--- title: "Ultrafiltration and Selective Reabsorption in the Nephron" description: "The glomerulus filters blood by the same pressure argument as any capillary, only harder, and the tubule then takes back what the body needs. Filtration and reabsorption are separate steps with separa" canonical: https://lightmysky.com/learn/science/ultrafiltration-and-selective-reabsorption-in-the-nephron-mt_vSKfoJN6zh source: https://lightmysky.com/learn/science/ultrafiltration-and-selective-reabsorption-in-the-nephron-mt_vSKfoJN6zh.md retrieved: 2026-09-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Ultrafiltration and Selective Reabsorption in the Nephron The glomerulus filters blood by the same pressure argument as any capillary, only harder, and the tubule then takes back what the body needs. Filtration and reabsorption are separate steps with separate mechanisms. Subject: Science · Area: The Human Body · Ages 17 to 18 Page: https://lightmysky.com/learn/science/ultrafiltration-and-selective-reabsorption-in-the-nephron-mt_vSKfoJN6zh ## Ready when they can - Explains why pressure in the glomerulus is high enough to force filtration. - Names what is filtered out of the blood and what is too large to pass. - Describes how glucose is recovered from the filtrate and why it normally never reaches the urine. ## Lesson: How the nephron filters then reclaims Each nephron starts with a filter built for pressure. Blood enters the glomerulus through a wide afferent arteriole and leaves through a narrower efferent one, so blood backs up and pressure climbs. That pressure shoves fluid through the capillary wall, the basement membrane and the capsule lining into the tubule. Narrow the exit and pressure rises: pure plumbing. Size decides who passes into the filtrate. Water, salts, glucose, amino acids and urea are small enough to cross. Blood cells and plasma proteins are held back. The filtrate entering the tubule is therefore plasma minus the big stuff, and everything useful in it must be won back downstream. The proximal tubule then reclaims glucose in full. Carrier proteins there pull every glucose molecule back into the blood, powered by sodium co-transport, the same trick gut cells use. Carrier numbers are limited, but normal blood levels sit safely below their ceiling. That is why healthy urine contains no glucose at all. **Example.** Follow glucose to see the two steps stay separate. Filtration dumps every glucose molecule out at the glomerulus by pressure, with no carriers involved. Reabsorption wins it all back by carriers in the tubule. Only when blood glucose runs extremely high do the carriers saturate and glucose spills into the urine, which is why glucose in urine warns of diabetes. **Recap.** Pressure filters everything small out, then carriers reclaim all the glucose, so healthy urine has none. ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Tissue Fluid and Its Return to the Circulation](https://lightmysky.com/learn/science/tissue-fluid-and-its-return-to-the-circulation-mt_0jw7uXpK3C) - [Blood Glucose Control and Diabetes](https://lightmysky.com/learn/science/blood-glucose-control-and-diabetes-mt_eOzNtiBvX2) - [Co-transport and Coupled Movement Across Membranes](https://lightmysky.com/learn/science/co-transport-and-coupled-movement-across-membranes-mt_pzQ8n3FWNV) ## Opens up - [Acid-Base Balance: The Bicarbonate System and Compensation](https://lightmysky.com/learn/science/acid-base-balance-the-bicarbonate-system-and-compensation-mt_LFLgZre62C) - [Osmoregulation and the Action of ADH](https://lightmysky.com/learn/science/osmoregulation-and-the-action-of-adh-mt_Rz_MmL-Pa-)