Collecting Tubules and Ducts:

1. Structure. These differ from the nephrons in their embryonic origin and can be easily distinguished from the proximal and distal tubules in sections. Their block like lining cells have distinct intercellular borders; they are cuboidal in the smaller tubules and columnar in the larger leg, papillary) ducts of the medulla. Since their cytoplasm stains poorly, the cells appear clear or white.

2. Function. The cortical collecting tubules receive a reduced volume of hypotonic or isotonic urine from the nephrons and empty it into larger collecting ducts. These leave the cortex in medullary rays and enter the medulla, increasing in size until they open into a minor calyx through the tips of papillae.

2. Proximal convoluted tubule

This epithelial tube begins at the urinary pole of the rcnal corpuscle. Its lining is simple low columnar-to-cuboidal epithelium. The lining cells have abundant long microvilli. Together they form a brush border that partly obscures the lumen and increases the surface area available for absorption. The convoluted part of the proximal tubule lies in the cortex and empties into its straight portion (also called the thick descending limb of the loop of Henle), which has a similar epithelium and function. Together, the convoluted and straight portions of the proximal tubule measure about 14 mm, making this the longest portion of the nephron.