The Juxtamedullary Nephrons Extend Deep into the Inner Medulla
The anatomical arrangement of the renal tubules in the medulla is a crucial element of the urine-concentrating mechanism. The nephrons of the mammalian kidney are subdivided into superficial and juxtamedullary nephrons based on the location of their respective glomeruli (see Figure 41-1).
Superficial nephrons have short loops of Henle that extend only into the inner stripe of the outer medulla. Iuxtaniedullary nephrons have long loops of Henle that extend deep into the inner medulla. The JuxtameduIIary nephrons are particularly responsible for the kidney’s ability to concentrate urine at a much higher level than the osmolality of plasma.As discussed in Chapters 41 and 42, avian kidneys contain both mammalian-type and reptilian-type nephrons. The reptilian-type nephrons have glomeruli that are near the surface in the renal cortex and have no loops of Henle. The mammalian-type nephrons have glomeruli that are deeper in the cortex and have either short or long loops of Henle that extend into the medullary cone. The mammalian-type nephrons are believed to be at least partly responsible for the ability of birds to excrete urine that is hypertonic relative to plasma.