Reabsorption, Secretion, and How Urine Is Made

The Story

Filtration is only the first of three steps, and on its own it is wildly wasteful. The glomeruli filter about 180 liters of plasmaThe liquid component of blood. a day, yet a person makes only one to two liters of urine — meaning more than 99% of the filtrateThe fluid that is filtered from the blood into the nephron and will eventually become urine. is reclaimed. That reclaiming is tubular reabsorptionThe process of fluid moving back into capillaries from surrounding tissues due to colloid osmotic p, and it is most aggressive in the proximal convoluted tubuleThe first part of the nephron tubule where most reabsorption of water, ions, and nutrients occurs., which pulls back glucoseA simple sugar that is the main source of energy for cells., amino acids, most ionsCharged atoms or molecules., and the bulk of the waterThe universal solvent essential for life. and sodium(Na⁺): Major ECF cation; important for fluid balance, nerve function.. The loop of Henle sets up a salty medulla that lets the kidney concentrate urine; the distal tubule and collecting ductA duct in the nephron that collects urine from multiple nephrons and adjusts water reabsorption. fine-tune sodium, potassium(K⁺): Major ICF cation; essential for muscle and nerve function., and water under hormonal control. Alongside reabsorption runs the third step, tubular secretionThe process of moving substances from the blood into the nephron tubule to be excreted in urine., in which the tubule actively adds wastes, drugs, and excess ions (like hydrogen and potassium) from the blood into the filtrate. Urine, then, is what is left after the kidney filters everything, reclaims what the body needs, and dumps what it does not.

These three steps — filtrationThe process by which fluid moves out of capillaries into surrounding tissues due to hydrostatic pre, reabsorption, secretion — are the kidney’s entire logic, and keeping them straight is what lets a clinician localize a problem. Here is the diagnostic payoff for Stina. Her tubules are working: her glucose is not spilling, her electrolytes are reasonably maintained, the reabsorptive machinery is intact. What is failing is upstream, at the filter itself — large protein is crossing a barrier that should have held it. That contrast, between a competent tubule and a leaking glomerulusA network of capillaries in the nephron where blood filtration occurs., is precisely how we will later separate glomerular proteinuria from tubular proteinuria. The kidney does not break all at once; it breaks at a specific station, and reading which station is broken is reading the disease.

From Stina’s chart: Stina’s basicA solution with a pH above 7, having a lower concentration of H⁺ ions. chemistries stay near-normal even as protein spills — her tubules are still doing their reabsorptive work, which is exactly why the protein leak (a glomerular problem) stands out against intact tubular function.