The distal tubule starts shortly after the macula densa and extends to the point where multiple nephrons combine to form a cortical collecting duct in the cortex. This duct then enters the medulla and transforms into the outer medullary collecting duct and finally the inner medullary collecting duct.

Each segment of the nephron is made up of specialized cells that perform specific transport functions. Proximal tubule cells have an amplified apical membrane called the brush border, which is unique to the proximal tubule. The basolateral membrane is highly invaginated and contains many mitochondria. On the other hand, the descending and ascending thin limbs of the loop of Henle have underdeveloped apical and basolateral surfaces with few mitochondria. The thick ascending limb and the distal tubule cells have abundant mitochondria and extensive infoldings of the basolateral membrane.

The collecting duct consists of two types of cells: principal cells and intercalated cells. Principal cells have a moderately invaginated basolateral membrane and few mitochondria. They are responsible for the reabsorption of NaCl and the secretion of K+. Intercalated cells, which regulate acid-base balance, have a high density of mitochondria. One population of intercalated cells secretes H+ (absorbs HCO3-) while another population secretes HCO3-. The inner medullary collecting duct, the final segment of the nephron, is composed of cells with poorly developed apical and basolateral surfaces and few mitochondria.

Except for intercalated cells, all cells in the nephron have a nonmotile primary cilium in their apical plasma membrane. These primary cilia act as mechanosensors and chemosensors, sensing changes in fluid flow rate and compounds in the surrounding fluid. They initiate Ca++-dependent signaling pathways that control kidney cell function, proliferation, differentiation, and programmed cell death.
Figs. 33.2 33.3 ). The distal tubule begins a short distance beyond the macula densa and extends to the point in the cortex where two or more nephrons join to form a cortical collecting duct. The cortical collecting duct enters the medulla and becomes the outer medullary collecting duct and then the inner medullary collecting duct.
Each nephron segment is made up of cells that are uniquely suited to perform specific transport functions (see
Fig. 33.3 ). Proximal tubule cells have an extensively amplified apical membrane (the ultrafiltrate or urine side of the cell) called the brush border, which is present only in the proximal tubule. The basolateral membrane (the interstitial or blood side of the cell) is highly invaginated. These invaginations contain many mitochondria. In contrast, the descending and ascending thin limbs of the loop of Henle have poorly developed apical and basolateral surfaces and few mitochondria. The cells of the thick ascending limb and the distal tubule have abundant mitochondria and extensive infoldings of the basolateral membrane.
The collecting duct is composed of two cell types: principal cells and intercalated cells. Principal cells have a moderately invaginated basolateral membrane and contain few mitochondria (see
Fig. 33.3 ). Principal cells play an important role in reabsorption of NaCl (see ) and secretion of K+ (see ). Intercalated cells, which play an important role in regulating acid-base balance, have a high density of mitochondria (see
Fig. 33.3 ). One population of intercalated cells secretes H+ (i.e., reabsorbs HCO3 −), and a second population secretes HCO3 − (see ). The final segment of the nephron, the inner medullary collecting duct, is composed of inner medullary collecting duct cells, which have poorly developed apical and basolateral surfaces and few mitochondria.
All cells in the nephron except intercalated cells have in their apical plasma membrane a single nonmotile primary cilium that protrudes into the tubule fluid (
Fig. 33.4 ). Primary cilia are mechanosensors (i.e., they sense changes in the rate of flow of tubule fluid) and chemosensors (i.e., they sense or respond to compounds in the surrounding fluid), and they initiate Ca++-dependent signaling pathways, including those that control kidney cell function, proliferation, differentiation, and apoptosis (i.e., programmed cell death).