0805 - Tubular Reabsorption and Secretion - RM Flashcards
How is water handled in the nephron?
99% reabsorbed flows osmotically, using aquaporins, with around 99% reabsorbed. Nephron is most permeable from renal corpuscle to hairpin of loop of Henle. Impermeable in ascending limb (‘diluting section’ as it removes solute), and variably permeable in DCT and collecting duct.
How is Sodium handled in the nephron?
Used to establish osmotic gradient, with 99.5% reabsorbed. Transport is both active (Na+K+-ATPase) and passive. Most takes place in thin ascending Loop of Henle. PCT (66%) - Symported into cell with other substances (glucose, Aa, P etc), then actively transported to interstitial space - around ⅓ leaks back into lumen. Ascending limb (25%) - Thin ascending Limb (minimal) - Passive diffusion. - Thick ascending limb - paracellular (50%) and co-transporter (inhibited by loop diuretics). K+ leak generates charge gradient to attract Na+. Inhibited by loop diuretics. DCT (3%) - NaCl symporter - inhibited by thiazide diuretics - transcellular transport only. CCT (2%) - Na+ channels - ‘not that important’, inhibited by K+-sparing diuretics.
How is glucose handled in the nephron?
100% reabsorbed, almost all (98%) of which is absorbed in the PCT via SGLT-2 (low affinity, high capacity). Several drugs can inhibit this (for type II Diabetes). The remainder (2%) is reabsorbed post-PCT, via SGLT1. While healthy people reabsorb 100% of glucose, the transporters become saturated above 13.3mM (effects appear 8.3mM), with the remaining glucose excreted.
How is potassium handled in the nephron?
depends on dietary intake, but some is always secreted. While the proximal tubule consistently reabsorbs the bulk (80-90%) of K+, the distal nephron may reabsorb or secrete K+. In both cases, the medullary collecting duct reabsorbs more K+. Thus, urine K+ can range from 2-150% of original filtered load. K+ reabsorption in DCT relies on K+/H+ active exchanger, resulting in potential alkalosis where low K+ exists.
How is urea handled in the nephron?
Very complex and poorly understood. It is filtered, absorbed (in PCT), and secreted (in Loop of Henle), according to physiological demand (creates 50% of inner medulla osmolarity). In normal situations, at the end of the distal convoluted tubule, urea is 110% of filtered load, however 70% of filtered load is then reabsorbed by the inner medullary collecting duct. In all cases more urea is filtered than is excreted (normally about 40% is excreted, ranging from 15-70%). Increased urinary flow will lead to increased urea excretion.
How are amino acids handled in the nephron?
Freely filtered and are almost exclusively (99%) reabsorbed trans-cellularly in the proximal tubule. The remaining 1% is reabsorbed in the rest of the nephron. There are a wide variety of secondary transporters (generally Na+ or H+-driven) and AA exchangers to achieve this.
How is renal plasma flow measured?
Para-aminohippuric (PAH) acid, which is filtered and secreted to be excreted in its entirety at rates proportional to the filtration fraction (20% filtered, 80% secreted by active transport in peritubular capillaries). Many drugs (e.g. penicillin) and organic anions are handled the same way.
How does ADH work?
Increases the water permeability of the collecting duct with almost immediate effect by activating a second-messenger system that causes vesicles containing aquaporin-2 to fuse to the cell membrane. Over the longer term, it increases aquaporin-2 synthesis.
Define countercurrent
The flow of fluid in opposite directions in adjacent parts of the same structure.
Why is the Loop of Henle considered a countercurrent multiplier?
In ascending loop, Na is pumped out, increasing interstitial osmolality, drawing water out of descending loop. This cycle continues as new isotonic (300mOsm) fluid enters the descending loop and becomes more concentrated due to existing osmolality. When Na is pumped out of this fluid, it further increases the osmolality/concentration of both the fluid and the interstitial space, and the cycle repeats (NEED DIAGRAM).
