Читать книгу Disorders of Fluid and Electrolyte Metabolism - Группа авторов - Страница 27

Because the urine that emerges from the PT is isosmotic, the first nephron segment actually involved in urinary concentration is the thin descending limb of the loop of Henle (tDL). There are two types of tDL. The short loops originate in superficial and mid-cortical glomeruli and turn in the outer medulla. The long loops originate in deep cortical and juxtamedullary glomeruli and penetrate variable distances into the inner medulla. Short and long tDL are anatomically distinct; the long limbs in particular display considerable interspecies variability [12]. It is interesting that no correlation is apparent between a species’ maximal concentrating ability and the ratio of short and long loops. In fact, in rodents with highest urinary concentrations, the number of short loops is considerably greater than the number of long loops. Approximately, 15% of nephrons possess long loops in the human kidney; the other 85% of nephrons have short loops. The tDL is highly water permeable as it also has, in its first portion, abundant expression of AQP1 [13]. Thus, tubular fluid is concentrated as it descends primarily, but probably not exclusively, by the extraction of water.

Somewhat proximal to the hairpin turn, there is a transition from tDL to the thin ascending limb of the loop of Henle (tAL). This segment, as well as the remainder of the tAL, is water impermeable. The nature and precise site at which the movement of solutes (urea and sodium chloride) occur has not been fully defined. Active sodium transport has not been demonstrated convincingly, and this segment’s morphologic appearance with a few mitochondria does not suggest active metabolic metabolism.

The TALLH appears both structurally and functionally distinct from its thin counterpart. The epithelium is remarkably uniform among species with tall, heavily interdigitating cells with large mitochondria. The observation that fluid emerges into the early distal tubule hypotonic (about 100 mOsm/kg) supports the view that active sodium chloride transport out of this water-impermeable segment provides the single effect required for the operation of the countercurrent multiplier. The primary mechanism of chloride absorption in the TALLH is mediated by NKCC2.