passive reabsorption is really important in the second half of the proximal convoluted tubule because of these electrochemical gradients that have been produced by the symporters and the antiporters this allows chloride potassium calcium ions magnesium and urea to fuse across either through the cell or between the cells paracellular or trans cellular into the interstitial fluid and then into the peritubular capillary this is going to promote osmosis where salt goes water follows so water is going to want to follow it and the proximal convoluted tubule is especially permeable to water this is where the majority of water is reabsorbed because it has aquaporin channels in it the loop of henle is where we're going to independently regulate the volume and the osmolarity of the body fluids remember volume is referring to the water the fluid in it and the osmolarity is the concentration of solutes so in the descending loop of henle both water and some solutes move about 15 of water is reclaimed here but in the ascending limb and the thick part of it it's impermeable to water so only sodium and the other ions can move across the cell membrane and be reabsorbed into the um in this case the vasorecta so we can't have sodium uh we can't have water move across it's impermeable but here we have a thin porter that moves both sodium potassium and chloride the sodium then is going to be pumped out using the energy from atp into the interstitial fluid and from there into the into the vasorecta and the two chloride ions are going to passively move across the cell into the interstitial fluid and then into the vasa rectum the potassium is going to be recycled and put back into the fluid of the lumen so as fluid flows along the distal convoluted tubule we're going to have reabsorption of sodium chloride with sodium chlorides importers and we're also going to have reabsorption of calcium this is where the parathyroid hormone is going to act in the distal convoluted tubule remember the parathyroid hormone acts in three ways to increase blood calcium it activates osteoclasts to break down bone it increases reabsorption of calcium from the urine and it also is going to activate calcitriol so that we can absorb more calcium from our food by the end of the distal convoluted tubule we've reabsorbed about 95 of all the solutes in water the cells in the collecting duct then are going to make the very final adjustments and this is where we're going to do some regulation of the concentration of urine with antidiuretic hormone also aldosterone works here in the principal cells the principal cells are going to reabsorb sodium and um they are going to move to the slide here and show you sodium is going to the channels open and it diffuses through and potassium is going to we're going to have an antiporter here a sodium potassium pump excuse me where we're using atp to pump potassium in and sodium out then the potassium inside the cell some of it will diffuse out and some of it will diffuse into the lumen aldosterone increases sodium and water reabsorption and potassium secretion by synthesizing new pumps and new channels to do that so if we think about the hormones antidiuretic hormone is going to concentrate urine and increase blood volume and blood pressure aldosterone by putting sodium into the blood is going to also increase blood volume because water is going to follow and so it increases blood volume and blood pressure atrial natural peptide is the only one of the three that is going to decrease blood volume and blood pressure remember that's released because the a tree of the heart are being stretched they're being stretched your blood volume and blood pressure is too high so the negative feedback would tell you that that one has to decrease i also wanted to mention the intercalated cells are going to reabsorb potassium bicarbonate and this is how they do it so there are proton pumps inside these uh intercalated cells remember a hydrogen ion is a hydrogen that doesn't have an electron so it has one proton and that's what gives it the positive charge so we call them protons proton pumps move hydrogen ions so these proton pumps are going to use atp and they're going to force hydrogen into the lumen where the concentration of hydrogen is already high so they're going against the gradient so they have to use atp so here again we have carbon dioxide and water producing carbonic acid which dissociates into hydrogen ions and bicarbonate the bicarbonate is going to use a chloride antiporter to leave the cell will go into the interstitial fluid and be reabsorbed and this is going to help buffer the blood help decrease the acidity of the blood so hydrogen is secreted bicarbonate is added the um by doing this the intercalated cells help to regulate the ph body fluids and these sodium um not sodium excuse me hydrogen ion atpases or proton pumps are can concentrate the urine so that it's a thousand times more acidic than blood the urine is then going to be buffered itself because if it's too acidic it can damage the cells so it's buffered with phosphate and ammonia and both of these combine irreversibly with the hydrogen ions and excreted as the ammonium ion and it as phosphoric acid these are the main things this is a great little chart these are a lot of the main things that i want you to know about secretion and reabsorption filtration so here it talks about filtration how much is filtered here it tells you the main things that are reabsorbed and secreted in the proximal convoluted tubule in the loop of henle in the distal convoluted tubule in the principal cells in the late distal tubule and the collecting duct as well as the intercalated cells in the late distal tubule and collecting duct the goals for this lecture to find and describe the importance of tubular secretion and reabsorption describe where most reabsorption occurs differentiate between trans cellular and paracellular reabsorption describe why the phrase where salt goes water follows is so important in understanding kidney physiology differentiate between obligatory and facultative water reabsorption defined transport maximum primary active transport secondary active transport symporters anti-porters antiporters diffusion and glucosauria list the substances that are secreted and reabsorbed in each of the following regions the proximal convoluted tubule the loop of henle distal convoluted tubulin collecting dot and describe the functions of principal cells in interpolated cells it's also a good idea as well to understand on that last chart main things that are reabsorbed and secreted in each of the regions