<P> The initial collecting tubule is a segment with a constitution similar as the collecting duct, but before the convergence with other tubules . </P> <P> The "cortical collecting ducts" receive filtrate from multiple initial collecting tubules and descend into the renal medulla to form medullary collecting ducts . </P> <P> "Medullary collecting ducts" are divided into outer and inner segments, the latter reaching more deeply into the medulla . The variable reabsorption of water and, depending on fluid balances and hormonal influences, the reabsorption or secretion of sodium, potassium, hydrogen and bicarbonate ion continues here . Urea passively transports out of duct here and creates 500mOsm gradient . </P> <P> The outer segment of the medullary collecting duct follows the cortical collecting duct . It reaches the level of the renal medulla where the thin descending limb of loop of Henle borders with the thick ascending limb of loop of Henle </P>

Where are the collecting ducts in the kidney