Renal tubule

The Renal tubule is a long, complicated tubule, about 15 mm long and 55 μm in diameter. The tubular portion of the nephron is the continuation of the Bowman capsule.

1. Proximal convoluted tubule

The proximal convoluted tubule is the coiled part of the Bowman capsule. It's situated in the cortex. It continues as the descending limb of Henle's loop. The length of the proximal convoluted tubule is 14 mm and the diameter is 55 μ.

The proximal convoluted tubule is formed by a single layer of cuboid epithelial cells. The characteristic feature of these cells is the presence of hair-like projections directed towards the lumen of the tubule. Because of the presence of these projections, epithelial cells are called brush-bordered cells.

2. Loop of Henle

The fluid flows from the proximal tubule to the loop of Henle (or the loop of the nephron) which dips into the renal medulla.

Each loop consists of a descending limb and an ascending limb. The walls of the descending limb and the lower end of the ascending limb are very thin and are thus called the thin segment of the Henle loop. After the ascending limb of the loop returns partway back to the cortex, the surface becomes much thicker and is referred to as the thick segment of the ascending limb.

At the end of the thick ascending limb, there is a short segment with a plaque of specialized epithelial cells in its wall, known as the macula densa., macula densa plays an important role in the regulation of nephron function.

3. Distal tubule

The distal convoluted tubule is a continuation of the thick ascending segment and, like the proximal tubule, is located in the renal cortex. The length of the distal convoluted tubule is 14.5 to 15 mm. It is from 22 to 50 μ in diameter.

The distal convoluted tubule is lined with a single layer of cuboid epithelial cells without a brush border. The epithelial cells in the distal convoluted tubule are called intercalated cells (I cell). beyond the macula densa, the fluid reaches the distal tubule, followed by the connecting tubule and the cortical collecting tubule, which lead to the cortical collecting duct.

Initial sections of 8 to 10 cortical collecting ducts are joined together to create a single larger collecting duct, which runs down into the medulla and becomes a medullary collecting duct.

Collecting ducts merge into progressively larger ducts that are eventually drained into the renal pelvis via the tips of the renal papillae. There are about 250 of the very large collecting ducts in each kidney, each of which collects urine from about 4000 nephrons.