Renal Tubule and Collecting Duct

The renal tubule measures approximately 3 cm and consists of three main segments: the proximal convoluted tubule (PCT), the loop of Henle, and the distal convoluted tubule (DCT). Each segment has distinct epithelial cell types and structural features that support its specialized reabsorption and secretion functions.

The proximal convoluted tubule's apical surface is covered with microvilli, which significantly increase surface area for fluid and solute reabsorption and substance secretion. This structural adaptation allows the PCT to efficiently reabsorb glucose, amino acids, sodium, and water from the filtrate back into the bloodstream.

The descending limb of the loop of Henle is permeable to water but not solutes, causing filtrate concentration to increase. The ascending limb is impermeable to water but actively transports sodium and chloride out of the tubule, creating a countercurrent gradient essential for reabsorption and secretion in the loop of Henle and urine concentration.

Principal cells regulate water and sodium balance through antidiuretic hormone (ADH) receptors, with high ADH levels increasing water reabsorption to produce concentrated urine. Intercalated cells maintain blood pH by excreting or reabsorbing hydrogen ions or bicarbonate ions based on the body's acid-base status.

The distal convoluted tubule has fewer microvilli and thinner cuboidal cells than the proximal convoluted tubule because it performs selective reabsorption and secretion rather than bulk reabsorption. This structural difference reflects its specialized function in fine-tuning sodium, potassium, and calcium levels to adjust electrolyte balance.

The proximal convoluted tubule and initial descending limb of the loop of Henle contain cuboidal epithelial cells with microvilli. The descending thin limb has simple squamous epithelium, while the ascending limb consists of cuboidal or low columnar cells. The distal convoluted tubule comprises thinner cuboidal cells with fewer microvilli.

The collecting duct receives filtrate from multiple nephrons and fine-tunes water and electrolyte levels to determine final urine volume and composition. Principal cells respond to ADH to control water reabsorption, while intercalated cells adjust acid-base balance, allowing the kidney to produce either concentrated or dilute urine based on body needs.