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University of California - Irvine
Biological Sciences
Peter A.Bowler

e109 11/26/12 lec 22 Physiology in the news Excreted Tamiflu (flu-fighting drug) contaminating the rivers = might spread drug-resistant strains of seasonal and avian flu -Kidneys take compounds out of the blood and put them in the urine—no chemical modifications (that’s why medications appear in the urine) Asingle nephron -Fluid (filtrate) is modified chemically (by adding or subtracting components-not chemical reactions) as it moves from start (Bowman’s capsule) to end (collecting duct) to ureter to bladder -The four processes of the nephron are: 1. Filtration: movement from blood to lumen 2. Reabsorption: movement from lumen to blood 3. Secretion: movement from blood to lumen 4. Excretion: movement from lumen to outside the body -Filtration occurs ONLY in Bowman’s capsule -The blood enters the glomerulus (surrounded by Bowman’s capsule) via the afferent arteriole -Blood moves through the capillary bed and exits through the efferent arteriole (heads off to another capillary bed) -from blood to lumen or from capillary to Bowman’s capsule -Filtered substances pass through endothelial pores and filtration slits )(move from capillary lumen, passing through basal lamina, and entering into the lumen of Bowman’s capsule). The amount of material moving across is the amount filtered. -Filtered material includes water, small solutes (not cells, not large macromolecules—retained in the capillary lumen or blood) Volume of fluid = filtered/ time = glomerular filtration rate (GFR) =180 L/day (comes out of Bowman’s capsule)  enters proximal tubule (closed off tube)  enters loop of Henle  enters distal tube (several other tubes feed into this tube)  enters collecting duct to the bladder -Typical excretion rate would be 1.5 L/day for humans 180-1.5 L/day = 178.5 L/day is REABSORBED Reabsorption takes place at the proximal tubule, descending loop of Henle, ascending loop of Henle, distal tubule, and collecting duct -In addition to water, many small solutes (amino acids, sugars, other nutrients, and ions) are reabsorbed >99% of filtered water is reabsorbed in the kidney -Water moves from lower osmolarity to HIGHER osmolarity; need an appropriate osmotic difference between the lumen and the outside. +If solutes are transported, we have water following (movement of water) Reabsorption of solutes and water -anything in the lumen of the nephron ends up being excreted from the body 1. Na+ is reabsorbed by active transport. Once Na+ is driven out of the lumen into the extracellular fluid 2. Electrochemical gradient drives anion reabsorption. 3. Water moves from lower osmolarity to higher osmolarity (into the extracellular fluid) by osmosis, following solute reabsorption. Concentrations of other solutes increase as fluid volume in the lumen decreases, so they can move down their concentration gradients. 4. Permeable solutes are reabsorbed by diffusion through membrane transporters or by the parace
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