23 Renal Water regulation

23 Renal Water regulation - RENAL WATER REGULATION page 1...

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RENAL WATER REGULATION AC Brown page 1 A7d INTRODUCTION TO WATER EXCRETION A. Role of the Kidney: to adjust urine formation rate and urine concentration to maintain 1. body fluid osmolar concentration 2. body fluid volume 3. intravascular volume Note: the term "osmolar" refers to the total concentration of dissolved particles; the osmolar concentration determines the total osmotic pressure B. Mechanism: the ability to adjust urine volume and concentration depends on establishing an appropriate osmotic environment in the renal medulla; this is accomplished by those nephrons whose loops of Henle dips deep into the medulla -- the juxtamedullary nephrons Note: the fraction of total nephrons that are juxtamedullary governs the ability to survive in a water-deprived environment. Humans have about 20% juxtamedullary nephrons (and 80% cortical nephrons). Desert rats have almost 100% juxtamedullary nephrons. C. Juxtamedullary Nephron Functional Anatomy 1. descending thin limb 2. "hairpin" loop 3. ascending thin limb 4. ascending thick limb and initial segment of the distal tubule 5. parallel capillaries and venules (vasa recta)
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RENAL WATER REGULATION AC Brown page 2 A7d LOOP OF HENLE FUNCTION A. Countercurrent Multiplication 1. The ascending thick limb of the Loop of Henle has an active transport pump that reabsorbs Na + and Cl - ; however, it is impermeable to water. This results in an increased osmolality in the renal medulla interstitial fluid 2. The high accumulation of Na + and Cl - in the renal medulla interstitial fluid causes exchange of water and dissolved particles between the interstitium and the descending and ascending thin limbs, leading to trapping of most of the sodium and chloride in the medulla. As a result, interstitial fluid throughout the whole medulla becomes hyperosmotic and the fluid leaving the ascending thick limb and entering the distal tubule is hyposmotic. 3. Because this process occurs as the fluid is flowing along the loop of Henle and exchange takes place between the interstitium and fluid streams moving in opposite directions (descending and ascending) and because osmolality increases progressively with depth in the medulla, the mechanism is termed countercurrent multiplication . 4. The interstitial osmolality is further increased by the accumulation of urea (explained later). PT
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23 Renal Water regulation - RENAL WATER REGULATION page 1...

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