As the glomerular ultrafiltrate passes through the urinifer-ous and collecting tubules of the kidney, it undergoes changes that involve both active and passive absorption, as well as secretion.
• Certain substances within the ultrafiltrate are reabsorbed, some partially (e.g., water, sodium, and bicarbonate) and some completely (e.g., glucose).
• Other substances (e.g., creatinine and organic acids and bases) are added to the ultrafiltrate (i.e., the primary urine) by secretory activity of the tubule cells.
Thus, the volume of the ultrafiltrate is reduced substantially, and the urine is made hyperosmotic. The long loops of Henle and the collecting tubules that pass parallel to similarly arranged blood vessels, the vasa recta, serve as the basis for the countercurrent multiplier mechanism that is instrumental in concentrating the urine, thereby making it hyperosmotic.
Proximal Convoluted Tubule
The proximal convoluted tubule is the initial and major site of reabsorption
The proximal convoluted tubule receives the ultrafiltrate from the urinary space of Bowman's capsule. The cuboidal cells of the proximal convoluted tubule have the elaborate surface specializations associated with cells engaged in absorption and fluid transport. They exhibit the following features:
• A brush border, composed of relatively long, closely packed, and straight microvilli (Fig. 19.14)
• A junctional complex, consisting of a narrow, tight junction that seals off the intercellular space from the lumen of the tubule and a zonula adherens that maintains the adhesion between neighboring cells
• Plicae or folds located on the lateral surfaces of the cells, which are large flattened processes, alternating with similar processes of adjacent cells (see Fig. 19.14)
• Extensive interdigitation of basal processes of adjacent cells (Figs. 19.15 and 19.16)
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