Anatomical Structure
Morphologically, each nephron is a complex system of microscopic tubules. It is traditionally divided into several sequential segments, each performing a specific task in urine formation.
Key parts of the nephron include:
- Renal corpuscle. Consists of the glomerulus and the surrounding Bowman's capsule. This is the starting point where plasma filtration begins.
- Proximal convoluted tubule. Follows immediately after the capsule and serves as the main site for reabsorption of substances into the bloodstream.
- Loop of Henle. Has a U-shape and is divided into a thin segment (including descending and ascending limbs) and a thick segment.
- Distal convoluted tubule. The segment responsible for fine-tuning the composition of the tubular fluid.
- Collecting duct. Strictly speaking, this is the functional termination of the nephron where fluid from multiple distal tubules drains.
Proximal Tubule: Obligate Reabsorption
The most massive reabsorption process—obligate reabsorption—takes place in the proximal convoluted tubule. At this stage, the body reclaims the overwhelming majority of filtered components. The total volume of reabsorption here accounts for about 80% of the initial filtrate.
Ions such as $Na^+$, $K^+$, $Cl^-$, as well as water, essential amino acids, vitamins, and glucose molecules, are actively returned to the bloodstream. Water and ion absorption (sodium, potassium, calcium, and chloride) reaches 65–85%. Bicarbonates ($HCO_3^-$) are reabsorbed by 80–90%, and phosphates ($PO_4^{3-}$) up to 100%.
In addition to salvaging useful molecules, intensive tubular secretion occurs here. Epithelial cells purposefully secrete hydrogen ions ($H^+$), ammonia ($NH_3$), and para-aminohippuric acid (PAH) into the tubular lumen.
Loop of Henle and Urine Concentration
The Loop of Henle plays a critical role in urine concentration. Its descending and ascending limbs have fundamentally different histological and physiological characteristics.
The descending limb of the loop is lined by flat epithelial cells. This tissue is functionally passive: cells are poor in mitochondria and enzyme complexes. Due to the lack of necessary transport proteins, sodium reabsorption does not occur here. However, intercellular junctions in this segment form a membrane with pores permeable to fluid. Water passively leaves the tubular lumen into the interstitium along an osmotic gradient ($Na^+$ ions in the surrounding interstitial space literally "pull" water out).
The ascending limb, by contrast, is completely impermeable to water. However, robust electrolyte reabsorption ($Na^+$) takes place here. Thus, by separating the flows of water and salts, efficient concentration of the tubular fluid is achieved.
Distal Segments and Final Urine Formation
Facultative (regulated) reabsorption occurs in the distal convoluted tubule. Unlike the proximal segment, these processes strictly depend on the current needs of the body. In this segment, $Na^+$, $Cl^-$ and water are reabsorbed (the distal segment accounts for about 10% of water and sodium recovery).
The functional pathway ends in the collecting duct. This is where final urine is formed. Collecting ducts continue the reabsorption of $H_2O$ and $Na^+$, while secreting potassium ions ($K^+$), ammonia ($NH_3$), and hydrogen protons ($H^+$). After passing this stage, the fluid composition is fixed, and it is directed into the renal calyces and pelvis.