8.4 Renal_Urine formation_Tubular reabsorption and secretion

TUBULAR REABSORPTION (SECOND RENAL PROCESS)

Complex: Tubular reabsorption is trans-epithelial transport of solutes and water from the tubular lumen into peritubular capillaries, driven by electrochemical and osmotic gradients, like moving supplies across a conveyor belt back into circulation.
Simple: Useful stuff moves from urine-in-progress back into blood.

SODIUM (Na⁺) REABSORPTION — KEY DRIVER

Complex: About 99.6% of filtered Na⁺ is reabsorbed, mainly in the proximal tubule (≈65–70%) via Na⁺-K⁺ ATPase–dependent active transport, making Na⁺ the primary driver of water reabsorption, like salt pulling water with it.
Simple: Sodium is mostly taken back and water follows it.

HORMONAL CONTROL OF Na⁺ (ALDOSTERONE & ANP)

Complex: Aldosterone increases Na⁺ reabsorption in the distal tubule and collecting duct, while ANP inhibits Na⁺ reabsorption and increases GFR, acting like opposing volume knobs.
Simple: Aldosterone saves salt and water; ANP helps get rid of them.

GLUCOSE REABSORPTION (PROXIMAL TUBULE)

Complex: Over 99% of glucose is reabsorbed in the proximal tubule via Na⁺-glucose cotransporters (SGLT1/2) with a transport maximum (~400 mg/min), and glucose appears in urine when plasma levels exceed ~250 mg/dL, like a conveyor belt that overflows when overloaded.
Simple: All glucose is normally reabsorbed unless blood sugar is too high.

AMINO ACIDS, PEPTIDES & PROTEINS

Complex: Amino acids and small peptides are almost completely reabsorbed in the proximal tubule via cotransport and endocytosis, then broken down into amino acids inside cells, like recycling parts before reuse.
Simple: Protein pieces are taken back and reused.

CHLORIDE (Cl⁻) & UREA REABSORPTION

Complex: Chloride follows Na⁺ by passive diffusion across multiple nephron segments, while urea passively diffuses with ~50–60% reabsorbed, helping maintain the medullary gradient, like passengers following the main crowd.
Simple: Chloride and urea mostly follow sodium and water.

URIC ACID HANDLING

Complex: About 90% of filtered uric acid is reabsorbed in the proximal tubule, and high levels or acidic urine can lead to crystal formation and gout, like sugar crystallizing at the bottom of a cup.
Simple: Too much uric acid can form kidney stones and gout.

WATER (H₂O) REABSORPTION

Complex: About 80% of water is reabsorbed in the proximal tubule and descending loop of Henle passively, while the remaining ~20% is hormonally regulated by ADH via aquaporins in the distal tubule and collecting duct, like adjustable water valves.
Simple: Most water is always reabsorbed; the rest depends on ADH.

ANTIDIURETIC HORMONE (ADH / VASOPRESSIN)

Complex: ADH increases aquaporin insertion, raising water permeability and concentrating urine, while alcohol inhibits ADH and causes dilute urine, like turning a sponge on or off.
Simple: ADH saves water; alcohol blocks it.

TUBULAR SECRETION (THIRD RENAL PROCESS)

Complex: Tubular secretion moves substances from blood into tubules to regulate pH, potassium balance, and toxin removal, acting as a final cleanup step.
Simple: Extra waste is actively added to urine.

HYDROGEN (H⁺) SECRETION — ACID–BASE CONTROL

Complex: H⁺ is secreted in proximal, distal, and collecting tubules via Na⁺/H⁺ exchange to regulate blood pH, like adjusting acidity with a buffer.
Simple: The kidneys control acid levels by secreting hydrogen ions.

POTASSIUM (K⁺) SECRETION & ALDOSTERONE

Complex: Aldosterone increases Na⁺ reabsorption while enhancing K⁺ and H⁺ secretion in the distal nephron, so excess aldosterone can cause hypokalemia and alkalosis, like trading potassium for sodium.
Simple: More aldosterone means more potassium lost in urine.