Renal control of water and electrolyte homeostasis was studied in 10 healthy babies (gestation 29 to 34 weeks; birthweight 1.19 to 2.19 kg) while water intake was varied. Glomerular filtration rate and urine flow were estimated daily from spot plasma and urine samples for six days using a constant inulin infusion, and simultaneous sodium, potassium, osmolar, and free water clearances were calculated. The infusion was started at an average age of 14 hours. Each baby received a total fluid intake of 96 ml/kg daily on study days 1, 2, and 5, and about 200 ml/kg on study days 3, 4, and 6. Daily sodium intake was kept constant at 3 mmol/kg. At the end of the first study day the babies were undergoing a diuresis, but thereafter their estimated daily water balances remained stable regardless of intake. Glomerular filtration remained stable; alterations in urine flow reflected a change in the percentage of filtrate excreted. Plasma electrolytes and osmolality were stable throughout, and on study days 2 to 6 the urinary excretion rates of sodium, potassium, and other osmoles remained the same regardless of urine flow. The delivery of sodium to the distal tubule was estimated to be between 17 and 20% of the filtered load. Well preterm babies can cope with daily water intakes between 96 and 200 ml/kg from the third day of life.
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