**2.1 Distributed concept of ultrafiltration barrier**

The three potential barriers to both solute and water are (1) anatomic peritoneum (2) cellular-interstitial matrix surrounding the blood vessels (3) capillary endothelium. Blood vessels are the main source of UF and the water flow from the capillaries to the interstitium depends on the difference between the capillary luminal pressures and the effective pressure on the interstitial side. The concentration profile occurs due to diffusion of the small solutes via the tissue interstitium along with simultaneous uptake into the capillaries. The largest gradients of osmotic pressure will therefore be across blood vessels closest to the peritoneum.

**Figure 1A** describes the distributed concept of UF barrier and **Figure 1B** shows changes in membrane [4].

The distributed model proposes that "the influence of a specific capillary on PD transport is the function of that capillary's proximity to mesothelial to the dialysate interface". The proliferation of vessels near the interface increases the "effective" peritoneal surface area; especially during peritonitis and following exposure to high glucose containing fluids.
