Answer:
newborn infants. Bioassayable andimmunoassayable renin activities were elevated during the first six days after delivery. At 3 to 6 weeks of age, although elevated above that in adult control subjects, renin activity was lower than in the first postnatal week. Cord blood renin values were greater than those for maternal renin during labor. Renin substrate concentration was elevated in infants less than six days of age and also in 3- to 6-week-old infants. The enhanced activity of the renin-angiotensin system may contribute to the increased renal vascular resistance and the altered renal hemodynamics that occur in the newborn infant.