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The peritoneum and mechanism of peritoneal
dialysis
Contents:
The principle of peritoneal dialysis (PD) is similar to that of hemodialysis
,but in PD ,the patient’s peritoneum is used as the semipermeable membrane
between capillary blood and dialysis fluid which is introduced into the
peritoneal cavity through permanent catheter
Presently peritoneal
dialysis is home dialysis therapy for chronic end stage renal failure but can
also be a treatment option for patient with acute renal failure in hospital
sitting.
Peritoneal cavity and membrane:
In peritoneal dialysis the peritoneal cavity acts as the reservoir for
dialysate. The peritoneum serves as the semipermeable membrane across which
excess body fluid and uremic toxins are removed.
The peritoneum is a serous membrane consists of:
a-Parietal peritoneum ; lining the inner surface of the abdominal
and pelvic walls including the diaphragm.
b-Visceral peritoneum ; covering the abdominal organs.
The surface area of the peritoneum is approximately equal to body surface area
(1.73 msq). The peritoneum is closed sac in males but in females the fallopian
tubes and ovaries open into the peritoneal cavity.
Histologically human peritoneal membrane is made
of monocellular layer of mesothelium which rest on a continuous membrane
overlying blood vessels, lymphatics, collagen fibers and fibroblasts. Electron
microscopy studies shows numerous microvilli on the free surface of the
mesothlium which are absent in some areas. Methothelial cells and its microvilli
play an important role in peritoneal transport.
The methothelium situated
above the capillaries represent 2.6% of its surface area. However the presence
of wide distribution of lymph vessels which represent 4% of mesothelial surface
denote their importance in solute transport.
Physiological Considerations:
·
Diffusion
is the principle mechanism by which peritoneal dialysis removes waste products.
It is the exchange of solute between two solutions separated by a semipermeable
membrane. In peritoneal dialysis the first solution is the blood perfusing the
capillaries adjacent the peritoneum. The second solution is the dialysate in the
peritoneal cavity.
Diffusion rate is affected by
1-Concentration gradient; as the the concentration difference between the two
solution decreases, the net solute transport between them approach zero. Very
little net transfer of urea is occurring after 2 hours.
Blood flow rate through the peritoneum range between 70-100 ml/minute which is
2-3 times greater than the maximum clearance of urea i.e. transperitoneal
transfer of urea and other solute is not blood flow dependent or peritoneal
dialysis efficiency is maintained even in moderately hypotensive patients.
However impairment of peritoneal blood flow and clearance can occur in severely
hypotensive patients.
2-Molecular weight; smaller molecules moves more rapidly than larger, heavier.
In peritoneal dialysis some larger molecules are transported e g proteins which
is undesirable.
3-Membrane resistance; the permeability of the membrane can be altered by the
disease. Acute peritonitis increases the permeability to both solute and water.
On the other hand peritoneal sclerosis can lead to reduction in transport of
both solutes and water.
4-Physical activity and peristalsis stirring effect. Unstirred fluid slows the
diffusion rate because of the presence of relatively high concentration of the
solute next to the peritoneum.
·
Ultrfiltration [UF]
The primary driving force for UF in peritoneal dialysis is the Osmotic gradient
(Osmotic Ultrafiltration) . Osmosis is
the movement of the solvent (e.g. water) from the side of low concentration
to the side of higher concentration through a semipermeable membrane. The result
of this movement of water will be equalization of total solute concentration on
both sides of the membrane.
Osmotic UF during peritoneal dialysis is achieved by adding large amount
of glucose
to dialysis solution.
Peritoneal dialysis solution ordinarily contain 1.36%(1.5), 2.27%(2.5),
3.86%(4.25).
The osmotic pressure generated by glucose will draw water from the blood
and tissues
into the dialysate.
·
The hydrostatic UF effects
are of minor importance in peritoneal dialysis.
·
Lymphatic absorption
of peritoneal fluid occur when the peritoneum is filled with 1-2 liters of fluid
at a rate of 0.5-1 ml/minute or more in children and in the presence of
peritonitis
The net ultrafiltration is due to the balance
between [1] osmotic UF drawing fluid and solute into the peritoneal cavity
and[2] lymphatic absorption of dialysate and solutes.
·
Plasma protein removal
during peritoneal dialysis may serve as a means of excreting uremic toxins that
are tightly protein bound.
·
Sieving of sodium by peritoneal
membrane; When hypertonic dialysis
solution is used and the exchanges preformed rapidly the water extracted by UF
is hyonatremic relative to plasma. This huponatremic ultrfiltrate does not
equilibrate with plasma sodium during short dwell. As a result of that water is
removed from the body greater than sodium which tend to cause hyernatremia.
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Mechanism of peritoneal dialysis:
Dialysate is infused into
the peritoneal cavity via a catheter, allowed to dwell for a predetermined time
then drained (effluent); this process called an exchange. Dextrose is
used in the dialysate to create an osmotic gradient that causes water to be
moved into the peritoneal cavity and removed when the effluent is drained. Uremic
toxins and electrolytes are removed by diffusion from higher concentration
[blood] to lower concentration [dialysate]. Hypertonic dialysate enhance solute
removal by solute drag i.e. additional solute are removed or dragged along with
the ultrafiltrate by convective transport.
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