 reabsorption of bicarbonate ions, with concomitant secretion of hydrogen ions, leading to further acidification of the urine.  secretion of ammonium in response to the kidneys’ need to excrete acid and generate bicarbonate.
Aldosterone, secreted by the adrenal gland and released under stimulation by angiotensin II, increases the reabsorption of Na and secretion of K . These effects increase blood volume and blood pressure in response to increased blood Na concentration.
The collecting tubules as well as the cortical collecting ducts and medullary collecting ducts are composed of simple epithelium. The collecting tubules and cortical col lecting ducts have flattened cells, somewhat squamous to cuboidal in shape. The medullary collecting ducts have cuboidal cells, with a transition to columnar cells as the ducts increase in size. The collecting tubules and ducts are readily distinguished from proximal and distal tubules by virtue of the cell boundaries that can be seen in the light microscope (Plate 77, page 734). Two distinct types of cells are present in the collecting tubules and collecting ducts:  Light cells, also called collecting duct cells or CD cells, are the principal cells of the system. They are pale-staining cells with true basal infoldings rather than processes that interdigitate with those of adjacent cells. They possess a single primary cilium and relatively few short microvilli (Fig. 20.22). They contain small, spherical mitochondria. These cells possess an abundance of antidiuretic hormone (ADH)–regulated water channels, aquaporin-2 (AQP-2), which are responsible for water permeability of the collecting ducts. In addition, aquaporins AQP-3 and AQP-4 are present within the basolateral membrane of these cells.
 Dark cells, also called intercalated (IC) cells, occur in considerably smaller numbers. They have many mitochondria, and their cytoplasm appears denser. Microplicae, cytoplasmic folds, are present on their apical surface, as well as microvilli. The microplicae are readily observed with the SEM but may be mistaken for microvilli with the TEM (see Fig. 20.22). They do not show basal infoldings but have basally located interdigitations with neighboring cells. Numerous vesicles are present in the apical cytoplasm. The intercalated cells are involved in the secretion of H (-intercalated cells) or bicarbonate (-intercalated cells), depending on the whether the kidneys need to
FIGURE 20.22 • Scanning electron micrograph of a collecting tubule. This micrograph shows dark cells (asterisks), with numerous short lamellipodia or microridges on their surface, and light cells, each with a primary cilium on its free surface along with small microvilli. The terms light and dark refer to the staining character of sectioned cells and not to the density differences reflecting charge characteristics of the coated surface of the specimen. (Courtesy of Dr. C. Craig Tisher.) excrete acid or alkali. The -intercalated cell actively secretes H into the collecting duct lumen via ATP-depen dent pumps and releases HCO3 via Cl /HCO3 exchangers located in their basolateral cell membrane. The -intercalated cells have opposite polarity and secrete bicarbonate ions into the lumen of the collecting duct. Because of the nature of the diet and thus the need to excrete acid, the epithelium of collecting ducts contains more than -intercalated cells.
The cells of the collecting ducts gradually become taller as the ducts pass from the outer to the inner medulla and become columnar in the region of the renal papilla. The number of dark cells gradually decreases until there are none in the ducts as they approach the papilla.