Скачать или смотреть MRCP-1 Kidney Tubular Function Quiz WITH DR SHEEN

Kidney Tubular Function: The Modification Assembly Line
The renal tubules transform glomerular filtrate into urine through segment-specific reabsorption (reclaiming needed substances) and secretion (adding wastes/excess ions). Each segment has a specialized role in maintaining homeostasis.
Tubule Segment Primary Reabsorption Primary Secretion Key Regulatory Hormone
Proximal Convoluted Tubule (PCT) Bulk, isosmotic: 65% of Na⁺, Cl⁻, H₂O; 100% of glucose, amino acids, HCO₃⁻. Organic acids/bases (e.g., drugs, creatinine), H⁺. Angiotensin II (↑ Na⁺ reabsorption).
Loop of Henle Creates medullary gradient. Descending limb: H₂O only. Ascending limb: NaCl (via NKCC2), Ca²⁺, Mg²⁺. — —
Distal Convoluted Tubule (DCT) Fine-tunes NaCl (via NCC), actively reabsorbs Ca²⁺ (↑ by PTH). — PTH (↑ Ca²⁺ reabsorption).
Collecting Duct (CD) Final H₂O reabsorption (ADH-dependent), final Na⁺ reabsorption. K⁺ (principal cells), H⁺ (intercalated cells). ADH, Aldosterone (↑ Na⁺ reabsorption & K⁺ secretion).
Core Principles:

Mass Balance: The PCT reabsorbs the bulk, while later segments perform fine-tuning.

Hormonal Control: Critical in late DCT & CD (Aldosterone, ADH, PTH).

Gradient Creation: The Loop of Henle’s countercurrent multiplier establishes the osmotic gradient needed for urine concentration in the CD.

Clinical Correlate: Dysfunction at specific segments causes distinct syndromes:

PCT: Fanconi syndrome (glucose, amino acids, HCO₃⁻ in urine).

Loop of Henle: Bartter syndrome (salt-wasting, hypokalemia).

DCT: Gitelman syndrome (mimics thiazide diuretics; hypokalemia, hypocalciuria).

CD: Nephrogenic Diabetes Insipidus (ADH resistance) or Type 4 RTA (aldosterone resistance).

In essence, tubular function is a sequential, highly regulated process that balances efficient recovery of essential solutes with precise excretion of wastes and excess ions.