Increase It
Maximize concentration gradient
- More frequent exchanges (e.g., APD)
- Larger dwell volumes
Increase effective peritoneal surface area

12. Ultrafiltration What are the key factors?
Osmotic gradient (e.g. for glucose)
Reflection and UF coefficients
(NB – not discussed during this course)
Hydrostatic and oncotic pressure gradients

13. Peritoneal Fluid Absorption
Occurs directly via lymphatics
Also absorption into tissues with subsequent removal via lymphatics and capillaries
Difficult to measure but is about 1 to 2 mls per minute ( mls in 4 hours)

14. Net Ultrafiltration Net UF is actual UF minus fluid absorption
e.g. 1000mls – 200mls = 800 mls Net UF
Clinically we can only influence Net UF by: altering the osmotic gradient (e.g. from
1.36% to 2.27%),
or by
- changing the osmotic agent (e.g. from
glucose to icodextrin)

15. Pathways of Glucose Flow
Capillary
Peritoneal Space
Glucose transporter
mediated: minimal
Intercellular: >90%

16. Membrane Model
Membrane
PERITONEAL
DIALYSATE
BLOOD

17. What Happens to Fluid Removal with a 2L 4.25% PD Dwell?
Note: I/P = Intraperitoneal or inside peritoneal cavity
UF is maximal at the start of the dwell, approx. 15 ml/min
It quickly lessons as glucose diffuses out of the dialysate into the blood and as the UF dilutes the glucose
I/P volume increases until about 3 hours when UF rate falls to equal the constant fluid absorption rate of 1-2 ml/min
After this, the I/P volume reduces until it is less than 2L after 8-10 hours, leading to net fluid retention

18. Small Solute Clearance in PD Patients
Clearance is the quantity of plasma from which solute is cleared per unit time
In PD:
> total clearance = peritoneal + residual renal
Peritoneal clearance depends on:
> diffusion + UF – fluid absorption
and so varies during the course of the dwell period
Daily peritoneal clearance =
> daily dialysate drain volume x D/P ratio
(for the solute concerned over that day)

19. Determinants of Clearance Achieved on PD
Residual renal function
Body size (Volume or Body Surface Area)
Peritoneal solute transport rate
The prescription

20. What About Protein?
Protein losses occur via large pores, are greatest in high transporters and average 6 to 10 g/day
About 50% of losses are albumin and there is an inverse relationship to serum albumin
Fluid absorption during a dwell prevents losses being greater
Losses are not much affected by PD prescription, but increase during peritonitis

21. Total Removal of Protein in Different Transport Groups
3000
2500
2000 H
Total removal of protein, mg
1500
H-A
1000
L-A
500 L 60
120
180
240
300
360
Time, min
Wang et al. Nephrol Dial Transplant 13: , 1998

22. Conclusion
A knowledge of peritoneal anatomy and physiology is important in the management of PD patients
In particular, it helps to solve problems with clearance and ultrafiltration
It also improves understanding of the impact of new technologies such as cyclers, larger dwell volumes, new PD solutions, etc.