Highlights: Renal Concentrating Capacity in the Newborn. Development, clinical Disorders, Evaluation, and Management.

Polyuria, the excretion of an excessive volume of dilute urine, is a potentially lethal disorder in the neonate. Since body water accounts for roughly 60-70 percent of lean body mass in the newborn, loss of body water without adequate replacement may lead to circulatory collapse and death of the infant. The capacity of the kidneys to elaborate urine hypertonic to plasma or the concentrating capacity, involves many physiologic processes. These include countercurrent multiplication, recycling of filtered urea, antidiuretic hormone (ADH), solute delivery to the distal nephron, and the adenylate cylase (cyclic AMP) system. Although urine elaborated by the kidney in utero is normally hypotonic to plasma, development of the capacity to excrete a concentrated urine begins early in fetal life.