hi everyone welcome to bite size med where we talk about quick bite-sized concepts in medicine for study and rapid review this video is on the proximal convoluted tubule the kidney has millions of nephrons and each nephron has a glomerulus and a renal tubule the first part of the renal tubule is the proximal convoluted tubule which then leads into the loop of henle the distal convoluted tubule and finally the collecting duct the renal plasma gets filtered through the glomerulus the water and solutes that pass into the tubule can then get reabsorbed or secreted or neither and just get excreted the maximum reabsorption happens at the proximal tubule 65 of the filtered load of sodium and water are reabsorbed along with other solutes like glucose phosphate amino acids and bicarbonate because there's so much reabsorption happening here there are some structural changes that can aid it like the brush border which increases surface area for reabsorption and there are lots of mitochondria to provide energy for solute transport the tight junctions between the cells are a little loose in the proximal tubule when compared to the distal portions of the nephron there are slight differences between the early proximal tubule and the late proximal tubule this is a cell of the early pct with the tubular lumen the peritubular capillaries and the interstitium in between the luminal membrane is towards the tubular lumen and the basal lateral membrane is towards the interstitium sodium gets reabsorbed from the tubular lumen along with glucose amino acids phosphate and a few other solutes since both of them are moving in the same direction this is a co-transport for sodium and glucose it's called the sodium glucose co-transporter or sglt the type 2 is the one that's predominant in the kidney but where does it get its energy from on the opposite basolateral membrane there is a sodium potassium atpase pump which uses atp and pushes sodium out into the capillary and potassium is brought back into the cell so since sodium leaves the cell it creates a concentration gradient the low concentration in the cell pulls sodium from the other side and that creates an energy which brings substances like glucose along with it so this is a secondary active transport because the energy is coming from another primary active transport almost all the glucose and amino acids get reabsorbed here so by the time we reach the late pct there isn't enough to go with sodium but the negative ion that usually moves with sodium that's chloride gets concentrated in the lumen and in the later part of the pct sodium gets reabsorbed with chloride the pct is the site of glomerulotibular balance simply put this means if there's more sodium filtered there's more sodium reabsorbed the fraction of sodium reabsorbed remains the same the 65 percent so the ecf volume gets maintained along with sodium water gets passively reabsorbed in equal amounts the normal plasma osmolarity is around 300 milliosmoles per liter the reabsorption of equal water and solutes keeps the urine isosmatic at this point so 300 milliosmoles per liter glucose gets filtered by the glomerulus continuously and the carriers reabsorb it for substances like this there is a limit on how much the carriers can reabsorb and that's called the transport maximum when the plasma glucose concentration is within its normal limit so let's say 100 milligrams per 100 ml the filtered load is 125 milligram per minute so that's normal if the plasma concentration of glucose increases to about 200 milligrams that increases the filter load as well and that reaches 250 mg per minute so at this point a small amount of glucose starts appearing in the urine and this is called the threshold for glucose the transport maximum is the point at which all the carriers get saturated so they can no longer reabsorb glucose so again glucose will appear in the urine at this point so that's around milligram per minute now ideally these two should be the same the difference between the threshold and the transport maximum is because not all nephrons are the same some of them reach their transport maximum earlier than the others so glucose appears in the urine a little earlier than when the transport maximum is reached this period is called splay so in patients with uncontrolled diabetes mellitus the plasma glucose can go high enough to cross this threshold saturate the carriers and appear in the urine as glucose now back to the pct cell another substance that gets reabsorbed is bicarbonate this is important for acidification of urine carbon dioxide diffuses into the cell and binds to water with carbonic anhydrase it becomes carbonic acid which then dissociates into hydrogen ions and bicarb ions bicarb gets reabsorbed with sodium as a co-transport in this part of the nephron hydrogen ions get exchanged for sodium by the sodium hydrogen exchanger that's on the luminal membrane so the hydrogen ions land up in the lumen where it binds to bicarb which is in the glomerular filtrate and forms carbonic acid that then forms carbon dioxide and water and the carbon dioxide then diffuses back into the cell around 80 percent of bicarb gets reabsorbed this way that's about reabsorption but what about secretion though the pct does majority of the reabsorption it also secretes some organic acids and bases like bile salts urates and also para amino hyperic acid pah whose clearance measurement is used to determine renal plasma flow and that is the proximal convoluted tubule if you like this video give it a thumbs up and subscribe to my channel thanks for watching and i'll see you in the next one