Phys: Tubular Transport

Tubular reabsorption

The movement of filtered solutes and water from the tubular lumen across the tubular epithelium and into the peritubular capillaries or vasa recta.

Tubular secretion

The transfer of molecules from the peritubular capillary blood or tubular epithelial cells directly into the tubular lumen for excretion.

Transcellular route

The transport pathway where substances travel directly across the apical and basolateral cell membranes to navigate through the epithelial cell.

Paracellular route

The passive transport pathway where solutes and water move through the tight junctions and lateral intercellular spaces located between adjacent epithelial cells.

High-capacity transport epithelium

Specialized tubular epithelium, such as in the proximal tubule, characterized by dense apical microvilli (brush border) to expand surface area, folded basolateral membranes, abundant mitochondria for ATP generation, and transcytotic vesicles to move larger molecules.

Proximal convoluted tubule (PCT)

The initial tubular segment draining Bowman's space responsible for recovering roughly 65% of filtered sodium and water, 100% of filtered glucose and amino acids, 85% of bicarbonate, and 50% of urea, while secreting hydrogen ions, ammonium, organic acids, and toxins.

Sodium-glucose cotransporter (SGLT)

An apical secondary active symporter in the proximal tubule that couples the passive entry of sodium down its electrochemical gradient with the uphill transport of glucose into the cell.

Sodium-solute cotransporters

Apical symporters in the proximal tubule that harness the inward sodium gradient to co-transport amino acids, lactate, and phosphate into the tubule cell.

Sodium-hydrogen exchanger 3 (NHE3)

An apical antiporter in the proximal tubule that pumps hydrogen ions into the lumen in exchange for sodium entry.

Basolateral Na⁺/K⁺-ATPase

An active primary transport pump on the basolateral membrane that expels three sodium ions in exchange for two potassium ions, maintaining the low intracellular sodium concentration necessary to drive secondary active transport.

Carbonic anhydrase (CA)

The enzyme present on the apical brush border (CA-IV) and inside tubular cells (CA-II) that catalyzes the reversible conversion of carbonic acid into water and carbon dioxide, facilitating the reabsorption of 80% to 85% of filtered bicarbonate in the proximal tubule.

Descending thin limb (DTL)

The water-permeable segment of the loop of Henle rich in aquaporins where water exits into the hyperosmotic medullary interstitium, concentrating tubular fluid up to 1200 mOsm/kg H₂O.

Ascending thin limb (ATL)

The hairpin segment of the loop of Henle that is impermeable to water but allows passive outward diffusion of sodium and chloride, preventing water from re-entering and diluting the medullary gradient.

Thick ascending limb (TAL)

A specialized, mitochondria-rich segment of the loop of Henle that actively reabsorbs sodium, potassium, and chloride while remaining impermeable to water, rendering the exiting tubular fluid hypotonic.

Na⁺/K⁺/2Cl⁻ cotransporter (NKCC2)

An apical secondary active transport protein in the thick ascending limb that moves one sodium, one potassium, and two chloride ions from the lumen into the tubular cell.

Lumen-positive transepithelial potential

An electropositive charge in the lumen of the thick ascending limb generated by apical potassium recycling through ROMK channels, driving the paracellular reabsorption of cations including calcium, magnesium, sodium, and potassium.

Countercurrent multiplier

The active process mediated by the loop of Henle where opposing flows in the descending and ascending limbs, coupled with active solute transport in the ascending limb, establish a corticopapillary medullary osmotic gradient.

Countercurrent exchanger

The passive vascular mechanism carried out by the vasa recta that preserves the hypertonic medullary gradient by allowing blood to equilibrate solutes and water without washing out the medullary interstitium.

Early distal convoluted tubule (early DCT)

The tubular segment following the macula densa that actively reabsorbs sodium and chloride via apical Na⁺/Cl⁻ cotransporters and remains impermeable to water, functioning as a cortical diluting segment.

Late distal convoluted tubule & Cortical collecting duct (late DCT/CCD)

Tubular segments that perform fine-tuning of water, sodium, potassium, and proton excretion under hormonal control.

Principal cells

Specialized transport cells in the late distal tubule and collecting duct that reabsorb sodium and water and secrete potassium under the influence of aldosterone and antidiuretic hormone.

Epithelial sodium channel (ENaC)

An apical sodium-selective channel in principal cells that facilitates sodium reabsorption into the cell, creating a lumen-negative electrical gradient that promotes potassium secretion.

Intercalated cells

Specialized epithelial cells in the collecting duct responsible for regulating acid-base balance; Type A cells actively secrete protons into the lumen and recover bicarbonate, whereas Type B cells secrete bicarbonate.

Aldosterone

A mineralocorticoid hormone that acts on the late distal tubule and collecting duct to stimulate sodium reabsorption and promote potassium secretion.

Antidiuretic hormone (ADH / Vasopressin)

A hormone synthesized in the hypothalamus and released from the posterior pituitary in response to high plasma osmolality or low blood volume that increases water permeability across the collecting duct, increases inner medullary urea permeability, and enhances NKCC2 transporter expression.

Aquaporin-2 (AQP2)

Water channels stored in intracellular vesicles that are translocated and inserted into the apical membrane of collecting duct principal cells in response to ADH binding to basolateral V₂ receptors.

Aquaporin-3 and Aquaporin-4 (AQP3/AQP4)

Basolateral water channels in the collecting duct that provide an exit pathway for reabsorbed water into the interstitium and peritubular capillaries.

Urea recycling

A renal mechanism where urea is filtered, partially reabsorbed in the proximal tubule, concentrated along distal segments, transported out of the inner medullary collecting duct by urea transporters into the interstitium, and re-secreted into the thin loop of Henle to sustain the hyperosmotic medullary gradient.

Urea transporters (UT-A & UT-B)

Selective transporter proteins facilitating urea movement; UT-A1 and UT-A3 are stimulated by ADH in the inner medullary collecting duct, while UT-B facilitates urea transport in the vasa recta.

Nephritic syndrome

A glomerular disease presentation characterized by glomerular inflammation, hypertension, oliguria, and hematuria (cola-colored urine), commonly caused by IgA nephropathy (Berger's disease).

Nephrotic syndrome

A glomerular condition defined by damage to the filtration barrier resulting in massive proteinuria, severe hypoalbuminemia, peripheral edema, and hyperlipidemia.

Chronic kidney disease (CKD) stages

A 5-stage classification based on glomerular filtration rate (GFR)

Type 1 renal tubular acidosis (Distal RTA)

A disorder characterized by impaired distal hydrogen ion secretion resulting in an inability to acidify urine (urine pH > 5.5), hypokalemia, and a predisposition to renal stones.

Type 2 renal tubular acidosis (Proximal RTA)

A tubular disorder caused by impaired proximal bicarbonate reabsorption, leading to high urine pH initially (later < 5.5 once filtered load drops) and hypokalemia.

Type 4 renal tubular acidosis (Hyperkalemic RTA)

A disorder resulting from decreased aldosterone secretion or tubular resistance to aldosterone, leading to hyperkalemia and a urine pH < 5.5