Anatomical structure
Morphologically, each nephron is a complex system of microscopic tubules. Traditionally, it is divided into several consecutive segments, each performing a specific task in the process of urine formation.
Key parts of the nephron include:
- Renal corpuscle. Consists of a capillary 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 is the main site for returning substances to 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 for fine-tuning the composition of the future urine.
- Collecting duct. Strictly speaking, it represents the functional endpoint of the nephron, where fluid from multiple distal tubules drains.
Proximal tubule: obligatory reabsorption
The most extensive process of substance recovery unfolds in the proximal convoluted tubule — obligate (obligatory) reabsorption. At this stage, the body reclaims the vast majority of filtered components. The total volume of reabsorption here accounts for about 80% of the initial filtrate.
$Na^+$, $K^+$, $Cl^-$ ions, as well as water, vital amino acids, vitamins, and glucose molecules are actively returned to the bloodstream. According to physiological parameters, the absorption of water and ions (sodium, potassium, calcium, and chlorine) 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 takes place 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 the urine concentration process. Its descending and ascending limbs have fundamentally different histological and physiological characteristics.
The descending limb of the loop is lined with squamous epithelial cells. This tissue is functionally passive: cells are poor in mitochondria and enzyme systems. 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 freely leaves the tubular lumen and enters the interstitium passively, along an osmotic gradient ($Na^+$ ions in the surrounding tissue space literally "pull" water toward themselves).
The ascending limb, by contrast, is completely impermeable to water. However, potent reabsorption of electrolytes ($Na^+$) takes place here. Thus, due to the separation of water and salt flows, efficient concentration of the tubular fluid is achieved.
Distal segments and formation of definitive urine
In the distal convoluted tubule, facultative (regulated) reabsorption is carried out. Unlike the proximal segment, processes here strictly depend on the current needs of the body. In this segment, $Na^+$, $Cl^-$ ions 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 precisely where definitive urine is formed. In the collecting ducts, the reabsorption of $H_2O$ and $Na^+$ continues, and potassium ions ($K^+$), ammonia ($NH_3$), and hydrogen protons ($H^+$) are secreted. After passing this stage, the fluid composition is fixed, and it is directed into the renal pelvis.
Briefly: answers to topical questions
What types of nephrons are distinguished in the kidney depending on their location?
Depending on the localization of the renal corpuscle and the length of the loop of Henle, three types of nephrons are distinguished in the kidney.
- Short cortical nephrons — renal corpuscles lie in the cortex, the loop is short and does not descend deeply.
- Intermediate cortical nephrons — the main type (80%), medium-sized loop descends to the outer zone of the medullary pyramids.
- Juxtamedullary nephrons — renal corpuscles lie at the cortico-medullary junction, and the long loop penetrates almost the entire thickness of the pyramids.
How does the countercurrent multiplier system of the loop of Henle function?
The countercurrent multiplier system creates an osmotic gradient in the renal medulla to concentrate urine. The descending and ascending limbs work as a single unit:
- Ascending limb — active transport of sodium and chloride ions occurs, while epithelial cells do not let water through.
- Descending limb — passive reabsorption of water occurs.
The exit of ions from the ascending limb increases the osmotic pressure in the surrounding fluid, which promotes the passive exit of water from the descending limb along the gradient.
Which hormones regulate facultative reabsorption in the distal convoluted tubule and collecting duct?
Facultative reabsorption in the distal convoluted tubules and collecting ducts is regulated by aldosterone, antidiuretic hormone, and natriuretic hormone.
- Aldosterone — acts on the distal convoluted tubules and collecting ducts; enhances the reabsorption of Na+, Cl-, and H2O, activates Na+,K+-ATPase, water is passively reabsorbed following Na+.
- Antidiuretic hormone (vasopressin) — acts on the collecting ducts; increases the permeability of their walls to H2O via aquaporins and enhances water reabsorption.
- Natriuretic hormone — in the distal convoluted tubules, it inhibits Na+,K+-ATPase and reduces the reabsorption of Na+ and water.
Class and exam questions
Is sodium absorbed in the descending limb of the loop of Henle?
No, sodium reabsorption does not occur in this segment. The walls of the descending limb are formed by squamous epithelium, which lacks mitochondria and specialized enzyme systems for active ion transport.
What is the difference between obligate and facultative reabsorption?
Obligate reabsorption occurs in the proximal tubule and is mandatory, returning up to 80% of the filtrate. Facultative reabsorption takes place in the distal tubule, is regulated, and affects only about 10% of water and sodium.
How does water leave the descending limb of the nephron loop?
Water exits passively along an osmotic gradient through pores at the junctions of epithelial cells. It is drawn by sodium ions present in the interstitial fluid.
Where is definitive urine formed?
The process of definitive urine formation is completed in the collecting ducts, where final reabsorption of water and sodium occurs, as well as the secretion of protons, potassium, and ammonia.
What else to study on this topic
- Mechanisms of tubular secretion of hydrogen and ammonia ions
- Osmotic gradient of the renal interstitium
- Histological structure of the epithelium of various nephron segments
- The role of para-aminohippuric acid (PAH) in assessing renal function
- Normal values of the glomerular filtration rate (GFR)
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