Page 719 - Atlas of Histology with Functional Correlations

FIGURE 18.12 ■ Kidney medulla: terminal end of a papilla (longitudinal

section). Stain: hematoxylin and eosin. Medium magnification.

FUNCTIONAL CORRELATIONS 18.3 ■

Collecting Tubules, Collecting Ducts, and

Antidiuretic Hormone

Glomerular filtrate flows from the distal convoluted tubules to the collecting
tubules and collecting ducts. Under normal conditions, these tubules are not

permeable to water, and the urine remains dilute or hypotonic. However,
during excessive water loss or dehydration, antidiuretic hormone (ADH) is
released from the posterior lobe (neurohypophysis) of the pituitary gland.
The ADH induces the epithelium of collecting tubules and collecting ducts to
become highly permeable to water passage. The water-permeable cells in the

collecting ducts contain integral transmembrane pore proteins called
aquaporins that function as channels for water molecules. Aquaporins, also
known as water channels, increase water permeability and selectively

conduct water molecules in and out of the cells. The functions of aquaporins
are regulated by the ADH that binds to the receptors on the duct cells and

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