transport mechanisms. Sodium is mainly reabsorbed in the proximal and distal
tubules under the influence of aldosterone, which is produced by the adrenal
cortex more effectively in term than pre-term infants. The kidney response to
aldosterone is, similarly, better in term infants.
Physiologic mechanisms of sodium reabsorption in the proximal tubule include
Na/H exchange transporters, Na-P and Na-glucose as well as Na-aminoacid co-
transporters (at the apical side of cell) and by energy mediated Na-K-ATPase at
the basolateral membrane. Fig 1.
Renal fluid and electrolyte balance is not only possible by a fully functional
tubular system but by mature renal interstitium capable of concentrating urine.
The renal interstitium regulates how much water needs to be kept or eliminated
in the urine, therefore, the amount of water excreted in the urine will determine if
the urine is concentrated or diluted. A normal urine osmolality of 300 mOsm/L to
400 mOsm/L is considered normal in the term baby but can range from 50
mOsm/L to 800 mOsm/L depending on specific circumstances. [23] The ability of
the neonate to maintain a urine output in the range of 2-3 mL/kg/h reflects both a
mature tubular system and a normal capacity of urine concentration.
The described physiologic changes are somewhat different in premature infants.
Loss of water in the first week of life can approach 15-20% of the total body
weight and sodium urinary losses are usually higher. Fluid management and
electrolyte replacement in premature children should therefore be judicious and
guided by clinical and laboratorial parameters.
Ultrafiltration of plasma occurs at the glomerular level. The afferent arteriole
brings blood to the glomerular capillaries where it is filtrated through the
fenestrated glomerular endothelium and capsule of Bowman (podocytes). The
filtrated blood exits the glomerulus by the efferent arteriole. To allow adequate
filtration, there must be a difference in pressure across the Bowman’s capsule
(transmembrane pressure). Thus, constriction of the efferent arteriole (partially
closing the exit valve) elevates the pressure at the glomerular capillaries. In