Chapter 11: Urinary system
The major function of the proximal tubules (PT) is to reabsorb fluid, electrolytes and nutrients. There is a convoluted portion, which is tortuous and often seen as rings of epithelial cells lining a basement membrane, and a straight portion which is more linear running from the cortex to the medulla. This PT is responsible for roughly 98% of glucose and 65% of sodium reabsorption. The PT is located in the cortex and is composed of simple columnar epithelium joined together by tight junctions. The cell-cell junctions are permeable to small molecules yet permit transport from the tubular lumen into the interstitium in a paracellular manner. The apical brush border is a characteristic and distinguishing feature of the PT. The brush border is composed of microvilli which facilitate reabsorption of small peptides and simple sugars (monosaccharides and disaccharides).