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- it acts as countercurrent multiplier (uses energy to form conc gradients resulting in movement of substances)
- descending loop leads from PCT, conc of Na+ and Cl- in tissue fluid of medulla gets higher and higher moving from cortex to pyramids (due to activity of ascending limb of loop)
- filtrate entering descending limb of loop isotonic with blood, as it travels down, water enters tissue fluid via osmosis, it then moves down conc gradient into blood of surrounding cappilaries
- descending loop impermeable to Na+ and Cl- so no AT, fluid at bend is hypertonic to blood in capillaries
- first section of ascending loop very permeable to Na+ and Cl-, so diffuses out, then when filtrate isotonic to fluid, Na+ and Cl- actively pumped out into medulla tissue fluid (now high Na+ and Cl- concs in medulla tissue), ascending loop impermeable to water so it doesnt follow Na+ and Cl- down conc gradient, now filtrate hypotonic to blood and tissue fluid has high conc of ions (for kidney to form urine thats more conc than blood)
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