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Nephron

The nephron is the microscopic structural and functional unit of the kidney, consisting of a renal corpuscle and a renal tubule that work together to filter blood, reabsorb essential nutrients and water, secrete wastes, and regulate electrolyte balance to produce urine.^[1]^[2] Each human kidney contains approximately one million nephrons, which collectively process about 180 liters of blood filtrate per day while returning vital substances to the bloodstream.^[2]^[1] The renal corpuscle, located in the kidney's cortex, serves as the initial filtration site and comprises the glomerulus—a network of capillaries—and Bowman's capsule, which surrounds it to collect the filtrate.^[1] The filtrate passes from the corpuscle into the renal tubule, a series of segments including the proximal convoluted tubule (PCT), loop of Henle, distal convoluted tubule (DCT), and collecting duct.^[1] In the PCT, about 60-65% of the filtered water, sodium chloride, and nutrients such as glucose and amino acids are reabsorbed into the peritubular capillaries via active and passive transport mechanisms.^[1] Nephrons exist in two main types: cortical nephrons, with short loops of Henle confined to the cortex and comprising the majority (about 85%), and juxtamedullary nephrons, with long loops extending deep into the medulla to facilitate urine concentration.^[1] The loop of Henle establishes a countercurrent multiplier system that creates an osmotic gradient in the medulla, enabling the kidneys to produce urine ranging from dilute to highly concentrated based on hydration status.^[1] In the DCT and collecting ducts, fine-tuning occurs through hormone-regulated reabsorption of sodium, water, and calcium, as well as secretion of potassium, hydrogen ions, and other wastes, ensuring acid-base homeostasis and blood pressure regulation.^[1]^[2] Overall, these processes maintain fluid and electrolyte balance, excrete metabolic wastes like urea and creatinine, and support systemic homeostasis.^[2]

Overview

Definition and Location

The nephron is the microscopic structural and functional unit of the kidney, consisting of a renal corpuscle and a renal tubule that together enable the processes of blood filtration, reabsorption, secretion, and excretion to form urine.^[1] This unit is essential for the kidney's role in waste removal and homeostasis.^[1] Nephrons are embedded within the renal cortex and medulla of each kidney, with approximately one million nephrons present in the human kidney.^[3] Each nephron measures 30-50 mm in length, allowing for efficient organization within the kidney's architecture.^[4] Nephrons exist in two main types—cortical and juxtamedullary—differing primarily in the depth of their loops within the kidney.^[1] Through its operations, the nephron processes about 180 liters of glomerular filtrate daily, ultimately producing 1-2 liters of urine while maintaining fluid volume, electrolyte concentrations, and acid-base balance in the body.^[5]^[6]

Types of Nephrons

Nephrons are classified into two primary types based on the position of their renal corpuscles within the kidney cortex and the extent to which their loops of Henle penetrate the medulla: cortical nephrons and juxtamedullary nephrons. Cortical nephrons have their glomeruli situated in the outer two-thirds of the renal cortex, with short loops of Henle that remain confined to the cortex or extend only slightly into the outer medulla. In contrast, juxtamedullary nephrons feature glomeruli located near the junction of the cortex and medulla, along with long loops of Henle that descend deep into the inner medulla, sometimes reaching the papillary tip.^[1]^[7] In humans, cortical nephrons constitute approximately 85% of the total nephron population, while juxtamedullary nephrons account for the remaining 15%. Structurally, cortical nephrons are associated with peritubular capillaries that form a dense network around their tubules for efficient reabsorption in the cortex. Juxtamedullary nephrons, however, are supplied by both peritubular capillaries in the cortex and vasa recta—straight, hairpin-shaped vessels that parallel the long loops of Henle into the medulla to maintain the medullary osmotic gradient without dissipating it. These vascular differences support the specialized roles of each nephron type in renal physiology.^[8]^[9] Functionally, juxtamedullary nephrons enable the kidney's ability to produce highly concentrated urine through the countercurrent multiplier mechanism, where active transport of ions in the ascending limb of the loop of Henle creates a steep osmotic gradient in the medulla, facilitating maximal water reabsorption via aquaporins in the collecting ducts. Cortical nephrons, with their shorter loops, primarily handle bulk filtration and reabsorption of solutes and water in the cortex, contributing less to medullary hypertonicity but supporting overall glomerular filtration rate. The relative prevalence of these nephron types varies by species; for instance, humans rely mostly on cortical nephrons for routine homeostasis, whereas desert-adapted mammals like kangaroo rats possess a much higher proportion of juxtamedullary nephrons—often exceeding 50%—to conserve water in arid conditions by enhancing urine concentration capabilities.^[10]^[11]^[12]

Structure

Renal Corpuscle

The renal corpuscle, also known as the Malpighian corpuscle, serves as the initial filtration site within the nephron, consisting of a capillary tuft called the glomerulus enclosed by Bowman's capsule. This structure is located in the renal cortex and measures approximately 200 μm in diameter. The glomerulus is a network of interconnected capillaries that facilitates the selective filtration of blood plasma, while Bowman's capsule surrounds it to collect the resulting filtrate in the space between its layers.^[13]^[14] The glomerulus features a fenestrated endothelium with pores approximately 60 nm in diameter, allowing passage of plasma components while retaining blood cells. Beneath this lies the glomerular basement membrane, a 240-270 nm thick acellular layer composed of collagen IV, laminin, and proteoglycans, which provides structural support and contributes to charge- and size-selective filtration. Podocytes, specialized epithelial cells of the visceral layer of Bowman's capsule, envelop the glomerular capillaries; their interdigitating foot processes form filtration slits measuring 30-40 nm wide, bridged by slit diaphragms that further refine the filtration barrier.^[14] Bowman's capsule is a double-layered cup-shaped structure: the parietal layer consists of simple squamous epithelium that lines the outer boundary, while the visceral layer comprises podocytes that directly contact the glomerular capillaries. The space between these layers, known as Bowman's space, receives the ultrafiltrate from the glomerulus and connects to the proximal convoluted tubule for further processing. Mesangial cells, located within the mesangium between glomerular capillaries, provide structural support, exhibit phagocytic activity to clear trapped residues and immune complexes from the basement membrane, and possess contractile properties that help regulate capillary surface area.^[14]^[15] Blood enters the glomerulus via the afferent arteriole, which branches into the capillary tuft, and exits through the narrower efferent arteriole, generating a high hydrostatic pressure essential for filtration. This vascular arrangement, supported by mesangial cells, ensures efficient perfusion while maintaining the integrity of the filtration apparatus.^[14]

Renal Tubule

The renal tubule constitutes the elongated extension of the nephron beyond the renal corpuscle, receiving the glomerular filtrate at the urinary pole of Bowman's capsule and processing it through a series of interconnected segments before delivery to the collecting duct system. This structure measures approximately 30 to 55 mm in total length in human nephrons, with juxtamedullary nephrons featuring longer loops of Henle that extend deep into the medulla, increasing the overall length compared to cortical nephrons.^[16] Lined throughout by a continuous simple epithelium, the tubule displays progressive morphological variations in cell height, organelle density, and surface specializations that reflect segmental differentiation.^[1] The proximal convoluted tubule (PCT), confined to the renal cortex, initiates the tubular pathway with its tightly coiled configuration. It is composed of low cuboidal epithelial cells adorned with a dense brush border of microvilli on the apical surface, enhancing luminal contact area, alongside abundant basal-lateral infoldings packed with elongated mitochondria for energy support. These cells also harbor numerous endocytic vesicles, contributing to their robust structural profile.^[1] Descending from the cortex into the medulla, the loop of Henle forms a U-shaped hairpin turn, comprising thin and thick portions with distinct epithelial architectures. The descending thin limb features flattened squamous-like cells with sparse, short microvilli and few organelles, forming a relatively permeable barrier. In contrast, the ascending thick limb returns toward the cortex with taller cuboidal cells exhibiting more pronounced basolateral interdigitations, increased mitochondrial density, and occasional microvilli, providing a more robust cellular framework. Length variations in the loop are pronounced in juxtamedullary nephrons, where it may extend deep into the inner medulla, contrasting with the shorter loops of cortical nephrons.^[1] The distal convoluted tubule (DCT), resuming in the cortex near its originating glomerulus, presents a less coiled path lined by cuboidal to low columnar epithelial cells lacking a prominent brush border but displaying subtle apical microvilli. These cells show extensive basolateral membrane amplification with densely packed mitochondria, imparting a palisade-like appearance, particularly in the early DCT; the late segment incorporates specialized intercalated cells with electron-dense cytoplasm. The DCT spans about 5 mm in humans, underscoring its compact morphology.^[1]^[17] Transitioning seamlessly, the connecting tubule merges the DCT with the collecting duct, featuring a heterogeneous epithelium of principal-like connecting tubule cells with moderate basolateral infoldings and fewer mitochondria, interspersed with intercalated cells exhibiting prominent cytoplasmic density on electron microscopy. This segment maintains cuboidal cell height while bridging cortical regions.^[1]

Vascular and Regulatory Components

The vascular supply to the nephron is essential for its filtration and reabsorption functions, beginning with the afferent arteriole that delivers oxygenated blood from interlobular arteries in the renal cortex directly to the glomerular capillaries.^[18] This arteriole branches from larger renal arteries and enters the glomerulus at the vascular pole, where blood pressure is modulated to facilitate ultrafiltration.^[19] Following filtration, the efferent arteriole exits the glomerulus, carrying protein-rich blood away under higher pressure due to its narrower diameter compared to the afferent vessel.^[1] Post-glomerular blood flow diverges based on nephron type. In cortical nephrons, which constitute the majority, the efferent arteriole gives rise to a dense network of peritubular capillaries that closely surround the proximal and distal convoluted tubules in the cortex, enabling efficient exchange of solutes and water reabsorbed from the filtrate.^[18] These capillaries restore plasma volume depleted during glomerular filtration and return blood to the venous system via interlobular veins.^[19] In contrast, juxtamedullary nephrons feature efferent arterioles that extend deeper into the medulla, forming the vasa recta—specialized capillaries arranged in hairpin loops that parallel the loops of Henle.^[18] These loops consist of descending and ascending limbs bundled together, structurally adapted to minimize disruption of the medullary osmotic gradient through countercurrent exchange principles inherent in their parallel arterial and venous flows.^[19] The juxtaglomerular apparatus (JGA) represents a critical regulatory structure at the vascular pole of the renal corpuscle, integrating tubular and vascular elements for localized control.^[20] It comprises three main components: the macula densa, juxtaglomerular cells, and extraglomerular mesangium. The macula densa consists of 15–20 specialized epithelial cells at the distal end of the thick ascending limb of the loop of Henle (part of the distal convoluted tubule), positioned in direct contact with the afferent arteriole's smooth muscle wall.^[20] These cells feature tall, columnar morphology with prominent apical microvilli exposed to tubular fluid, allowing sensing of filtrate composition.^[18] Juxtaglomerular cells are modified smooth muscle cells located in the terminal portion of the afferent arteriole and, to a lesser extent, the efferent arteriole, containing secretory granules that store regulatory proteins.^[20] The extraglomerular mesangium, also known as lacis cells, forms a supportive network of irregularly shaped cells and extracellular matrix between the macula densa, afferent arteriole, and glomerular mesangium, providing structural continuity and potential pathways for intercellular communication.^[20] This arrangement at the vascular pole enables close apposition of tubular and vascular components, facilitating tubuloglomerular feedback through direct physical contacts.^[18] Overall, the nephron's vascular components are anatomically integrated with the tubular segments, with arterioles and capillaries running parallel to the tubules to optimize reabsorption and secretion without excessive dilution of the peritubular fluid.^[19] This parallel orientation ensures that reabsorbed substances from the filtrate can be efficiently taken up by the adjacent capillaries, maintaining electrochemical gradients essential for nephron operation.^[18]

Function

Glomerular Filtration

Glomerular filtration is the initial step in urine formation, involving the ultrafiltration of blood plasma across the glomerular capillary wall into Bowman's space within the renal corpuscle.^[5] This process produces an ultrafiltrate that is essentially protein-free and isotonic to plasma, allowing water and small solutes like ions, glucose, and urea to pass while retaining larger molecules and cells.^[5] The driving force for filtration is primarily the hydrostatic pressure within the glomerular capillaries, which averages about 55 mmHg due to the high-resistance efferent arteriole.^[21] This outward pressure is opposed by the hydrostatic pressure in Bowman's space (approximately 15 mmHg) and the colloid oncotic pressure in the glomerular capillaries (around 30 mmHg), yielding a net filtration pressure of roughly 10 mmHg.^[21] These Starling forces ensure continuous filtration along the glomerular capillaries, with the net pressure decreasing slightly from afferent to efferent ends due to rising oncotic pressure as water is filtered.^[5] The glomerular filtration barrier, composed of fenestrated endothelial cells, the glomerular basement membrane (GBM), and podocyte foot process slit diaphragms, exhibits both size and charge selectivity.^[22] Size selectivity excludes molecules larger than approximately 70 kDa, such as most plasma proteins, while allowing passage of smaller solutes.^[22] Charge selectivity arises from negatively charged proteoglycans in the GBM and glycocalyx, which repel anionic proteins like albumin, further preventing their filtration.^[23] The glomerular filtration rate (GFR), which quantifies the volume of filtrate produced per unit time, averages 125 mL/min (or about 180 L/day) in healthy adults, representing roughly 20% of the renal plasma flow.^[5] GFR is determined by the equation GFR = K_f × (P_G - P_B - π_G + π_B), where K_f is the hydraulic filtration coefficient reflecting the barrier's permeability and surface area, P_G is glomerular capillary hydrostatic pressure, P_B is Bowman's space hydrostatic pressure, π_G is glomerular capillary oncotic pressure, and π_B is Bowman's space oncotic pressure (typically negligible).^[24] This formula highlights how changes in pressures or K_f directly impact filtration volume. GFR is tightly regulated to maintain constancy despite fluctuations in systemic blood pressure, primarily through intrinsic autoregulation mechanisms.^[25] The myogenic response involves direct constriction of afferent arteriolar smooth muscle in response to increased wall tension from elevated pressure, thereby stabilizing glomerular hydrostatic pressure.^[25] Tubuloglomerular feedback, mediated by the macula densa cells in the distal tubule, senses increased NaCl delivery due to higher filtration and releases vasoconstrictive signals (such as adenosine) to constrict the afferent arteriole, reducing GFR back to baseline. These mechanisms operate within a mean arterial pressure range of 80–180 mmHg to protect the glomerulus from hemodynamic damage.^[26]

Tubular Processing

The glomerular filtrate entering the proximal tubule undergoes extensive modification through reabsorption and secretion, reclaiming essential solutes and water while eliminating waste. In the proximal tubule, approximately 65% of filtered sodium ions (Na⁺), water, glucose, and amino acids are reabsorbed, preventing their loss in urine and maintaining plasma homeostasis.^[27] This bulk reabsorption is isosmotic, meaning the reabsorbate has the same osmolarity as the original filtrate, and is obligatory, driven by solute transport rather than hormonal regulation.^[28] The primary driver is the basolateral Na⁺/K⁺-ATPase pump, which extrudes Na⁺ from the tubular cell into the interstitium, establishing a low intracellular Na⁺ concentration that powers apical Na⁺ entry.^[29] Apical Na⁺ entry occurs via secondary active transporters, such as the sodium-glucose linked transporter (SGLT2 and SGLT1) for glucose and Na⁺ cotransport, ensuring nearly complete reabsorption of filtered glucose under normal conditions.^[30] Amino acids are similarly reclaimed through Na⁺-coupled cotransporters. Water follows osmotically through aquaporin-1 (AQP1) channels in both apical and basolateral membranes, facilitating transcellular flux, while paracellular water movement occurs via solvent drag through relatively leaky tight junctions.^[28] Solute reabsorption employs both transcellular (energy-dependent, through cells via pumps and channels) and paracellular (passive, between cells driven by electrochemical gradients) pathways; for instance, chloride ions (Cl⁻) use both routes, with Na⁺ reabsorption generating the lumen-negative potential that enhances paracellular Cl⁻ flux.^[31] Bicarbonate (HCO₃⁻) reabsorption, critical for acid-base balance, accounts for about 80-90% of filtered load in the proximal tubule and involves apical H⁺ secretion via the Na⁺/H⁺ exchanger 3 (NHE3), which combines with filtered HCO₃⁻ to form carbonic acid, dissociating into CO₂ and water for intracellular reclamation and basolateral export as HCO₃⁻.^[32] Additionally, the proximal tubule serves as a site of gluconeogenesis, producing glucose from precursors like lactate and glutamine using enzymes such as phosphoenolpyruvate carboxykinase, contributing up to 25% of systemic glucose during fasting.^[33] Secretion in this segment eliminates organic acids and bases (e.g., drugs like penicillin) via basolateral uptake through organic anion transporters (OATs) and organic cation transporters (OCTs), followed by apical efflux, aiding toxin clearance.^[34] In the distal convoluted tubule, finer adjustments occur, with reabsorption of about 20% of remaining Na⁺ via the thiazide-sensitive Na⁺-Cl⁻ cotransporter (NCC) on the apical membrane, coupled to basolateral Na⁺/K⁺-ATPase activity.^[35] This segment is aldosterone-sensitive, as the hormone upregulates NCC and serum/glucocorticoid-regulated kinase 1 (SGK1) to enhance Na⁺ retention during volume depletion.^[36] Calcium (Ca²⁺) reabsorption, approximately 10% of filtered load, is transcellular and mediated by the apical transient receptor potential vanilloid 5 (TRPV5) channel, regulated by parathyroid hormone and klotho to maintain bone and plasma Ca²⁺ levels.^[37] Water handling here is facultative but limited, with low permeability independent of antidiuretic hormone (ADH), relying on aquaporin-2 expression primarily in downstream segments; thus, the distal convoluted tubule contributes minimally to water reabsorption compared to the proximal tubule.^[38] Overall, these processes distinguish transcellular active transport, which dominates energy-intensive solute recovery, from paracellular passive diffusion, optimizing efficiency in the proximal tubule while enabling precise regulation in the distal convoluted tubule.^[31]

Urine Concentration and Regulation

The urine concentration mechanism in the nephron relies on the establishment of a hyperosmotic gradient in the renal medulla, which enables the kidney to produce urine with osmolality ranging from approximately 50 mOsm/L (dilute) to 1200 mOsm/L (concentrated), far exceeding plasma osmolality of about 300 mOsm/L.^[39]^[40] This medullary hyperosmolarity, reaching up to 1200 mOsm/L at the papillary tip, is generated and maintained primarily through the countercurrent multiplier system in the loop of Henle and enhanced by urea recycling.^[39] The juxtaglomerular apparatus (JGA) further contributes to regulation by sensing tubular NaCl levels and modulating renin release to activate the renin-angiotensin-aldosterone system (RAAS), which influences blood volume and pressure to support overall fluid balance.^[20] In the loop of Henle, the countercurrent multiplier mechanism creates the corticomedullary osmotic gradient essential for urine concentration. The descending thin limb is highly permeable to water due to the presence of aquaporin-1 (AQP1) channels in both apical and basolateral membranes, allowing passive water efflux into the hypertonic interstitium while being relatively impermeable to solutes, which concentrates the tubular fluid as it descends.^[41] In contrast, the ascending limb is impermeable to water, lacking aquaporins, but actively reabsorbs NaCl to dilute the tubular fluid and generate the osmotic gradient.^[42] Specifically, in the thick ascending limb, the Na⁺-K⁺-2Cl⁻ cotransporter (NKCC2) on the apical membrane drives solute reabsorption, transporting ions against their electrochemical gradients using the sodium gradient established by basolateral Na⁺/K⁺-ATPase:

\ce{Na+ + K+ + 2Cl- \rightarrow[NKCC2] cytoplasm}

This process establishes a transverse osmotic gradient of 200 mOsm/L per horizontal level, amplified longitudinally by countercurrent flow to produce the full 200–1200 mOsm/L medullary gradient.^[43]^[44] The collecting duct fine-tunes urine osmolality through vasopressin (antidiuretic hormone, ADH)-regulated water reabsorption and urea recycling. In the presence of ADH, binding to V2 receptors on principal cells triggers cAMP-mediated insertion of aquaporin-2 (AQP2) channels into the apical membrane, increasing water permeability and allowing passive reabsorption down the medullary gradient to concentrate urine.^[45] Without ADH, the collecting duct remains impermeable to water, producing dilute urine. Urea recycling further enhances the gradient: in the inner medullary collecting duct, ADH upregulates urea transporters (UT-A1), facilitating urea reabsorption into the interstitium, where it diffuses into the ascending thin limb to contribute up to 50% of the inner medullary osmolality.^[46] The JGA, located at the vascular pole of the renal corpuscle, integrates tubular and vascular signals for regulatory feedback. Macula densa cells in the distal tubule sense luminal NaCl concentration via NKCC2 and other transporters; decreased NaCl delivery (e.g., due to low glomerular filtration rate) stimulates adenosine and nitric oxide signaling to juxtaglomerular cells, promoting renin release.^[20] Renin initiates the RAAS cascade, leading to angiotensin II-mediated vasoconstriction and aldosterone-induced Na⁺ reabsorption, which helps maintain extracellular volume and supports the medullary gradient for urine concentration.^[47]

Clinical Significance

Nephron Dysfunction and Diseases

Nephron dysfunction refers to impairments in the structural or functional components of the nephron, leading to disrupted filtration, reabsorption, secretion, and overall renal homeostasis. These dysfunctions can arise from acute or chronic insults, resulting in reduced glomerular filtration rate (GFR) and altered tubular handling of solutes, which manifest as azotemia, electrolyte derangements, and fluid imbalances. The primary mechanisms involve damage to glomerular endothelium, podocytes, or tubular epithelia, often triggered by ischemia, toxins, or immune processes.^[48] Acute kidney injury (AKI) represents a sudden decline in nephron function, categorized by etiology into prerenal, intrinsic, and postrenal types. Prerenal AKI stems from hypoperfusion of the renal cortex, reducing GFR without structural nephron damage; common causes include volume depletion or heart failure, leading to decreased delivery of filtrate to tubules.^[49] Intrinsic AKI involves direct nephron injury, such as acute tubular necrosis (ATN) from ischemia or nephrotoxins, where tubular epithelial cells swell, detach, and obstruct lumens, further impairing reabsorption and causing intratubular backleak of filtrate.^[49] Postrenal AKI results from obstruction distal to the nephron, such as ureteral stones, causing retrograde pressure that dilates tubules and compresses vessels, ultimately reducing GFR across affected nephrons.^[49] Chronic kidney disease (CKD) involves progressive nephron loss, where surviving nephrons undergo compensatory hyperfiltration, exacerbating damage through glomerular hypertension and hypertrophy. In diabetic nephropathy, hyperglycemia induces mesangial expansion, podocyte injury, and basement membrane thickening in the glomerulus, leading to proteinuria and eventual nephron sclerosis.^[50] Hypertensive nephropathy primarily affects tubules and interstitium, with sustained high pressure causing arteriolar hyalinosis, ischemia to medullary nephrons, and tubular atrophy, contributing to interstitial fibrosis.^[51] CKD is defined by GFR below 60 mL/min/1.73 m² for over three months, reflecting widespread nephron attrition.^[52] Specific nephron-targeted disorders include glomerulonephritis, an immune-mediated condition damaging the glomerular filtration barrier through antibody deposition or complement activation, resulting in hematuria, reduced GFR, and endocapillary proliferation.^[53] Nephrotic syndrome arises from podocyte effacement and slit diaphragm disruption, increasing glomerular permeability to proteins and causing massive proteinuria exceeding 3.5 g/day, hypoalbuminemia, and edema due to impaired selective filtration.^[54] Tubular disorders like Fanconi syndrome involve proximal tubule dysfunction, impairing reabsorption of glucose, amino acids, phosphate, and bicarbonate, often from inherited transporter defects or acquired toxins, leading to glycosuria, phosphaturia, and metabolic acidosis.^[55] Pathophysiologically, nephron dysfunction culminates in reduced GFR and tubular maladaptations, such as impaired sodium and water reabsorption, fostering volume overload or depletion. Tubular injury disrupts potassium secretion in the distal nephron, promoting hyperkalemia through diminished aldosterone responsiveness or reduced distal flow.^[56] Globally, CKD affects approximately 10% of the adult population, with rising incidence linked to diabetes and hypertension; recent studies from 2023-2025 highlight SGLT2 inhibitors' role in preserving nephron integrity in diabetic nephropathy by mitigating glomerular hyperfiltration and tubular glucose toxicity.^[57]^[58]

Diagnostic and Therapeutic Implications

Assessment of nephron function relies on several diagnostic tools that evaluate glomerular filtration and tubular integrity. Serum creatinine levels serve as a primary marker for estimating glomerular filtration rate (GFR), with the 2021 Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) creatinine equation (without race) providing the current standardized calculation:

eGFR = 142 \times \min\left(\frac{S_{cr}}{\kappa}, 1\right)^{\alpha} \times \max\left(\frac{S_{cr}}{\kappa}, 1\right)^{-1.200} \times 0.9938^{\mathrm{Age}} \times \left(1.012 \text{ if female}\right) \ \text{mL/min/1.73 m}^2,

where S_{cr} is serum creatinine in mg/dL, \kappa = 0.7 (females) or $0.9 (males), and \alpha = -0.241 (females) or -0.302 (males).^[59] This equation enables early detection of reduced nephron filtration capacity, guiding clinical management. Urinalysis detects proteinuria, which indicates glomerular barrier dysfunction, and urinary casts, such as hyaline or granular types, that reflect tubular damage or stasis.^[60] Renal biopsy remains the gold standard for histological evaluation of nephron structures, allowing direct visualization of glomerular, tubular, and vascular abnormalities through light, immunofluorescence, and electron microscopy.^[61] Imaging modalities complement biochemical tests by assessing nephron-related anatomy. Renal ultrasound is the initial imaging choice, non-invasively measuring kidney size, echogenicity, and corticomedullary differentiation to identify structural changes like atrophy or hydronephrosis affecting nephron function.^[62] For vascular evaluation, computed tomography (CT) or magnetic resonance imaging (MRI) detects perfusion deficits or stenoses in renal arteries that impair juxtaglomerular apparatus signaling and overall nephron viability.^[63] Therapeutic interventions target specific nephron components to preserve function or mitigate damage. Angiotensin-converting enzyme (ACE) inhibitors and angiotensin II receptor blockers (ARBs) modulate the renin-angiotensin-aldosterone system (RAAS), reducing efferent arteriolar constriction to protect the juxtaglomerular apparatus and lower intraglomerular pressure, thereby slowing progression in proteinuric states.^[64] Diuretics, particularly loop agents like furosemide, inhibit the Na-K-2Cl cotransporter (NKCC2) in the thick ascending limb of the loop of Henle, promoting natriuresis and relieving tubular overload in edematous conditions.^[65] In advanced nephron failure, dialysis serves as a renal replacement therapy, artificially performing filtration and solute clearance to substitute for lost nephron capacity.^[66] Emerging strategies aim to directly address nephron deficits at the genetic and cellular levels. Gene therapy for inherited tubular disorders, such as those involving cystinosis or Bartter syndrome, uses viral vectors to deliver corrective genes to proximal tubule cells, restoring transport functions in preclinical models and early trials as of 2025.^[67] Stem cell-based regeneration of nephrons, employing induced pluripotent stem cell-derived progenitors, has shown promise in preclinical studies by 2025, demonstrating integration into damaged kidney tissue and partial restoration of filtration in animal models of acute injury.^[68] Monitoring nephron response often involves calculating the fractional excretion of sodium (FENa) to differentiate prerenal from intrinsic causes of acute kidney injury (AKI):

FENa = \left( \frac{U_{Na}/P_{Na}}{U_{Cr}/P_{Cr}} \right) \times 100,

where U_{Na} and P_{Na} are urine and plasma sodium concentrations, and U_{Cr} and P_{Cr} are urine and plasma creatinine levels; values below 1% suggest preserved tubular reabsorption, while higher values indicate nephron impairment.^[69]

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Glomerular filtration: the charge debate charges ahead
The concept that the glomerular filtration barrier exhibits charge selectivity is a basic tenet of renal physiology. Heparan sulfate is a major contributor ...
[25]
A simple model for demonstrating the factors affecting glomerular ...
Mar 3, 2018 · The rate of filtration, or the glomerular filtration rate (GFR), is determined by the equation GFR ⫽ Kf ⫻ net filtration pressure, where Kf is ...
[26]
Molecular Mechanisms of Renal Blood Flow Autoregulation - PMC
This review focuses on current thought regarding the molecular mechanisms underlying myogenic control of renal vascular tone.
[27]
Mysteries of Renal Autoregulation | Hypertension
Whole kidney autoregulation reflects regulatory input from the myogenic and tubuloglomerular feedback mechanisms at the very least. Recently ...
[28]
Proximal Nephron - PMC - PubMed Central - NIH
Proximal tubules are responsible for reabsorbing approximately 65% of filtered load and most, if not all, of filtered amino acids, glucose, solutes, and low ...
[29]
Aquaporins in the kidney: physiology and pathophysiology
The proximal tubules are responsible for 70% of the filtered water reabsorption, the tDLH is responsible for ~15%, and the connecting tubules and collecting ...
[30]
Na/K-ATPase Signaling Tonically Inhibits Sodium Reabsorption in ...
Through its classic ATP-dependent ion-pumping function, basolateral Na/K-ATPase (NKA) generates the Na+ gradient that drives apical Na+ reabsorption in the ...Missing: aquaporins | Show results with:aquaporins
[31]
Proximal tubule hypertrophy and hyperfunction - Oxford Academic
Hyperglycaemia enhances glucose and sodium reabsorption via SGLT2 and SGLT1. Intracellular glucose accumulation in PT cells leads to hypertrophy, ...
[32]
Tubular Transport: Core Curriculum 2010 - PMC - NIH
Solutes can utilize both transcellular and paracellular routes, either across the same nephron segment (Cl− in the proximal tubule), or in different parts ...
[33]
Bicarbonate reabsorption in proximal renal tubule - PubMed
Aug 25, 2014 · Taken together, NHE3 and H(+)-ATPase are responsible for about 80% of HCO3(-) reabsorption in the proximal tubule. The remaining 20% is likely ...
[34]
The sugar daddy: the role of the renal proximal tubule in glucose ...
Under healthy conditions (left), gluconeogenesis (blue arrows/enzyme names) is the prevalent metabolic pathway performed in the renal proximal tubule.
[35]
Proximal Tubular Secretory Clearance: A Neglected Partner of ...
Feb 28, 2018 · Proximal tubular secretion is capable of rapidly removing retained solutes and medications from the circulation, including protein-bound ...
[36]
Control of sodium and potassium homeostasis by renal distal ...
Distal convoluted tubules (DCT), which contain the Na-Cl cotransporter (NCC) inhibited by thiazide diuretics, undergo complex modulation to preserve Na+ and K+ ...
[37]
Regulation of distal tubule sodium transport: mechanisms and roles ...
Jul 27, 2022 · Aldosterone is the key hormonal factor regulating Na+ transport in the distal nephron, and for coupling Na+ reabsorption with K+ secretion.
[38]
PTH modulation of NCC activity regulates TRPV5 Ca2+ reabsorption
Kidney tubule reabsorption of filtered Ca2+ occurs primarily in the proximal tubule (PT), thick ascending limb (TAL), distal convoluted tubule (DCT), and ...Missing: aldosterone | Show results with:aldosterone
[39]
Renal water transport in health and disease - PMC - PubMed Central
Jun 9, 2022 · The ascending limb of Henle and the distal convoluted tubule are impermeant to water and are responsible for urine dilution. The water balance ...
[40]
The Physiology of Urinary Concentration: an Update - PubMed Central
The renal medulla produces concentrated urine through the generation of an osmotic gradient extending from the cortico-medullary boundary to the inner medullary ...
[41]
Urine Osmolality - an overview | ScienceDirect Topics
Normal plasma osmolality is 275–300 milliosmoles/kg (mOsm/kg) of water while urine osmolality is 50–1200 mOsm/kg of water. Although plasma and urine osmolality ...Missing: hyperosmolarity | Show results with:hyperosmolarity
[42]
Renal aquaporins - Kidney International
At least four aquaporins are expressed in the kidney where they mediate rapid water transport across water-permeable epithelia and play critical roles in ...
[43]
The importance of the thick ascending limb of Henle's loop in renal ...
Feb 15, 2018 · The renal epithelium along the descending limb is highly permeable to water but not to solutes.29 Here, the increasing interstitial ...<|control11|><|separator|>
[44]
Physiology and pathophysiology of the renal Na-K-2Cl cotransporter ...
In total, 20–25% of the filtered NaCl is reabsorbed along the thick ascending limb of the loop of Henle (TAL), whereas virtually no water is reabsorbed in this ...
[45]
Physiological role of the loop of Henle in urinary concentration
According to this hypothesis, the ioops of Henle serve as a countercurrent multiplier system to generate and maintain a medullary osmotic gradient which ...
[46]
Vasopressin and the Regulation of Aquaporin-2 - PMC
This short review will focus on the mechanisms involved in vasopressin-mediated regulation of AQP2 in the renal collecting duct by the above two processes.
[47]
Long-Term Regulation of Renal Urea Transporters during Antidiuresis
This creates an intra-renal urea recycling process that minimizes the loss of urea from the renal medulla and maintains the medullary osmotic gradient (Fig. 1).
[48]
Physiology, Renin Angiotensin System - StatPearls - NCBI Bookshelf
Delivery of sodium and chloride to the distal convoluted tubule that is sensed by the macula densa. Increased beta-sympathetic flow acting through the beta-1 ...
[49]
Pathophysiology of Acute Kidney Injury - PMC - PubMed Central - NIH
The primary causes of AKI include ischemia, hypoxia or nephrotoxicity. An underlying feature is a rapid decline in GFR usually associated with decreases in ...
[50]
Acute Kidney Injury - StatPearls - NCBI Bookshelf - NIH
Nov 25, 2023 · Postrenal pathophysiology is usually related to urinary reflux, causing decreased renal perfusion, tubular atrophy, and interstitial ...
[51]
Diabetic Nephropathy - StatPearls - NCBI Bookshelf - NIH
[3]. Endothelial Cell Damage. Endothelial cell damage is one of the earliest pathological changes in diabetic nephropathy. This damage generates reactive ...Etiology · Pathophysiology · Evaluation · Treatment / Management
[52]
Pathogenesis and Damage Targets of Hypertensive Kidney Injury
A large number of studies related to hypertensive kidney injury have focused more on the damage in the capillary tuft causing nephroangiosclerosis and ...
[53]
Chronic Kidney Disease - StatPearls - NCBI Bookshelf
Jul 31, 2024 · Kidney damage refers to pathologic abnormalities suggested by imaging studies or renal biopsy, abnormalities in urinary sediment, or increased ...
[54]
Glomerulonephritis - StatPearls - NCBI Bookshelf - NIH
Jun 26, 2023 · Glomerulonephritis encompasses a subset of renal diseases characterized by immune-mediated damage to the basement membrane, mesangium, or capillary endothelium.Introduction · Pathophysiology · Evaluation · Differential Diagnosis
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Nephrotic Syndrome - StatPearls - NCBI Bookshelf
It is caused by increased permeability through the damaged basement membrane in the renal glomerulus. It results from an abnormality of glomerular permeability ...
[56]
Fanconi Syndrome - StatPearls - NCBI Bookshelf
Jun 24, 2025 · Fanconi syndrome is a disorder of the proximal renal tubule ... tubular damage and progressive kidney dysfunction.[24]. In inherited forms ...Pathophysiology · History and Physical · Differential Diagnosis · Prognosis
[57]
Hyperkalemia - StatPearls - NCBI Bookshelf
Tubular dysfunction due to aldosterone deficiency or insensitivity can also cause hyperkalemia.
[58]
Epidemiology of chronic kidney disease: an update 2022 - PMC - NIH
Mar 18, 2022 · The age-standardized global prevalence of CKD stages 3–5 in adults aged ≥20 years in the same study was 4.7% in men and 5.8% in women. A more ...
[59]
Dual-faced guardians: SGLT2 inhibitors' kidney protection and ... - NIH
Jun 14, 2025 · As explained earlier, SGLT2 inhibitors have nephroprotective effects on the progression of diabetic kidney disease as well as nondiabetic ...Missing: nephrons | Show results with:nephrons
[60]
A more accurate method to estimate glomerular filtration rate from ...
The equation developed from the MDRD Study provided a more accurate estimate of GFR in our study group than measured creatinine clearance or other commonly ...
[61]
Proteinuria - StatPearls - NCBI Bookshelf - NIH
Sep 4, 2023 · Proteinuria can serve as an indicator of early renal disease. It marks an increased risk of renal damage secondary to hypertension and cardiovascular disease.
[62]
Approach to Kidney Biopsy: Core Curriculum 2022
Feb 4, 2022 · The kidney biopsy is an essential tool for diagnosis of many kidney diseases. Obtaining an adequate biopsy sample with appropriate ...
[63]
Kidney Ultrasound for Nephrologists: A Review - PMC - NIH
Ultrasound enables the assessment of both structural (form and size) and functional (perfusion and patency) aspects of kidneys, both of which are especially ...
[64]
Renal Imaging: Core Curriculum 2019
Feb 15, 2019 · Ultrasonography is the first-choice modality for evaluation of acute or chronic kidney failure. It is well suited for evaluating whether kidney ...
[65]
Renin-Angiotensin System Blockade in Advanced Kidney Disease
Both ACE inhibitors and ARBs induce what could be considered “functional” side effects linked to RAS inhibition, specifically hyperkalemia due to inhibition of ...
[66]
Loop Diuretics - StatPearls - NCBI Bookshelf
May 22, 2023 · Loop diuretics induce its effect by competing with chloride to bind to the Na-K-2Cl (NKCC2) cotransporter at the apical membrane of the ...
[67]
Hemodialysis - StatPearls - NCBI Bookshelf - NIH
Dialysis is a form of renal replacement therapy. The kidney's role of filtration of the blood is supplemented by artificial equipment to remove excess water ...
[68]
Updated Gene Therapy for Renal Inborn Errors of Metabolism - PMC
Apr 29, 2025 · These therapies aim to correct the source of the defect in the genetic code so that patients may live full, unencumbered lives. In this review, ...
[69]
Human iPSC–derived nephron progenitor cells treat acute kidney ...
Apr 2, 2025 · Human induced pluripotent stem cells (hiPSCs) can be used to produce nephron progenitor cells (NPCs) that are capable of differentiating into ...
[70]
Fractional excretion of sodium, urea, and other molecules in acute ...
Oct 30, 2024 · Definition – The FENa measures the percent of filtered sodium that is excreted in the urine (see 'Formula for fractional excretion of sodium ( ...