Fact-checked by Grok 2 weeks ago

Reabsorption

Reabsorption is the process by which the kidneys reclaim water, ions, and essential nutrients from the glomerular filtrate in the nephron tubules back into the peritubular capillaries, preventing their excretion in urine and maintaining fluid-electrolyte balance.^[1] This selective retrieval occurs across the epithelial cells of the renal tubules via active and passive transport mechanisms, reclaiming approximately 99% of the filtered water and solutes daily.^[2] Without reabsorption, the body would lose vital substances, leading to dehydration and electrolyte imbalances.^[3] The primary site of reabsorption is the proximal convoluted tubule (PCT), where about 65-70% of filtered sodium, water, glucose, amino acids, and bicarbonate are reabsorbed through primary active transport via the Na+/K+-ATPase pump and secondary active transport for organic solutes.^[1] In the loop of Henle, the descending limb facilitates water reabsorption by osmosis due to the hyperosmotic medullary interstitium, while the thick ascending limb actively reabsorbs sodium, chloride, calcium, and magnesium without water, contributing to the countercurrent multiplier system that concentrates urine.^[1] The distal convoluted tubule (DCT) and collecting duct handle fine-tuning, reabsorbing additional sodium and calcium under hormonal control.^[2] Regulation of reabsorption is crucial for adapting to physiological needs, with hormones playing key roles: aldosterone promotes sodium reabsorption in the DCT and collecting duct to increase blood volume, antidiuretic hormone (ADH) enhances water permeability via aquaporin insertion in the collecting duct, and parathyroid hormone (PTH) stimulates calcium reabsorption in the DCT while inhibiting phosphate uptake.^[1] These mechanisms ensure precise control over plasma osmolarity, acid-base balance, and blood pressure, with disruptions leading to conditions like hyponatremia or hyperkalemia.^[3] Overall, reabsorption exemplifies the kidney's role in homeostasis, processing about 180 liters of filtrate daily to produce roughly 1.5 liters of urine.^[2]

Overview

Definition

Reabsorption is the biological process whereby water, ions, nutrients, and other solutes are transported from the lumen of tubular epithelia or from a formed filtrate back into the adjacent interstitial fluid and, ultimately, the bloodstream. This selective movement across polarized epithelial cells ensures the recovery of essential substances that would otherwise be lost, occurring primarily through transcellular and paracellular pathways in structures like renal tubules. In physiological contexts, reabsorption contrasts with the absorptive processes in absorptive epithelia by emphasizing reclamation from a pre-formed fluid compartment rather than direct uptake from external environments.^[1] Distinct from secretion, which entails the active or passive transfer of substances from the peritubular capillaries or interstitial fluid into the tubular lumen to facilitate elimination or pH regulation, reabsorption operates in the reverse direction to conserve resources. Filtration, meanwhile, represents the initial passive separation of plasma ultrafiltrate from blood across a semipermeable barrier, setting the stage for subsequent reabsorption without involving net addition or removal of solutes beyond size-based exclusion. These processes collectively govern the composition of bodily fluids in tubular systems.^[1] The term and foundational understanding of reabsorption emerged in early 20th-century renal physiology, with key advancements by researchers such as Homer Smith in the 1920s and 1930s, who utilized clearance techniques to quantify tubular transport and its role in solute recovery. Smith's work, including studies on fish and mammalian kidneys, established reabsorption as a critical adaptive mechanism, influencing subsequent research on epithelial transport.^[4]

Physiological Role

Reabsorption in the renal tubules serves a critical physiological function by reclaiming the vast majority of water and essential solutes from the glomerular filtrate, thereby preventing excessive urinary loss and sustaining bodily fluid volume and electrolyte composition. In a typical adult human, the kidneys filter approximately 180 liters of plasma daily, yet only about 1-2 liters are excreted as urine, meaning roughly 99% of the filtered water and solutes—such as sodium, glucose, and amino acids—are reabsorbed to avert dehydration and maintain plasma osmolarity and electrolyte balance.^[5]^[1] Beyond fluid and electrolyte homeostasis, reabsorption plays an essential role in regulating acid-base balance through the selective reclamation of bicarbonate ions (HCO₃⁻), which function as the primary extracellular buffer against pH fluctuations. The kidneys reabsorb nearly all of the filtered bicarbonate load—around 4,500 milliequivalents per day—via mechanisms involving carbonic anhydrase and proton secretion in the tubular cells, thereby regenerating and conserving this buffer to stabilize blood pH between 7.35 and 7.45 and compensate for metabolic or respiratory acid-base disturbances.^[6]^[7] This reabsorptive workload imposes a significant metabolic demand on the kidneys, as active transport processes, particularly the sodium-potassium ATPase pump, power the uphill movement of solutes against concentration gradients. Consequently, renal tubular reabsorption consumes about 7-10% of the total basal metabolic rate in humans, reflecting the organ's high oxygen utilization—equivalent to roughly 6-7% of whole-body oxygen consumption—predominantly allocated to solute recovery rather than basal cellular maintenance.^[8]^[9]

Mechanisms

Transport Processes

Reabsorption of substances in the renal tubules primarily occurs via two distinct pathways: the transcellular route, which traverses the epithelial cells, and the paracellular route, which passes between adjacent cells.^[10] The transcellular pathway facilitates the movement of ions, nutrients, water, and macromolecules across both the apical (luminal) and basolateral (peritubular) membranes of renal epithelial cells. At the apical membrane, passive diffusion through channels and facilitated transport via carriers allow entry of solutes such as sodium ions, while endocytosis enables the uptake of filtered proteins by forming clathrin-coated vesicles that internalize receptor-bound ligands for subsequent intracellular processing and degradation.^[11]^[12] On the basolateral membrane, primary active transport is mediated by pumps, notably the Na⁺/K⁺-ATPase, which hydrolyzes ATP to extrude sodium ions into the interstitium while importing potassium, thereby establishing electrochemical gradients that power secondary active and passive transport mechanisms across the cell.^[13] In contrast, the paracellular pathway involves passive diffusion of small solutes and water through the intercellular space, regulated by the selective permeability of tight junctions formed by proteins such as claudins and occludins. This route allows for the movement of ions like chloride and sodium between cells, contributing to overall reabsorption efficiency without direct energy expenditure by the cell.^[14]^[15]

Driving Forces

Reabsorption in the kidney is fundamentally driven by electrochemical and osmotic gradients that facilitate the movement of ions and water from the tubular lumen back into the bloodstream. Electrochemical gradients arise from differences in ion concentrations and electrical potentials across epithelial cell membranes, providing the energy for passive and secondary active transport processes. These gradients are maintained by primary active transport mechanisms, such as the Na+/K+-ATPase, which create low intracellular sodium concentrations and a negative membrane potential, thereby favoring ion influx from the lumen. For ions like sodium (Na+), the driving force is quantified by the electrochemical gradient, which combines the chemical concentration gradient and the electrical potential difference. The Nernst equation calculates the equilibrium potential (E) at which the electrical and chemical forces on an ion balance, preventing net movement:

E = \frac{RT}{zF} \ln \left( \frac{[\text{ion}]_{\text{out}}}{[\text{ion}]_{\text{in}}} \right)

where R is the gas constant (8.314 J/mol·K), T is the absolute temperature in Kelvin, z is the ion's valence, F is the Faraday constant (96,485 C/mol), and [ion]_out and [ion]_in are the extracellular and intracellular concentrations, respectively. In renal physiology, this equation applied to Na+ illustrates how the low intracellular Na+ concentration (typically 10-15 mM versus ~140 mM extracellular) generates a favorable inward electrochemical gradient of approximately -60 to -70 mV, driving Na+ entry through channels and cotransporters. This gradient powers the reabsorption of up to 99% of filtered Na+, with the equilibrium potential ensuring that Na+ movement aligns with the membrane's overall potential under physiological conditions.^[16] Osmotic gradients serve as the primary driving force for water reabsorption, generated by the active reabsorption of solutes that lowers luminal osmolarity relative to the interstitium. The osmotic pressure (π) difference across the membrane is described by the Van't Hoff equation:

\pi = iCRT

where i is the van't Hoff factor (number of particles per solute molecule), C is the solute concentration, R is the gas constant, and T is the absolute temperature. In the kidney, this pressure drives water movement through aquaporin channels, with reflection coefficients (σ, ranging from 0 to 1) accounting for partial solute permeability; for impermeable solutes like urea in certain contexts, σ = 1, yielding full osmotic effectiveness. For example, a 2-15 mOsmol/L hypotonicity in the lumen can produce an osmotic pressure of ~39-290 mmHg (using 19.3 mmHg per mOsm/L at 37°C), sufficient to reabsorb the majority of filtered water isosmotically via aquaporin-mediated pathways. This process ensures that water follows solute reabsorption, maintaining fluid balance without excessive energy expenditure.^[17]^[18] Solvent drag contributes to reabsorption by coupling water flow to solute movement through paracellular pathways, where the bulk flow of water entrains solutes via frictional forces. When solutes are actively reabsorbed transcellularly, the resulting osmotic gradient induces paracellular water flux through selective pores, such as those formed by claudin proteins, pulling hydrated cations (e.g., Na+) along at ratios of approximately 500-1000 water molecules per ion. This convective mechanism enhances paracellular reabsorption of permeable solutes, amplifying overall efficiency without requiring direct energy input for the dragged species, though its quantitative contribution depends on pore selectivity and flow rates. Recent analyses indicate that while molecular details remain under investigation, solvent drag operates as a passive biophysical enhancer of solute-water coupling in permeable epithelia.^[19]^[20]

Nephron Sites

Proximal Tubule

The proximal convoluted tubule serves as the primary site for bulk reabsorption in the nephron, reclaiming the majority of filtered substances to prevent their loss in urine. Approximately 65% of the filtered sodium (Na⁺) and water is reabsorbed here, along with about 80% of bicarbonate (HCO₃⁻), ensuring efficient conservation of these essential components.^[21] Nearly 100% of filtered glucose and amino acids are also reabsorbed in this segment under normal physiological conditions, driven by active transport mechanisms that couple solute uptake with sodium entry.^[21] Key transporters facilitate this process, with the sodium-glucose linked transporter 2 (SGLT2) playing a central role in the apical membrane of early proximal tubule cells by mediating Na⁺-glucose cotransport at a 1:1 stoichiometry, responsible for about 90% of glucose reabsorption.^[22] For bicarbonate handling, carbonic anhydrase enzymes (primarily CAII in the cytosol and CAIV on membranes) catalyze the conversion of CO₂ and H₂O to H⁺ and HCO₃⁻, enabling H⁺ secretion via Na⁺/H⁺ exchangers and subsequent HCO₃⁻ reabsorption through basolateral Na⁺-HCO₃⁻ cotransporters.^[23] These transporters, powered by the basolateral Na⁺/K⁺-ATPase, underscore the proximal tubule's high-capacity reabsorptive function.^[21] Reabsorption in the proximal tubule occurs isosmotically, with water following solutes passively through aquaporin-1 channels to maintain tubular fluid osmolarity at approximately 300 mOsm/L, equivalent to plasma.^[24] This balanced process, involving paracellular and transcellular pathways, prevents osmotic gradients and supports the nephron's overall fluid homeostasis without altering the concentration of the remaining filtrate.^[21]

Loop of Henle

The loop of Henle plays a crucial role in renal reabsorption by establishing an osmotic gradient in the renal medulla, enabling the concentration of urine through a countercurrent multiplier system. This structure consists of a descending limb and an ascending limb, with distinct permeability properties that facilitate the reabsorption of water and solutes in a coordinated manner. In the descending limb, water is reabsorbed passively, while in the ascending limb, ions are actively transported out without water following, which progressively dilutes the tubular fluid and amplifies the medullary hyperosmolarity. The thin descending limb is highly permeable to water due to the presence of aquaporin-1 (AQP1) channels on both apical and basolateral membranes, allowing passive reabsorption of water driven by the hyperosmotic interstitial fluid in the medulla. This process accounts for approximately 20% of total water reabsorption in the nephron and contributes to increasing the osmolarity of the tubular fluid as it descends, thereby enhancing the medullary hyperosmolarity essential for overall urine concentration. In contrast, the limb has low permeability to ions and urea, minimizing solute loss during this phase. The thick ascending limb actively reabsorbs sodium chloride via the apical Na⁺-K⁺-2Cl⁻ cotransporter (NKCC2), which handles 25–30% of the filtered NaCl load, with potassium recycled through ROMK channels to maintain the transporter's activity. This segment is impermeable to water owing to the absence of aquaporins in the apical membrane and low overall transepithelial water permeability, preventing osmotic equilibration and resulting in the dilution of tubular fluid to hypotonic levels (around 100 mOsm/L). The active ion transport without water reabsorption is key to generating the "single effect" that drives the countercurrent system. The countercurrent multiplier mechanism in the loop of Henle operates as a mathematical model where the active transport in the thick ascending limb creates a small transverse osmotic gradient (approximately 200 mOsm/kg H₂O) between the ascending and descending limbs at each horizontal level. Over multiple iterations along the vertical length of the loop, this single effect is multiplied longitudinally, establishing a steep corticomedullary osmotic gradient that can reach up to 1200 mOsm/kg H₂O at the papillary tip in humans. This gradient, built through repeated cycles of solute removal in the ascending limb and water equilibration in the descending limb, provides the driving force for water reabsorption in subsequent nephron segments under the influence of antidiuretic hormone.

Distal Tubule and Collecting Duct

The distal convoluted tubule (DCT) plays a key role in the fine-tuning of electrolyte reabsorption, primarily handling sodium and calcium ions through specific apical transporters. Sodium reabsorption in the DCT occurs via the thiazide-sensitive Na⁺-Cl⁻ cotransporter (NCC), which facilitates the uptake of approximately 5-10% of the filtered NaCl load across the apical membrane, driven by the electrochemical gradient established by basolateral Na⁺/K⁺-ATPase activity.^[25]^[26] Calcium reabsorption in the late DCT and connecting tubule is mediated by the transient receptor potential vanilloid 5 (TRPV5) channel, which serves as the rate-limiting step for active transcellular Ca²⁺ transport, accounting for about 10% of the filtered calcium load under regulated conditions.^[27]^[28] In the collecting duct, reabsorption is highly regulated and segment-specific, involving two main cell types: principal cells and intercalated cells. Principal cells mediate sodium reabsorption through the epithelial sodium channel (ENaC) on the apical membrane, which allows selective Na⁺ entry, coupled with basolateral extrusion via Na⁺/K⁺-ATPase, contributing to the final 2-5% of filtered sodium recovery and influencing extracellular fluid volume.^[29] These cells also control water reabsorption via aquaporin-2 (AQP2) water channels, whose insertion into the apical membrane under hormonal regulation increases the duct's permeability, enabling the reabsorption of up to 10-20% of the filtered water load depending on physiological needs.^[30]^[24] Intercalated cells in the collecting duct primarily handle acid-base and potassium balance. Type A intercalated cells secrete H⁺ into the lumen via apical H⁺-ATPase and H⁺/K⁺-ATPase pumps, which also facilitate K⁺ reabsorption during states of potassium depletion, while type B cells promote bicarbonate secretion to fine-tune pH homeostasis.^[31] This regulated ion handling in the distal nephron ensures precise adjustments to maintain electrolyte and fluid balance, contrasting with the more obligatory reabsorption in upstream segments.

Reabsorbed Substances

Water and Solutes

In the kidneys, water reabsorption is critical for maintaining extracellular fluid volume and osmotic balance, with approximately 99% of the 180 liters filtered daily by the glomeruli being reclaimed, resulting in only 1-2 liters excreted as urine.^[24] This high efficiency occurs through obligatory reabsorption in the proximal tubule, where roughly 65-70% of filtered water follows solute uptake isosmotically, and facultative reabsorption in the collecting duct, which adjusts to antidiuretic hormone levels for precise volume regulation.^[24] Sodium, the principal extracellular cation, undergoes extensive reabsorption to sustain blood pressure and fluid homeostasis, with about 99% of the filtered load—equivalent to roughly 25,000 mmol per day—being recovered along the nephron.^[32] This process is predominantly powered by the basolateral Na+/K+-ATPase pump in tubular cells, which extrudes sodium in exchange for potassium, establishing a favorable electrochemical gradient for apical sodium entry via cotransporters and channels.^[32] Chloride reabsorption closely mirrors sodium handling to preserve electroneutrality, achieving approximately 99% recovery of the filtered amount through a combination of paracellular diffusion driven by electrochemical gradients and transcellular transport in various nephron segments.^[33] Potassium reabsorption is site-dependent and variable, totaling 90-100% of the filtered load to support cellular function and membrane potentials, with major uptake in the proximal tubule and loop of Henle offset by distal secretion as needed.^[34] Bicarbonate reabsorption, vital for acid-base equilibrium, reclaims 85-90% of the filtered load—primarily to buffer pH—via proton secretion and carbonic anhydrase-mediated conversion in the proximal tubule.^[35]

Organic Compounds

In the kidney, reabsorption of organic compounds such as glucose, amino acids, and urea occurs primarily in the proximal tubule through carrier-mediated and passive mechanisms, ensuring efficient recovery of these essential molecules from the glomerular filtrate to maintain metabolic homeostasis.^[36] These processes are highly specific, with glucose and amino acids employing sodium-dependent cotransporters for active uptake, while urea relies on passive diffusion facilitated by concentration gradients and urea transporters.^[37] Glucose, a key energy substrate, is almost completely reabsorbed under normoglycemic conditions, with over 99% of the filtered load (approximately 140–160 g/day) recovered in the proximal tubule.^[36] This reabsorption is mediated by sodium-glucose cotransporters on the apical membrane: SGLT2, which handles 80–90% in the early proximal tubule segments (S1/S2), and SGLT1, which reabsorbs the remaining 10–20% in the late segment (S3).^[36] Glucose then exits the basolateral membrane via facilitative transporters like GLUT2. The renal threshold for glucose reabsorption is approximately 10 mM (180 mg/dL) plasma concentration, below which 100% is reabsorbed; above this level, the transport maximum (Tm) of about 300–350 mg/min is exceeded, leading to glucosuria, as seen in uncontrolled diabetes mellitus.^[38] This threshold can vary slightly due to factors like tubular flow rate but establishes a critical safeguard against nutrient loss.^[39] Amino acids are reabsorbed with near-complete efficiency, recovering over 98% of the filtered load to prevent essential nutrient wasting.^[40] This process involves multiple sodium-dependent cotransporters in the proximal tubule, tailored to specific amino acid groups; for instance, B0AT1 (SLC6A19) mediates uptake of neutral amino acids such as leucine, valine, and tryptophan in the early proximal tubule, often in association with accessory proteins like collectrin for proper membrane expression.^[41] Basolateral exit occurs via exchangers like LAT2/CD98hc. Disruptions in these systems, such as mutations in SLC6A19 causing Hartnup disorder, result in renal aminoaciduria and pellagra-like symptoms due to impaired reabsorption of neutral amino acids.^[41] Urea, the primary nitrogenous waste product, undergoes partial reabsorption of about 50% of the filtered load, primarily through passive mechanisms that support renal concentrating ability.^[42] In the proximal tubule, around 40–50% is reabsorbed passively along with water, while in the inner medullary collecting duct, vasopressin-regulated urea transporters (UT-A1 and UT-A3) facilitate diffusion driven by medullary hypertonicity, enhancing permeability up to 10-fold during antidiuresis.^[37] This reabsorption contributes significantly to the osmotic gradient in the renal medulla, where urea concentrations can reach 600–1,200 mOsm/kg, and enables urea recycling: reabsorbed urea diffuses into the interstitium, enters thin descending limbs of the loop of Henle via UT-A2, and recirculates to maintain hypertonicity without net loss.^[37] In conditions like dehydration, this recycling amplifies the countercurrent mechanism, optimizing water conservation.^[37]

Regulation

Hormonal Mechanisms

Hormonal mechanisms play a central role in regulating renal reabsorption to maintain electrolyte and fluid balance, primarily through endocrine signals that target specific nephron segments. Key hormones such as aldosterone, antidiuretic hormone (ADH, also known as vasopressin), parathyroid hormone (PTH), and angiotensin II modulate the reabsorption of sodium, water, phosphate, and other solutes, respectively, by altering transporter expression and activity via receptor-mediated pathways.^[43] Aldosterone, a mineralocorticoid produced by the adrenal cortex, enhances sodium reabsorption in the distal tubule and collecting duct to promote extracellular fluid volume expansion and potassium excretion. It binds to mineralocorticoid receptors in principal cells, translocating to the nucleus to upregulate transcription of genes including serum- and glucocorticoid-regulated kinase 1 (SGK1), which phosphorylates Nedd4-2 and prevents its ubiquitin-mediated degradation of the epithelial sodium channel (ENaC). This results in increased ENaC density on the apical membrane, facilitating sodium entry and subsequent water retention via osmotic gradients.^[43]^[44] Antidiuretic hormone (ADH), released from the posterior pituitary in response to increased plasma osmolality, primarily regulates water reabsorption in the collecting duct to concentrate urine and prevent dehydration. ADH binds to V2 receptors on the basolateral membrane of principal cells, activating adenylate cyclase and increasing cyclic AMP levels, which triggers protein kinase A-mediated phosphorylation of aquaporin-2 (AQP2) vesicles. This promotes AQP2 translocation and insertion into the apical membrane, enhancing water permeability and reabsorption through osmotic equilibration with the hypertonic medullary interstitium.^[45]^[46] Parathyroid hormone (PTH), secreted by the parathyroid glands in response to low serum calcium or high phosphate levels, inhibits phosphate reabsorption in the proximal tubule to promote phosphaturia and maintain mineral homeostasis. PTH activates PTH1 receptors coupled to G proteins, stimulating adenylate cyclase and the cAMP/PKA pathway, which leads to downregulation of the sodium-phosphate cotransporter NPT2a (also known as SLC34A1) via endocytosis and lysosomal degradation from the apical brush border membrane. This reduces phosphate uptake, shifting its excretion to balance serum levels.^[47]^[48] Angiotensin II (Ang II), generated via the renin-angiotensin-aldosterone system in response to reduced renal perfusion or low blood pressure, stimulates sodium reabsorption primarily in the proximal convoluted tubule to support extracellular fluid volume retention. Ang II binds to angiotensin type 1 (AT1) receptors on the basolateral membrane of proximal tubular epithelial cells, activating phospholipase C, increasing intracellular calcium, and enhancing the activity of the Na+/H+ exchanger (NHE3) as well as the Na+/K+-ATPase pump. This promotes sodium and bicarbonate uptake, contributing to blood pressure regulation during hypovolemia.^[49]

Neural and Local Factors

Renal reabsorption is modulated by neural influences, primarily through sympathetic innervation of the kidney, which provides rapid adjustments to sodium handling in response to systemic needs such as blood pressure maintenance. The renal sympathetic nerves, originating from the thoracolumbar spinal cord, densely innervate the juxtaglomerular apparatus, afferent and efferent arterioles, and tubular segments, particularly the proximal tubule. Activation of these nerves releases norepinephrine, which acts via α-adrenergic receptors to enhance sodium reabsorption in the proximal tubule by stimulating the Na+/H+ exchanger (NHE3), thereby increasing bicarbonate and sodium uptake into tubular cells.^[50]^[51] This mechanism contributes to volume retention during conditions like hypovolemia or stress, with studies showing that acute sympathetic stimulation can increase proximal sodium reabsorption by up to 20-30% in experimental models.^[52] Local intrarenal factors also play a crucial role in fine-tuning reabsorption through feedback mechanisms that maintain glomerular-tubular balance without relying on extrinsic hormonal signals. A key example is tubuloglomerular feedback (TGF), mediated by the macula densa cells in the distal tubule, which sense luminal NaCl concentration via the Na+-K+-2Cl- cotransporter (NKCC2). When NaCl delivery to the macula densa increases, these cells release signaling molecules such as adenosine and ATP, which constrict the afferent arteriole, reducing glomerular filtration rate (GFR) and thereby decreasing the filtered load of sodium to limit excessive reabsorption downstream.^[53]^[54] This negative feedback loop indirectly enhances overall tubular reabsorption efficiency by matching filtration to reabsorptive capacity, preventing salt wasting or overload.^[55] Counter-regulatory local actions within the kidney can oppose sodium retention, as seen with atrial natriuretic peptide (ANP), which, despite its endocrine origin, exerts paracrine effects through intrarenal production and autocrine signaling in tubular cells. ANP inhibits sodium reabsorption primarily in the inner medullary collecting duct by reducing epithelial sodium channel (ENaC) activity and vasopressin-stimulated water permeability, promoting natriuresis during volume expansion.^[56]^[57] This local modulation complements but operates independently of broader hormonal pathways, providing site-specific control over reabsorption rates.

References

[1]
Physiology, Renal - StatPearls - NCBI Bookshelf - NIH
Jul 24, 2023 · The kidney regulates plasma osmolarity by modulating the amount of water, solutes, and electrolytes in the blood. It ensures long term acid-base ...
[2]
Processes of the Kidneys - UC Berkeley MCB
Reabsorption is the movement of water and solutes from the tubule back into the plasma. Reabsorption of water and specific solutes occurs to varying degrees ...
[3]
4.2. The Kidneys and Osmoregulatory Organs – Animal Physiology
Reabsorption of water and some key electrolytes are regulated and can be influenced by hormones. Sodium (Na+) is the most abundant ion and most of it is ...
[4]
Homer W. Smith's contribution to renal physiology - PubMed
Homer Smith was, for three decades, from the 1930s until his death in 1962, one of the leaders in the field of renal physiology.Missing: term | Show results with:term
[5]
Filtration, Reabsorption and Oxygen in The Kidney - SpringerLink
Since only one to three liters of fluid are usually excreted as urine each day by normal humans, this demands that 98-99% of the 150 liters is reabsorbed by ...
[6]
Physiology, Acid Base Balance - StatPearls - NCBI Bookshelf - NIH
The renal system affects pH by reabsorbing bicarbonate and excreting fixed acids. Whether due to pathology or necessary compensation, the kidney excretes or ...
[7]
Kidney metabolism and acid–base control: back to the basics - PMC
After two decades of debate on the source of renal ammonia, glutamine was identified by Donald Van Slyke et al. in 1943 as the main fuel of ammoniagenesis ...
[8]
Relationship between energy requirements for Na+reabsorption and ...
In the intact organism, while the kidney is only ∼1% of total body weight, it utilizes approximately 10% of the whole body O2consumption (Q-O2).Missing: expenditure percentage
[9]
Solute Transport, Energy Consumption, and Production in the Kidney
The kidneys use about 7% of the oxygen consumed by the body [32], and the average basal metabolic rate for a person is about 80 W [9]. Hence, the average ...<|separator|>
[10]
Paracellular transport as a strategy for energy conservation by ... - NIH
Mar 3, 2017 · We propose that paracellular transport exists in the proximal tubule because it allows transepithelial transport to be accomplished in a more ...
[11]
Endocytic Receptors in the Renal Proximal Tubule | Physiology
Aug 1, 2012 · In the normal kidney, reabsorption of filtered proteins occurs almost exclusively by receptor-mediated endocytosis in the proximal tubule. The ...
[12]
Receptor-Mediated Endocytosis in the Proximal Tubule - PMC
This review describes our current understanding of the endocytic pathway and the multiligand receptors that mediate LMW protein uptake in PT cells.Multiligand Receptors Of The... · Endocytic Uptake In... · Literature Cited
[13]
Sodium-Potassium-Adenosinetriphosphatase-Dependent Sodium Transport in the Kidney: Hormonal Control | Physiological Reviews | American Physiological Society
No readable text found in the HTML.<|control11|><|separator|>
[14]
Paracellular transport in the collecting duct - PMC - NIH
The paracellular pathway through the tight junction provides an important route for chloride reabsorption in the collecting duct of the kidney.
[15]
Paracellular transport as a strategy for energy conservation by ...
Transepithelial transport can occur by the transcellular or paracellular route. Transcellular transport occurs through the epithelial cell, passing sequentially ...
[16]
Tubular Sodium Transport | Abdominal Key
May 29, 2016 · The Nernst equation describes the equilibrium condition for a membrane permeable only to a single ionic species: where R is the gas constant ...
[17]
https://doi.org/10.1111/j.1523-1755.2004.09104.x
[18]
https://www.derangedphysiology.com/files/Physiology%2520-%2520Fluids%2520and%2520Electrolytes%2520-0.3%2520Osmotic%2520pressure%2520and%2520oncotic%2520pressure.pdf
[19]
https://doi.org/10.1007/s00424-022-02773-w
[20]
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 ...
[21]
Biology of the proximal tubule in body homeostasis and kidney ... - NIH
Glucose reabsorption and SGLT2 inhibition. Filtered glucose is reabsorbed by the sodium-glucose transporters SGLT2 (SLC5A2) in S1–2 and SGLT1 (SLC5A1) in S3 ...
[22]
Critical Analysis of the Effects of SGLT2 Inhibitors on Renal Tubular ...
Jul 24, 2023 · Activation of carbonic anhydrase promotes sodium and bicarbonate reabsorption in the proximal tubule and in the cortical collecting duct, ...
[23]
Renal water transport in health and disease - PMC - PubMed Central
Jun 9, 2022 · The water reabsorption process is mostly performed along the proximal tubule and the thin descending limb of the loop of Henle, which reabsorb ...
[24]
The thiazide-sensitive Na+-Cl− cotransporter: molecular biology ...
The thiazide-sensitive Na + -Cl − cotransporter is the major salt reabsorption pathway in the distal convoluted tubule, which is located just after the macula ...
[25]
cotransporter (NCC) in response to a high-salt diet - PubMed - NIH
The distal convoluted tubule (DCT) apical Na(+)-Cl(-) cotransporter (NCC) is responsible for the reabsorption of 5-10% of filtered NaCl and is the target ...
[26]
TRPV5: an ingeniously controlled calcium channel - ScienceDirect
Nov 2, 2008 · The epithelial Ca 2+ channel TRPV5 constitutes the rate-limiting step of active Ca 2+ reabsorption in the kidney.
[27]
TRPV5 in renal tubular calcium handling and its potential relevance ...
Jun 11, 2019 · The distal convoluted tubule and the connecting tubule control the final Ca2+ reabsorption (∼10%) via a 3-step transcellular pathway (Figure 1).
[28]
Collecting Duct Principal Cell Transport Processes and Their ...
May 29, 2014 · Electrogenic Na+ reabsorption via ENaC is balanced by K+ secretion through ROMK and Cl− reabsorption through multiple pathways (not shown). The ...
[29]
Molecular mechanisms regulating aquaporin-2 in kidney collecting ...
Water reabsorption in the collecting duct principal cells is controlled by vasopressin, a peptide hormone that induces the osmotic water transport across the ...
[30]
Collecting Duct Intercalated Cell Function and Regulation - PMC
Therefore, type A intercalated cells in the collecting duct participate in active potassium reabsorption via the H+/K+-ATPases (Figure 2), and they undergo ...
[31]
Integrated Control of Na Transport along the Nephron - PMC
Renal fractional Na reabsorption with plausible 10-fold changes in Na intake vary only between approximately 97% and 99.7%. Thus, even with extreme intake ...
[32]
Basic Renal Processes for Sodium, Chloride, and Water
Jun 14, 2016 · This is more than 60% of the filtered chloride and very similar to the fractional reabsorption of sodium. Later segments reabsorb almost all ...
[33]
Targeting renal glucose reabsorption to treat hyperglycaemia
In the absence of SGLT2, the renal reabsorptive capacity for glucose declines to ~80 g/day (the residual capacity of SGLT1), i.e. the safety valve opens at a ...
[34]
The Physiology of Urinary Concentration: an Update - PubMed Central
The tubular fluid emerging from the end of the proximal tubule and entering the outer medulla is isotonic to plasma (about 290 mOsm/kg H2O). That tubular fluid ...
[35]
Novel Hypothesis to Explain Why SGLT2 Inhibitors Inhibit Only 30 ...
Since renal Tmax equals ∼450 g/24 h, these estimates suggest that the SGLT2 Tmax is ∼300–320 g/day, while SGLT1 Tmax is ∼120–140 g/24 h.
[36]
Validation of a Novel Method for Determining the Renal Threshold ...
In diabetic rats, the SGLT2 inhibitor canagliflozin lowered mean RTG from 415 to 94 mg/dL (23–5 mM) (7). The availability of a simple method to estimate RTG ...
[37]
Cooperation of Antiporter LAT2/CD98hc with Uniporter TAT1 ... - NIH
Renal reabsorption accounts for >98% of recovery of most circulating amino acids (AAs) filtered in the glomerulus. To this end, AAs are actively transported ...
[38]
The role of the neutral amino acid transporter B0AT1 (SLC6A19) in ...
SLC6A19 requires either collectrin or angiotensin‐converting enzyme 2 for surface expression in the kidney and intestine, respectively.
[39]
Impacts of active urea secretion into pars recta on urine ...
Altogether, the results of this simulation suggest that active urea secretion improves urine concentrating ability by amplifying urea cycling within the medulla ...Missing: osmolarity | Show results with:osmolarity
[40]
Regulation of sodium transport by ENaC in the kidney - PMC
Aldosterone is the pivotal hormone regulating sodium transport in the collecting duct via its genomic effects on the mineralocorticoid receptor. Aldosterone ...
[41]
The role of the ENaC-regulatory complex in aldosterone-mediated ...
Aldosterone acts through the mineralocorticoid receptor to alter the transcription of specific genes, including SGK1 and GILZ1.
[42]
Vasopressin–aquaporin-2 pathway: recent advances in ...
Feb 4, 2019 · In the kidney, vasopressin binds to the type 2 vasopressin receptor (V2R) and increases osmotic water transport through the regulation of the ...
[43]
Vasopressin and the Regulation of Aquaporin-2 - PMC
The water permeability is controlled by vasopressin through regulation of the water channel, aquaporin-2 (AQP2). Two processes contribute: 1) regulation of AQP2 ...
[44]
Acute Down-regulation of Sodium-dependent Phosphate ... - NIH
Our findings suggest that the acute down-regulation of NPT2a expression by PTH ligands involves mainly the cAMP/PKA signaling pathway and are thus consistent ...
[45]
PTH-mediated inhibition of the renal transport of phosphate - PMC
May 15, 2013 · PTH, acting through the PTH1 receptor, inhibits phosphate transport in the renal proximal tubule by a NHERF-1 dependent mechanism.
[46]
Role of α2-Adrenoceptors in Hypertension: Focus on Renal ...
Nov 9, 2020 · In proximal tubules, acute increase in sympathetic norepinephrine has been shown to stimulate sodium hydrogen exchanger 1 and 3 and, thereby, ...
[47]
α2A-Adrenoceptors Modulate Renal Sympathetic ...
Feb 21, 2020 · NE, released from renal sympathetic nerves, induces renal vasoconstriction, stimulates renin release, increases sodium (Na+) reabsorption by ...
[48]
exchanger in immortalized SHR proximal tubular epithelial cells
Studies have shown that renal nerves, acting through α-adrenoceptors, enhance proximal tubular sodium reabsorption in the kidney. These studies suggest that ...Missing: innervation | Show results with:innervation
[49]
Tubuloglomerular Feedback Synchronization in Nephrovascular ...
TGF is a mechanism in which the luminal sodium-chloride (NaCl) concentration is sensed at the macula densa, and a signal is relayed to the afferent arteriole to ...
[50]
Oxidative status in the macula densa modulates tubuloglomerular ...
An increase in the NaCl delivery to the macula densa promotes release of adenosine and/or ATP, leading to afferent arteriolar constriction and decrease in the ...
[51]
Tubuloglomerular Feedback - Mechanistic Insights from Gene ... - NIH
It has been proposed that adenosine is produced and released by macula densa cells as a by-product of increased NaCl transport and ATP utilization. Whereas this ...
[52]
Protective Renal Effects of Atrial Natriuretic Peptide - NIH
May 28, 2021 · In the kidney, ANP stimulates diuresis and natriuresis by different mechanisms ( · (2) At tubular level: ANP inhibits sodium reabsorption ...Missing: Na+ | Show results with:Na+
[53]
Localization of corin and atrial natriuretic peptide expression in ...
Jun 24, 2016 · These results suggest that corin-mediated pro-ANP activation may occur in renal segments and that locally produced ANP may act in an autocrine manner.Missing: Na+ | Show results with:Na+