Renal Corpuscle

As discussed earlier, the renal corpuscle consists of a network of capillaries called the glomerulus that is surrounded by Bowman’s (glomerular) capsule. Bowman’s capsule surrounds the glomerulus to form a lumen, and captures and directs this filtrate to the proximal convoluted tubule (PCT). Podocytes are the cells that cover the glomerular capillaries by extending finger-like arms  called pedicels over them (Figure 9.7). Between the pedicels there are specialized slits that allow for filtration of the blood. As blood passes through the glomerulus, 10 to 20 percent of the plasma filters between these sieve-like fingers to be captured by Bowman’s capsule and funneled to the PCT. The glomerular capillaries have fenestrations, or “windows” in their walls that allow the filtrate to exit the capillary and be filtered by the pedicels. The fenestrations prevent filtration of blood cells or large proteins, but allow the passage of most other blood components, like water, electrolytes, and small molecules.

Proximal Convoluted Tubule (PCT)

Filtered fluid collected by Bowman’s capsule enters into the PCT. It is called convoluted due to its twisting path. A single layer of cells line the tubule and have folding of their cell membranes which creates microvilli, forming a brush border. These microvilli create a large surface area to maximize the absorption and secretion of solutes (Na⁺, Cl^–, glucose, etc.), which is the main function of this portion of the nephron. These cells actively transport ions across their membranes, so they have many mitochondria to produce sufficient ATP.

Loop of Henle

The descending and ascending portions of the loop of Henle are continuations of the same tubule which descends into the deep cortex and medulla and returns to the cortex to form the distal convoluted tubule. Its function will be discussed in a subsequent section.

Distal Convoluted Tubule (DCT)

The DCT, like the PCT, is twisty, but it is shorter than the PCT. These cells are not as active as those in the PCT; thus, they have fewer microvilli. These cells also require ATP to pump ions against their concentration gradient, so you will find of large numbers of mitochondria, although fewer than in the PCT.

Collecting Ducts

The collecting ducts are continuous with the nephron but not technically part of it. In fact, each duct collects filtrate from several nephrons for final modification. Collecting ducts merge as they descend into the medulla and empty at papillae. In states when the body needs to keep more water, a hormone called antidiuretic hormone (ADH) signals the cells of the collecting ducts to produce protein channels called aquaporins, which selectively allow water to be reabsorbed instead of excreted in the urine.

Aquaporin Water Channel

Positive charges inside the channel prevent the leakage of electrolytes across the cell membrane, while allowing water to move due to osmosis.