Pediatric Nutrition in Practice

Fluid and Electrolytes 57

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effectively managed with oral rehydration thera-
py, using a prescribed f luid and electrolyte oral
rehydration solution (ORS). Because malnutri-
tion increases the frequency, severity, and dura-
tion of diarrhea, f luid and electrolyte replace-
ment and nutritional therapy are critical ele-
ments for recovery [2].

Regulation of Sodium Balance

Sodium absorption occurs in the gastrointestinal
tract and excretion primarily by the kidney, with
small amounts excreted in sweat and feces. In
pathologic conditions, especially diarrheal dis-
ease, normal gastrointestinal mechanisms of ho-
meostasis become disturbed and can result in
large, sometimes life-threatening fluid and elec-
trolytes losses. Systems regulating renal sodium
chloride (NaCl) and water excretion operate by a
negative feedback loop consisting of an afferent
(sensory) component, an efferent (messenger)
component, and an effector organ [3]. The renal
response aims at reconstituting the ECF volume
by decreasing the glomerular filtration rate and
thus the filtered load of Na + and, even more criti-
cally, by promoting tubular reabsorption of Na +
utilizing the various mechanisms of Na + trans-
port including exchangers, channels, and co-
transporters. Receptors located in the renal juxta-
glomerular apparatus detect a reduced ECF vol-
ume and Na + concentration and stimulate renal
Na + retention via the renin-angiotensin cascade.

Regulation of Body Water

Plasma osmolality is the primary driver for thirst
and, therefore, water intake, although under con-
ditions of a reduced ECF volume such as severe
dehydration, a low blood volume assumes a great-
er role and will override tonicity [4]. Arginine va-
sopressin secreted from pituitary neuronal cells
in response to signals from osmoreceptors binds

to epithelial-cell basolateral receptors of the oth-

erwise water-impermeable collecting tubule of

the nephron, stimulating the insertion of water

channels (aquapores) into the apical cell surface

and subsequent extraordinary movement of wa-

ter from the lumen to the cell interior.

Gastrointestinal Regulation of Fluids and

Electrolytes

The permeability of the tight junction between

epithelial cells decreases distally so that the jeju-

num is the most and the distal colon and rectum

the least permeable to the passive movement of

electrolytes and water [5]. Ions traverse the epi-

thelium via the transcellular or paracellular routes

throughout the length of the bowel by passive or

active transport mechanisms. Passive movement

of fluids follows, with paracellular transport being

the main mechanism of flow in the small bowel

and transcellular flow predominating where the

epithelia are tightly aligned and less permeable, as

in the distal colon. Cotransportation of Na + with

certain nutrients including glucose and amino ac-

ids at the apical surface of the upper villus in the

small intestine is responsible for most Na + and

water absorption following a meal or ingestion of

an ORS ( fig.  1 ). The carrier specific for Na-glu-

cose cotransportation, SGLT-1, is preserved in

most diarrheal diseases and forms the basis for

oral rehydration therapy [6]. In fasted states or

between meals, most NaCl is transported from

the lumen via exchange (Na + /H – and Cl – /HCO 3 ).

While sodium transport drives f luid absorp-

tion, Cl – excretion is the driving force for f luid

secretion. Cl is taken up along the basolateral

membra ne of t he epit hel ia l cel l by t he elec t roneu-

tral Na + /K + /2Cl – cotransporter and accumulates

within the cell above its electrochemical equilib-

rium ( fig. 2 ). Once within the cell, Cl exits into

the intestinal lumen via Cl channels that open in

response to regulatory agonists that invoke sec-

ond messenger systems.

Koletzko B, et al. (eds): Pediatric Nutrition in Practice. World Rev Nutr Diet. Basel, Karger, 2015, vol 113, pp 56–61
DOI: 10.1159/000367869