A) Segmental distribution of AVP V1a receptor (adapted from references 13, 14 and 31). AVP, acting on V1a receptors in the macula densa, regulates renin secretion; acting on V1a receptors in the vasa recta, reduces blood flow to the inner medulla and minimizes solute escape from the medullary interstitium; and, acting on V1a receptors on the luminal side of collecting duct principal cells, stimulates synthesis of prostaglandins that attenuate V2-mediated antidiuretic action and inhibit sodium transport. B) Segmental distribution of AVP V2 receptor (adapted from reference 12). AVP acting on V2 receptors contribute to urinary concentration by inserting AQP-2 into the apical cell membrane of collecting duct principal cells within minutes and up-regulating AQP-2 gene expression on a longer-term; activating UT-A1 and UT-A3 in the terminal part of the inner medullary collecting duct; stimulating ENaC sodium transport in cortical and outer medullary collecting ducts, and increasing Na-K-2Cl cotransporter expression and sodium reabsorption in TAL. C) Vascular and tubular routes of urea recycling within the kidney. Only a long loop of Henle is depicted for simplicity. Urea delivery to the inner medulla and transit in ascending vasa recta are shown in green. The pathways allowing urea to return to the inner medulla are indicated by red arrows for the vascular route and by blue arrows for the tubular route (adapted from reference 38).38 C, cortex; OS, outer stripe of the outer medulla; IS, inner stripe of the outer medulla; IM, inner medulla; AA, arcuate artery; ILA, interlobular artery; aa, afferent arteriole; ea, efferent arteriole; M, mesangium; MTAL, medullary thick ascending limb of Henle; CTAL, cortical thick ascending limb of Henle; MD, macula densa; DCT, distal convoluted tubule; CNT, connecting tubule; CCD, cortical collecting duct; MCD, medullary collecting duct; DVR, descending vasa recta; AVR, ascending vasa recta; UT-A1/3, vasopressin-regulated urea transporters A1/3; UT-B, urea transporter B; UT-A2, urea transporter A-2.