Without nephron loops, the collecting duct could not produce hypertonic urine. The production of hypertonic urine requires an osmotic gradient from the corticomedullary junction to the apex of the medullary pyramid, and this gradient is produced by the nephron loops.
A simple squamous epithelium would not be as capable as a transitional epithelium of the great stretching and thinning, and relaxation and thickening, associated with filling and emptying of the bladder.
A urachal fistula results in the emission of urine through the navel when the bladder contracts. Whenever the baby urinates, it expels urine through both the urethra and the navel.
Such an anatomical arrangement would result in hydronephrosis, because as the bladder filled and its fundus rose in the abdominopelvic cavity, the ureters entering from above would be pushed upward and probably become kinked. Like a kink in a garden hose, this would obstruct fluid flow and result in high pressure proximal to the obstruction—in this case, in the upper ureter and renal pelvis.
The proximal convoluted tubule is much longer than the distal, and has more cross-sectional surface area because of its long, abundant microvilli. The distal convoluted tubule is shorter and has a smoother epithelial surface, therefore much less surface area. This difference is correlated with the fact that the PCT reabsorbs about 65% of the water in the glomerular filtrate, whereas the DCT reabsorbs about 15%.
