**3.5 Excretion**

Drug excretion by the kidney is controlled by glomerular filtration, tubular secretion, and tubular re-absorption (Buxton & Benet, 2011). Because these processes mature at different rates in the pediatric population, age can affect systemic exposure for drugs where renal excretion is a dominant pathway of elimination. Consideration should also be given to the maturation of other excretory pathways, including biliary and pulmonary routes of excretion. Glomerular filtration rate (GFR) in the newborn, normalized to body surface area, is only about 20% of the adult value, and tubular function is also reduced. Accordingly, plasma elimination half-lives of renal eliminated drugs are longer in neonates than in adults. In babies born at term, renal function increases to values similar to those in young adults in less than a week, and indeed continues to increase to a maximum of approximately twice the adult value at 6 months of age. The increase in renal function occurs more slowly in premature infants. Renal immaturity in premature infants can have a very large effect on drug elimination. For example, in premature newborn babies the antibiotic Gentamicin has a plasma half-life of 18 hours or greater, compared with 1-4 hours for adults, and approximately 10 hours for babies born at term. It is, therefore, necessary to reduce and/or space out doses to avoid toxicity in premature babies.

Drugs and their metabolites are excreted through sweat, urine, stools, or enzymes. By the time kidney functions develop, disposing drugs via urinary system is limited. The glomerular filtration speed and the circulation in kidneys are 30-40% of adults; this ratio is even smaller for infants delivered before 34 weeks. Following the first two weeks, the glomerular filtration speed is doubled, eventually reaches to adult-levels in 2.5 – 5 months (Çetinkaya &Tengir, 2006). Glomerular filtration speed meets adult levels in 6 – 12 months (Pala & Baktr, 2011). The half-lives of drugs also change (Pala & Baktr, 2011).
