U RINE F ORMATION IN THE N EPHRON 9.2

4 P ROCESSES IN THE F ORMATION OF U RINE Glomerular Filtration Moves water and solutes from blood plasma into nephron Creates filtrate Tubular Reabsorption Removes useful substances and returns them to blood (eg. sodium) Tubular Secretion Moves additional wastes and excess substances from the blood into the filtrate Water Reabsorption Removes water from filtrate and returns it to blood

G LOMERULAR F ILTRATION Factors contributing to filtration: Permeability of capillaries Glomerular capillaries have many pores in their walls Prevent blood cells and proteins from passing through – most other substances can pass Blood Pressure 4X greater in glomerulus than in other capillary beds in the body Provides force for filtration 180 L / day of filtrate Very similar to plasma at this point

T UBULAR R EABSORBTION : P ROXIMAL T UBULE 65% of filtrate is reabsorbed in the proximal tubule Involves active and passive transport Cells of proximal tubule are loaded with mitochondria to aid in transport Focuses on transporting Na+, glucose Negative ions follow Water follows by osmosis

T UBULAR R EABSORPTION : P ROXIMAL T UBULE ( CONTINUED ) Loop of Henle Function: absorb water and ions Descending loop  very permeable to water, slightly permeable to ions Medulla is salty  drives movement of water by osmosis Concentration of Na+ inside Loop of Henle increases Ascending loop  becomes more permeable to solutes Thin-walled section: Na+ move along concentration gradients, passing into blood vessels Thick-walled section: Na+ is now moved out of filtrate by active transport Helps to maintain salty environment of medulla

T UBULAR R EABSORPTION : D ISTAL T UBULE Reabsorption of ions decreases the concentration of the filtrate, causing water to be reabsorbed by osmosis Potassium and hydrogen ions are actively secreted into the distal tubule from the bloodstream Other substances (eg. Drugs like penicillin) are secreted into the distal tubule

R EABSORPTION FROM THE C OLLECTING D UCT Passive reabsorption of water in collecting duct High concentration of ions outside the collecting duct, in the medulla If blood plasma is too concentrated, permeability to water in the duct will increase Filtrate is ~4X more concentrated by the time it exits the duct Filtrate is now called urine ****TABLE 9.2, Pg 315 – excellent study tool