• Water is filtered and then reabsorbed by osmosis.
• The major site of water reabsorption is the proximal tubule
• In the proximal tubule water is absorbed iso osmotically with ions In the loop of Henle, ion and water reabsorption occur in different limbs.
• In the loop of Henle more ions are reabsorbed than water, so the medulla becomes concentrated and the tubular fluid becomes dilute
• There is no significant water reabsorption in the distal tubule In the collecting ducts water can leave the ducts to enter the medullary interstitium
• ADH controls the insertion of aquaporin water channels which are necessary for water reabsorption in the collecting duct
• The posterior pituitary makes ADH
• Vasa recta are looped blood vessels which maintain the concentrated nature of the medullary interstitium

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The year of the pores
Schiepatti A, Remuzzi G, 1996
Lancet 348 suppl II 13

Pathophysiology of the aquaporin water channels.
King LS, Agre P,
Annu Rev Physiol 1996: 58: 619-648

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C. A. O'Callaghan 2004