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
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). Dessert rats have almost 100% juxtamedullary nephrons.
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)
