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Each kidney contains about one million nephrons. In Bowman's capsule, the glomerulus filters water, glucose, amino acids, and urea from blood into the tubule (ultrafiltration). As filtrate moves through the proximal tubule, loop of Henle, and distal tubule, essential molecules are reabsorbed into blood while wastes are concentrated. The final urine is stored in the bladder and excreted.
The kidney's job is selective filtration: dump nearly everything from the blood into a tube, then carefully retrieve what the body needs, leaving behind what it doesn't. Understanding this process becomes intuitive once you connect it to your prerequisites. From osmosis and tonicity, you know that water moves passively across membranes toward regions of higher solute concentration. From active transport, you know that cells can move molecules against concentration gradients using ATP-powered pumps. The kidney exploits both mechanisms in sequence across the nephron — a microscopic tube roughly 5 cm long that acts as the functional unit of filtration.
The process begins at Bowman's capsule, where the glomerulus — a tightly coiled capillary bed — sits under high hydrostatic pressure. This pressure literally pushes water and small solutes (glucose, amino acids, ions, urea, creatinine) out of the blood into the capsular space. Large molecules like proteins and blood cells are too big to pass through the filtration membrane, so they stay in circulation. About 180 liters of this filtrate are produced daily — far more than the 1–2 liters of urine actually excreted. This means the tubule must reabsorb the vast majority of what was filtered.
As filtrate flows into the proximal tubule, cells lining the tube aggressively reclaim glucose, amino acids, sodium, and other valuable solutes using active transport — your prerequisite concept in action. Water follows osmotically, reducing filtrate volume substantially. Next comes the loop of Henle, which creates a salt concentration gradient in the surrounding kidney tissue. The descending limb is permeable to water (which flows out into the increasingly concentrated medullary interstitium), while the ascending limb actively pumps out sodium and chloride without letting water follow. This counter-current arrangement builds the steep osmotic gradient that drives the concentrating step downstream — directly connected to the osmolarity regulation of the collecting duct you already studied.
In the distal tubule and collecting duct, fine-tuning occurs under hormonal control. ADH (antidiuretic hormone) inserts water channels (aquaporins) into collecting duct cells, allowing water to flow out into the hyperosmotic medulla and producing concentrated urine. Without ADH, water cannot leave and dilute urine results. Aldosterone stimulates sodium reabsorption in the distal tubule, which draws water with it and raises blood pressure. The homeostatic negative feedback loops from your prerequisites operate here: if blood pressure drops, the renin-angiotensin-aldosterone system amplifies sodium retention; if blood osmolarity rises, ADH secretion increases water reabsorption. The final urine — concentrated in urea, creatinine, and excess ions — drains into the renal pelvis, flows down the ureter, and is stored in the bladder until voided.