The Nephron and the Glomerular Filtration Barrier

The Story

The nephronThe functional unit of the kidney that filters blood and produces urine. is the kidney’s functional unit, and there are about a million of them per kidney. Each begins as a tuft of capillariesThe smallest blood vessels where gas, nutrient, and waste exchange occurs between blood and tissues., the glomerulusA network of capillaries in the nephron where blood filtration occurs., cradled inside a cup called Bowman’s capsule; together they form the renal corpuscleThe structure in the nephron that consists of the glomerulus and Bowman’s capsule, where filtratio, where blood is filtered. The filtrateThe fluid that is filtered from the blood into the nephron and will eventually become urine. then runs through a long, looping tubule — the proximal convoluted tubuleThe first part of the nephron tubule where most reabsorption of water, ions, and nutrients occurs., the loop of Henle that dives into the medulla and climbs back, the distal convoluted tubuleThe part of the nephron where final adjustments to ion and water balance occur under hormonal contro, and finally the collecting ductA duct in the nephron that collects urine from multiple nephrons and adjusts water reabsorption. — each segment modifying the fluid before it becomes urine. Blood enters the glomerulus through an afferent arterioleThe small artery that carries blood into the glomerulus of the nephron. and leaves through a narrower efferent arterioleThe small artery that carries blood away from the glomerulus after filtration., and that narrowing builds the pressureThe force exerted by gases in the respiratory system, affecting airflow and gas exchange. that drives filtrationThe process by which fluid moves out of capillaries into surrounding tissues due to hydrostatic pre. The glomerulus is, in essence, a pressurized sieve.

What makes that sieve trustworthy is its three-layer filtration barrier, and this is the structure at the center of Stina’s whole story. Plasma crossing from blood into Bowman’s space must pass, in order, through: the fenestrated capillaryCapillaries with pores (fenestrations) that allow rapid exchange of fluids and small solutes (found endotheliumThe innermost layer of blood vessels, composed of simple squamous epithelial cells, which reduces f, riddled with pores that stop blood cellsThe basic structural and functional units of life.; the glomerular basement membrane, a dense, negatively charged gel that is the main barrier to protein; and the filtration slits between the foot processes of podocytesSpecialized cells in the glomerulus that form filtration slits, preventing large molecules from ente, octopus-like cells wrapping the capillaries. Together these layers filter by size and by charge — small moleculesGroups of atoms bonded together. and waterThe universal solvent essential for life. pass freely, while large, negatively charged plasmaThe liquid component of blood. proteinsLarge molecules made of amino acids with various functions in the body. like albuminA plasma protein that helps maintain osmotic pressure and transport substances. are held back. A healthy barrier keeps essentially all protein in the blood. So when protein appears in Stina’s urine and then climbs into the nephrotic range, the message is unambiguous: this three-layer barrier is failing. The next pages explain what it normally does with the fluid that does get through — and then, what was wrecking the barrier itself.

From Stina’s chart: Stina’s quantified urine protein climbs over months from a trace into the heavy, nephrotic range — a leak pointing squarely at the glomerular filtration barrier this page dissects.