Page 452 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

440 FLUID THERAPY

intake, stomach juice, saliva, pancreatic juice, and bile) is intestinal epithelium. 20 Passive absorption of water or
presented to the small intestine each day. Approximately electrolytes can be transcellular (i.e., through the cyto-
1.35 L is absorbed in the jejunum, 1 L in the ileum, and plasm of the cells) or paracellular (i.e., via the lateral inter-
300 mL in the colon, with 50 mL remaining in the cellular spaces and tight junctions between enterocytes),
feces. 123 From these figures, it can be calculated that and transfer occurs down a chemical or electrical gradient

þ
the jejunum absorbs 50%, the ileum 38%, and the colon (e.g., passive transport of Na and Cl in the jejunum
11% of the fluid presented to the intestinal tract and ileum).
(Figure 18-6). Approximately 1% of the water presented Active transport involves transport against a concen-
to the GIT is lost in feces. The variation in absorptive effi- tration gradient and requires energy input (e.g., Na þ
þ
þ
þ
ciency along the intestinal tract is a function of enterocyte transport driven by the Na ,K pump). The Na ,K -
þ
pore size, membrane potential difference, and the type of ATPase is present in all enterocytes (see Figures 18-3
transport processes associated with each intestinal seg- and 18-6) and maintains the electrochemical Na gradi-
þ
þ
ment. 20,82,120 Whereas the jejunal epithelium is “leaky” ent required not only for net transepithelial Na move-
and transfers a large amount of fluid (isotonic absorp- ment but also for the transport of many other
tion), the tight epithelial junctions of the distal colon solutes. 120 Solvent drag is the term used to describe
allow a high transepithelial voltage gradient to develop, solute movement secondary to water flow (e.g., NaCl
and net solute transfer occurs against this gradient. 120 transport in the jejunum via the paracellular route).
The absorption of water is passive in the small and large The relative importance of each transport system is site
intestines and follows the transport of solutes across the dependent (see Figures 18-3 and 18-6), and the location
Ileum Colon
Jejunum 300 mL
1350 mL 1000 mL

600-mL
diet Feces
35 mL
20-kg dog

2100 mL
endogenous
secretions

Mucosal resistance Leaky Mod. leaky Tight

Potential difference (PD) 3 mV 6 mV 20 mV

Absorptive Mechanism
+
+
Sodium Na /nutrient Na /nutrient
+
+
+
Na /H + Coupled Na /Cl – Coupled Na /Cl –
Potassium Passive K + Passive K + Passive K +
Active K +
Chloride PD-dependent Cl – PD-dependent Cl – PD-dependent Cl –

+
+
Coupled Na /Cl – Coupled Na /Cl –
–
–
HCO -dependent Cl – HCO -dependent Cl –
3
3
Figure 18-6 Fluid and electrolyte absorption in the gastrointestinal tract. (Adapted from Burrows
CF. Chronic diarrhea in the dog. Vet Clin North Am 1983;13:521; Chang EB, Rao MC. Intestinal water and
electrolyte transport: mechanisms of physiological and adaptive responses. In: Johnson LR, editor. Physiology
of the gastrointestinal tract, 3rd ed. New York: Raven Press, 1994: 2027–2081; and Sellin JH. Intestinal
electrolyte absorption and secretion. In: Sleisinger MH, Fordtran JS, Feldman M, et al, editors. Gastrointestinal
and liver disease pathophysiology, diagnosis and management, 6th ed, Vol 2. Philadelphia: WB Saunders, 1998:
1451–1471.)