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Why is only a small fraction of filtered fluid excreted as urine?

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Human kidneys — renal system Diagram showing the location of the kidneys in the abdominal cavity and their attachment to major arteries and veins.

Only a small fraction of the fluid filtered by the glomerulus is excreted as urine because the nephron tubules perform extensive, selective reabsorption to maintain homeostasis.12

While the glomerulus produces a large volume of ultrafiltrate—approximately 170 liters per day—the body must retain essential substances like water, glucose, and various salts. As this filtrate travels through the proximal convoluted tubule, the majority of water and solutes are reabsorbed back into the blood. For instance, about 70 percent of water and sodium are reclaimed in the proximal tubule alone, with further adjustments occurring in the loop of Henle, distal tubule, and collecting ducts.123

This process ensures that substances required by the body are conserved, while waste products and excess electrolytes are concentrated for excretion. Ultimately, the tubules act as a regulatory system, modifying the initial filtrate so that only about 1.5 liters of fluid is typically excreted as urine daily.12