The Urinary System Urine formation Tubular Reabsorption and Secretion 23 1 Tubular Reabsorption and Secretion After filtration comes the process of converting the glom filtrate into urine by tubular reabsorption back into blood and secretion from blood urine In contrast to glom filtration reabsorption and secretion i e mvmnt of substances is not all by bulk flow of fluid only some by bulk flow 23 2 Proximal Convoluted Tubules PCT Tubular reabsorption is the process of reclaiming H2O and useful solutes from tubular fluid and returning them to the blood via the peritubular cap Side facing toward lumen inside tubule or facing the filtrate tubular fluid is luminal side apical side of epithelial cell Side facing inward towards tissues peritubular cap is basolateral side of epith cell PCT reabsorbs most of the glomerular filtrate PCT also secretes some substances into tubule fluid Greatest length many microvilli and abundant mitochondria for active transport 23 3 PCT Reabsorbs the most chemical types of any part of nephron transcellular route through epithelial cells of PCT Individual transcellular mechanisms responsible for the reabsorption of some individual solutes paracellular route between epithelial cells of PCT Leaks occurs through the tight jxn btw cells bulk flow and solvent drag with paracellular route water carries along a variety of dissolved solutes these materials enter tissue fluid at the base of the tubule epithelial cells and are taken up by peritubular capillaries 23 4 Sodium Reabsorption Sodium reabsorption is most important reabsorption process creates an osmotic gradient that drives the reabsorption of water and other solutes Na reabsorption is an active process which occursa in all tubular segments except thin segment of descending limb of nephron loop A concentration gradient is maintained in PCT with about 10x higher Na conc in tubular fluid than in cytoplasm of tubule epthelial cells In turn drives diffusion of Na from tubular fluid into the tubule epithelial cells Na K pumps in the basolateral membrane of the epithelial cells stop the accumulation of sodium in the epithelial cells ATP consuming active transport pumps pumps Na out into the tissue fluid near peritubular capillary pumping K back into tubule epithelial cells from tissue fluid 23 5 Sodium Reabsorption Thus a conc gradient is maintained with lower Na conc in the epithelial cells which allows sodium to diffuse downhill into epithelial cells from tubular fluid in lumen 10 fold diff The Na K pumps are used in basolateral membrane to maintain the Na conc gradient in ALL Na reabsorbing segments of the renal tubule In PCT two types of transport proteins on the luminal side of epithelial cells implement sodium uptake Symports which allow Na moving by diffusion to simultaneously bind Na to another solutes and bring it along at the same time Na symports depend on the Na concentration gradient maintained by the Na K pumps do not consume E Na H antiports using energy move Na into PCT epithelial cells while pumping H out in opposite direction into PCT tubular fluid Angiotensin II increases Na H antiport activit in PCT which has a strong 23 6 influence on Na reabsorption Glucose Reabsorption in PCT Glucose is cotransported into tubule cells of PCT with Na by symports called sodium glucose transport proteins SGLTs Glucose moves with sodium as sodium moves passively down its gradient from high Na in lumen to low Na in tubule epithelial cells Uses Potential E to move Na glucose taken along too Amino acids also move into tubule cells via sodium symports Later glucose moves out of tubule cells down its gradient into tissue fluid by facilitated diffusion Normally all glucose is reabsorbed from tubule fluid with none left in the urine reclaim them Kidneys do not regulate glucose or amino acid levels just 23 7 Reabsorption in the PCT Cl follow the positive sodium ions by electrical attraction Various antiports absorb Cl into epithelial tubule cells Cl and K move out of epithelial cell through the basolateral membrane into tissue fluid by a K Cl symport Many electrolytes move through by the paracellular route with water blue arrows Peritubular capillary Tissue fluid Tubule epithelial cells Tubular fluid Glucose Na K K Cl ATP Na K pump ADP Pi K Cl symport Tight junction Glucose Na Na H Cl Anions H2O Sodium glucose transport protein SGLT Symport Na H antiport Cl anion antiport Aquaporin Solvent drag Transcellular route Paracellular route Brush border H2O urea uric acid Na K Cl Mg2 Ca 2 Pi 23 8 Reabsorption in the PCT Nitrogenous wastes primarily leave by diffusion Urea diffuses through the tubule epithelium with water in tubular fluid After filtration about of the urea that was originally in blood and is now in tubular fluid is reabsorbed by diffusion mainly by the paracellular route in PCT and collecting duct Some urea is also secreted into tubular fluid at PCT and Urea cycles secreted and reabsorbed but some is lower descending limb always excreted out The PCT reabsorbs nearly all the uric acid but later secrete all of it back out 23 9 Bicarbonate Reabsorption Bicarbonate reacts with H in tubule fluid to form CO2 and water CO2 not the bicarbonate diffuses into tubule epithelial cell where it reacts with water to form carbonic acid and then bicarbonate and H The bicarbonate diffuses out of cell into tissue fluid and then diffuses into capillary while the H is pumped into tubular fluid by Na H antiport 23 10 Transport Maximum The transport maximum Tm is the maximum rate of absorption and it is reached when the binding sites on the transport proteins in membrane are saturated Tm is limited by number of transport proteins in the plasma membrane relative to the amount of substance in filtrate For ex if too much glucose is in the blood then more is filtered into urine than can be reabsorbed back into tissues 23 11 Water Reabsorption Significant function of the kidneys Most water in filtrate is reabsorbed by the PCT water follows solutes by osmosis through both paracellular and transcellular routes As solutes leave the tubular lumen osmolarity lowers or H2O conc increaes in lumen and as solutes move into interstitial fluid osmolarity increases or H2O conc loweres mvmnt of solutes down conc gradient moves water The higher water concentration in lumen causes diffusion of water out of lumen through tubule cells or between tubule cells into tissue fluid transcellular through water channels called aquaporins 23 12 Water