Proximal convoluted tubule

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The proximal convoluted tubule (PCT) is the first tubular component of the nephron, the functional filtration unit of the kidney. It arises as a continuation of the flattened urinary (Bowman’s) space from the tubular pole of the renal corpuscle.

The proximal convoluted tubule is located within the renal cortex and as its name suggests, it is the most coiled section of the renal tubule. It measures about 14 mm in length and continues into the medulla as the loop of Henle. The proximal convoluted tubule is first to receive the ultrafiltrate from the urinary space of Bowman’s capsule and serves as the primary site for initial reabsorption.

Histologically, the proximal convoluted tubule is lined by simple cuboidal epithelium with a prominent brush border (tall microvilli) on its luminal
surface to enhance its reabsorption capacity. Additionally, these cells possess junctional complexes including tight junctions and zonula adherens, as well as extensive basolateral interdigitations that ensure cell-to-cell adhesion. Furthermore, these cells are rich in mitochondria which provide the energy, needed for its active transport processes and gives the cytoplasm a bright eosinophilic appearance on light microscopy. 

The primary function of the proximal convoluted tubule is to actively reabsorb nearly all organic nutrients such as glucose, polypeptides and amino acids, electrolytes especially sodium (Na+), all plasma proteins, vitamins, and 60–65% of the water from the tubular fluid into blood in the peritubular capillaries. This is then transported into general circulation. Other organic anions and cations such as hydrogen (H+), and ammonium (NH4+) are in turn secreted into the filtrate.