--- title: "Ultrafiltration and Selective Reabsorption in the Nephron" description: "Glomerular pressure drives filtration through a selective barrier. Proximal tubule cells return glucose to blood using sodium co-transport and facilitated diffusion. Glucose in urine can reflect exces" 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: 2026-11-12 --- > **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 Glomerular pressure drives filtration through a selective barrier. Proximal tubule cells return glucose to blood using sodium co-transport and facilitated diffusion. Glucose in urine can reflect excessive filtered glucose or reduced tubular transport. 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 how arterial blood pressure and arteriole resistance support glomerular filtration. - Identifies small substances that enter filtrate and large plasma proteins that are mainly retained in blood. - Traces glucose from filtrate through proximal tubule cells back to blood and explains why glucose in urine alone does not diagnose diabetes. ## Lesson: How the nephron filters then reclaims Glomerular filtration uses pressure differences, as tissue-fluid formation does. Blood enters through the afferent arteriole and leaves through the efferent arteriole. Arterial blood pressure and resistance to blood leaving help maintain high glomerular pressure. Resistance in both arterioles affects it. Pressure in Bowman's capsule and the osmotic effect of retained plasma proteins oppose filtration. Fluid crosses the capillary endothelium, basement membrane and filtration slits between podocyte processes into Bowman's capsule. The healthy barrier retains blood cells and most large plasma proteins. Water, ions, glucose, amino acids and urea enter filtrate readily. Only part of the plasma passing through the glomerulus is filtered; not every glucose molecule in that blood leaves it. *(drawing: The boxes show barrier order from blood towards Bowman's capsule, not microscopic shapes.)* Net pressure drives glomerular filtration. Glucose reabsorption begins in the proximal tubule. A sodium-potassium pump uses ATP to export sodium, keeping it low inside the cell. Sodium enters from filtrate down its electrochemical gradient through a sodium-glucose co-transporter, bringing glucose in. Glucose exits through another carrier by facilitated diffusion into tissue fluid, then enters nearby capillaries. Water reabsorption uses osmosis. *(drawing: Co-transport brings glucose into the cell. Facilitated diffusion takes it out towards tissue fluid and blood.)* At normal blood glucose levels, the proximal tubule reabsorbs almost all filtered glucose. Urine normally contains only a tiny amount, usually too little for a routine urine test to detect. Glucose transport has a maximum rate. If filtered glucose arrives faster than that rate, some remains in the fluid that becomes urine. Filtration and reabsorption are separate processes. **Example.** A urine test detects glucose. High blood glucose, including that caused by diabetes, is one possible explanation: the filtered load may exceed glucose reabsorption capacity. Reduced or blocked tubular glucose transport can also allow glucose into urine without high blood glucose. The urine result alone cannot diagnose diabetes. Blood measurements and other information are needed. **Recap.** Pressure forms filtrate. Tubule transport returns glucose to blood. Glucose in urine can have more than one cause. *Lesson, questions and flashcards written for this dataset. Anatomy and Physiology sections 25.4, 25.5 and 25.6 were used for factual study.* ## 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) - [Co-transport and Coupled Movement Across Membranes](https://lightmysky.com/learn/science/co-transport-and-coupled-movement-across-membranes-mt_pzQ8n3FWNV) ## Optional connections - [Blood Glucose Control and Diabetes](https://lightmysky.com/learn/science/blood-glucose-control-and-diabetes-mt_eOzNtiBvX2) ## 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-)