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The Nephron: Loop of Henle

figure Henle’s loop consists of a thin descending limb, a thin ascending limb characterized by a thin flat squamus epithelial cell, and a thick ascending limb with somewhat taller squamous epithelial cells. The thin limb begins at the end of the pars recta, descends into the medulla, makes a hairpin turn becoming the thin ascending limb which parallels the course of the descending limb back towards the cortex. At the junction of the outer and inner medulla the epithelial cells of the ascending limb become cuboidal , the tubule diameter increases and the segment becomes the thick ascending limb of Henle’s loop. At this point it is necessary to distinguish between three types of nephrons.

  1. Superficial cortical nephrons have glomeruli located in the outermost cortex, have short loops of Henle extending only a short way into the medulla, and lack thin ascending limbs.

  2. Mid cortical nephrons have glomeruli located between the superficial and the juxtamedullary nephrons, and may have short or long loops of Henle.

  3. Juxtamedullary nephrons have glomeruli located in the cortex just above the junction of cortex and medulla and all have long loops of Henle which descend deep into the medulla.

The thin descending limb is water permeable and relatively impermeant to solutes. Both thin and thick ascending limbs are impermeable to water. The thin ascending limb is permeable to solutes and the thick ascending limb has a high capability for active salt reabsorption. The loop of Henle and the closely associated vasa recta play a critical role in the mechanism for urinary concentration or dilution. Salt reabsorption in the water impermeable thick ascending limb dilutes the tubular fluid, concentrates the medullary ISF and generates a cortico-medullary osmolar gradient in the renal ECF which provides the force for passive water reabsorption from the collecting duct. The permeability of the collecting duct is determined by ADH which increases water permeability of the duct. When the duct is permeable the urine is concentrated, when it is impermeable the urine is dilute.

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