Figure 20-8 Tubular concentration of urine. The juxtamedullary nephrons have long loops of Henle associated with the vasa recta. Dashed arrows represent passive movement of fluid or solutes along concentration or osmolar gradients; solid arrows represent active transport. Tubular fluid enters the distal proximal tubule iso-osmotic with plasma (300 mOsm/kg) (1). In the descending limb of Henle (2), water rapidly diffuses out into the increasingly hypertonic medulla and is removed by the vasa recta, and as a consequence, the tubular fluid becomes hypertonic, largely because of concentration of sodium chloride (NaCl). Urea diffuses in from the hypertonic interstitium, further increasing tubular fluid osmolality (1200 mOsm/kg). In the thin ascending loop of Henle (3), NaCl passively diffuses into the interstitium along its concentration gradient, but water is trapped in the water-impermeable tubule, which progressively decreases tubular fluid osmolality. Urea passively diffuses into the tubular fluid (urea recycling). Tubular dilution is accelerated by active reabsorption of NaCl in the thick ascending loop (the diluting segment) and proximal distal tubule (4). The fluid entering the distal tubule is quite hypo-osmotic (100 mOsm/kg). In the collecting segment (5), the osmolality of the tubular fluid returns to that of plasma (300 mOsm/kg), but unlike the contents of the proximal tubule, the solute component consists largely of urea, creatinine, and other excreted compounds. Increased plasma antidiuretic hormone (ADH) renders the cortical and medullary collecting ducts (6) permeable to water, which passively diffuses into the hypertonic medullary interstitium. Even though some urea diffuses out into the medulla, the maximal osmolality of concentrated urine (7) approaches that of the hypertonic medullary interstitium, about 1200 mOsm/kg. In the absence of ADH, the collecting ducts remain impermeable to water, and the urine is diluted. (From Stanton BA, Koeppen BM: Control of body fluid osmolality and volume. In Berne RM, Levy MN [eds]: Physiology, 4th ed. St Louis, CV Mosby, 1998, pp 715–743.)