Page 679 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

666 SPECIAL THERAPY

as decorin and biglycan, and phosphatidylcholine- Convection occurs when solutes are carried along with
containing lamellar bodies that allow free movement of the bulk flow of water during ultrafiltration. This move-
the visceral organs during respiration and gastrointestinal ment can occur even when the concentrations of solute
peristalsis. 48,61 Mesothelial cells play a role in a number of on either side of the semipermeable membrane would
otherprocesses,includingantigenpresentation,controlof not promote diffusion of the solute. This effect does
inflammation, tissue repair, coagulation, and fibrino- not play an important role in PD; it is more important
lysis. 13,35 It is generally believed, however, that the in hemodialysis, where this process can be mechanically
mesothelium does not represent a significant barrier to manipulated (see Chapter 29).
water transport. 61 The anatomic structures that appear Transport of water and solutes across the peritoneal
toplaythemostimportantroleinfluidandsolutetransport membrane is best explained by the three pore theory.*
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are the walls of the capillaries and the extracellular matrix Large pores, 100 to 200 A in diameter, correspond to
locatedinthesubmesothelial cell connectivetissue. 27,28,63 clefts in the endothelium and allow the transport of
Peritoneal capillaries are composed primarily of macromolecules such as albumin. They are present in very
nonfenestrated endothelial cells supported by a basement small numbers, accounting for less than 0.01% of the total
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membrane. Endothelial cells contain aquaporins, which pore surface area. Small pores, 20 to 25 A in diameter,
are 20-kDa cellular membrane proteins that are responsi- also correspond to clefts in the endothelium. They pres-
ble for water transport. Intercellular clefts between ent in large numbers, representing more than 90% of the
endothelial cells also play a role in solute transport. 36 pore surface area, and allow the passage of low molecular
Although the anatomic surface area of the peritoneum weight substances such as urea. creatinine, and glucose.
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is large, the effective surface area—that area involved in Ultrasmall pores, 4 to 6 A in diameter, are aquaporin I
fluid and solute movement—is considerably smaller. This channels found within peritoneal capillary and mesothe-
discordance is because transport of water and solutes is lial cells, and transport only water. 21,40,50–52,59,61
primarily dependent on the surface area of peritoneal Aquaporins are a family of transmembrane polypeptides
capillaries, rather than the mesothelium. 3,10,36 that permit water transport across the cellular membrane
in response to an osmotic gradient. 21,51,59,71 Aquaporin
FLUID AND SOLUTE expression in mesothelial cells can be induced by exposure
TRANSPORT of the cells to hyperosmotic solutions. 50
It has also been postulated that the location and the
The mechanisms by which fluids and solutes are number of peritoneal capillaries affect the rate of trans-
transported across the peritoneal membrane involve sev- port of fluid and solutes. 10,41,56 Those capillaries that
eral physical processes, including diffusion, convection, are located farther from the mesothelium would partici-
and ultrafiltration. Diffusion can be defined as the ten- pate less in the transport process.
dency for solutes to disperse within the available space. 11 In PD, diffusion is responsible for the transfer of urea,
Solutes move by osmosis from a space with a higher creatinine, and other small solutes from the compartment
concentration of that solute to one with a lower concentra- in which they are present in high concentration (plasma in
tion. When this movement occurs across a semipermeable peritoneal capillaries) to that in which they have low or no
membrane, the rate of diffusion is governed by concentration (dialysate). The other factor affecting dif-
thepermeabilityofthemembrane,theavailablesurfacearea fusion is the ability of a membrane to transport the spe-
for diffusion, and the concentration of solute on either side cific solute that is directly proportional to the effective
of the membrane. Diffusion is most rapid when the two peritoneal surface area, and inversely proportional to
solutions have markedly different solute concentrations, the overall resistance. The mass transfer area coefficient
and the rate of movement of solute slows as the (MTAC) is the theoretical clearance rate that would occur
concentrations become more equal. Diffusion continues if the concentration gradient for a solute is infinitely high.
until the solutions on either side of the semipermeable The osmotic gradient, MTAC, and rate of diffusion are
membrane are of equal solute concentration. highest at the beginning of a dwell cycle, when the con-
Ultrafiltration is the removal of fluid (water) during centration gradient is highest. 61,68 The rate of removal of
PD. The rate of ultrafiltration is dependent on the a substance by diffusion is not only related to osmotic
osmotic or oncotic gradient between peritoneal capillary gradients but also to the size of the molecule and to
plasma and dialysate, as well as the effective peritoneal the area available for diffusion. Urea has a relatively low
surface area and capillary blood flow. 61,68 In PD, ultrafil- molecular weight of 60 and diffuses more rapidly than
tration is accomplished by instilling fluid into the perito- creatinine, which has a molecular weight of 113. Larger
neal cavity that is of higher osmolality than that of plasma. molecules such as albumin (MW 69,000) are dependent
Ultrafiltration is frequently desired when performing PD on diffusion through larger pores, and the rate is
because animals are often overhydrated as the result of comparably slower (Figure 28-2).
fluid administration.
*References 3, 15, 26, 55, 61, 63, 68.