the kidneys are very important organs the functional units of the kidneys are the nephrons and we have millions of these in each kidney the nephron the kidneys are important in forming urine and there are four main steps in urine formation these are filtration reabsorption secretion and excretion in this video we'll focus mainly on reabsorption and secretion it's important to understand some terminology though reabsorption refers to the movement of water and solutes from the nephron back into circulation secretion is the movement of solutes and stuff from circulation back into the nephron back inside the nephron tubular so let's just recap some anatomy first here you have the nephron the afferent arteriole rings in blood brings in plasma to the head of the nephron where the vessels here will form what's called the glomerulus the vessels will then exit the head of the nephron via the efferent arteriole the afferent arteriole will then form the Vaser rectum which are basically capillaries which will follow the tube of the nephron the Vaser rector allow for reabsorption of things back into circulation while at the same time allow for secretion of things from the circulation back inside the nephron tube you'll now the tubules of the nephron has a few sections after the head of the nephron called the Bowman's capsule you have the proximal convoluted tubules the loop of Henle the distal convoluted tubules and then the collecting duct these different segments or sections of the nephron are responsible for the reabsorption of different electrolytes and substances as well as water but also these different segments of the nephron may absorb the same things but in varying amounts so let's focus on the reabsorption of things first beginning with the proximal convoluted tubules here you get reabsorption of sodium close Ryde potassium glucose amino acids which are the protein building units urea and bicarbonate as well as water of course in the descending limb of the loop of Henle water reabsorption takes place in the ascending limb of the loop of Henle so that going up on sodium chloride and potassium reabsorption occurs the loop of Henle here has a descending limb and is ascending limb as we talked about the distal convoluted tubules are responsible for the reabsorption of sodium chloride potassium calcium magnesium and bicarbonate the collecting duct here is responsible for the reabsorption of sodium chloride urea and water now different segments of the nephrons also allow for the secretion of things back inside the tube you'll from the circulation specifically from the Vaser Ekta now the proximal convoluted tubules for the secretion of creatinine certain drugs as well as hydrogen ions which are if you remember these guys are the acidic ones and the distal convoluted tubules secretion of hydrogen ions as well as well as potassium in the distal convoluted tubule there's actually an important transporter which is actually responsible for the exchange of sodium and potassium and this is how the potassium enters the tubular of the nephron and this transport is also a site where diuretics have an effect important things to take note here is that sodium and water play a key role in regulating the our blood pressure bicarbonate and hydrogen ions play a key role and acid-base balance so maintaining the pH of our body so let us now focus on each segment or each section of the nephron and look into it in a bit more detail firstly focusing on the reabsorption of things that occur in this area so let's look at what how the proximal convoluted tubule and the ascending limb of the loop of Henle the lumen here refers to the tubule of the nephron so inside the nephron tube and the cells here refer to the cells which line up the tube of the nephron and here is the Vaes erector which is a circulation in the proximal convoluted tubule sodium get reabsorbed together with glucose or amino acids the glucose or amino acid will then get reabsorbed back into circulation there is another transporter which uses an exchanger a sodium for hydrogen exchange the sodium is reabsorbed inside the cell and then is exchanged with potassium via the sodium potassium ATPase pump the sodium potassium ATPase pump is very important to remember and it is situated usually on the basal aspect of the cell so closer towards circulation the proximal convoluted tubule is important in regulating acid-base balance it is a site where hydrogen can be secreted as you can see here but also it's a site where I carbonate can be reabsorbed what happens is a chemical reaction takes place hydrogen ions inside the lumen of the nephron reacts with bicarbonate ions to form carbonic acid h2co3 through the enzyme carbonic anhydrase or CA carbonic acid gets converted to water h2o and carbon dioxide co2 carbon dioxide is a gas which can diffuse back into the cell water is also present inside the cell and so carbon dioxide can diffuse back into circulation because it's a gas further within the cell the same reaction can take place with these same enzyme carbonic anhydrase which makes carbonic acid again carbonic acid can then become hydrogen ions and bicarbonate and this cycle can continue and that is why when you have an increase in carbon dioxide you are more acidic because there's a shift for more hydrogen ion production and hence more acidity the hydrogen here can also come from circulation the bicarbonate can be reabsorbed into circulation to increase the pH of blood to make it more alkali if it has to and it does this with a symptom a type of transport which brings back into circulation bicarbonate and sodium and so bicarbonate reabsorption in the proximal convoluted tubule is sodium dependent it's important to note that other electrolytes are also reabsorbed in the proximal convoluted tubule which I have not actually drawn and these include calcium ions for example and majority of calcium is reabsorbed here the ascending loop of Henle not that descending loop of Henle is a site where electrolytes are also reabsorbed there is a try symptom a transporter which brings in one potassium to chloride and one sodium back inside the cell the potassium and chloride then go through another symptom which carries it back into circulation while the sodium gets exchanged for potassium through again the sodium potassium ATPase and so you can see this sort of repetition with the sodium potassium ATPase on a basal surface of the cell important to note here on the apical surface on the top of the cells of the loop of Henle there are passive channels which allow sodium to enter the cell from the lumen and also potassium to enter the lumen from the cell let us now focus on what happens in more detail at the distal convoluted tubule as well as a collecting duct I wrote here distal convoluted tubule and collecting duct because these pots the nephrons share some similar functions as well again you have the lumen of the nephron tubular and the cells which line up the tubular and here is the Basel rector which is a circulation essentially in the distal convoluted tubule sodium and chloride get reabsorbed via a symptom the chloride has its own channel from here back into circulation well sodium actually obviously gets exchanged for a potassium via again the sodium potassium ATPase on the basal surface of the cell the potassium can passively go through into the lumen of the nephron from the cell magnesium and calcium reabsorption is thought to occur via para cellular routes meaning in between the cells in between the tight junctions and it's not to occur via diffusion now towards the collecting duct there are important processes which take place and it is here in particular with a last bit of reabsorption and secretion takes place before the final urine product is produced from the lumen of the nephron sodium is reabsorbed in exchange for hydrogen ions and the reabsorption of sodium into circulation from inside the cell relies on hydrogen exchange so now you have sodium in circulation and you have hydrogen in the lumen whenever you have hydrogen in the lumen you know acid-base balance stuff occurs depending on the pH of the blood the acid-base regulation allows hydrogen's to interact with bicarbonate ions to become carbonic acid then again to carbon dioxide and water the carbon dioxide can diffuse easily inside the cell in and out because it's a gas within the cell however carbon dioxide can react with water again and the reverse reaction can take place water and carbon dioxide become carbonic acid and then again to bicarbonate and hydrogen ions bicarbonate reabsorption into circulation uses an exchanger a bicarbonate chloride exchanger and so the big difference to the reabsorption of bicarbonate here is that it relies on chloride rather than sodium and so it is not sodium dependent like the proximal convoluted tubule finally within the distal convoluted tubules and the collecting duct there are many channels which basically exchange sodium and potassium and not necessarily atp-dependent rather it is controlled by an important hormone called aldosterone aldosterone is a hormone produced and secreted by the adrenal glands specifically the adrenal cortex in response to low blood pressure an increase in circulating aldosterone means more sodium will be reabsorbed which means more water will be reabsorbed to increase blood pressure however this means more potassium will also be secreted there is a decrease in potassium reabsorption which will cause hypokalemia low potassium in the blood finally let us focus on what happens only in the collecting duct the apical surface of the cells here lining the collecting ducts have these special channels called aquaporins which allow for reabsorption of water this is the final concentrating area for urine which occurs in the collecting duct when the body reabsorbs water it will increase the molality of urine so it will increase the solute concentration in urine the number of aquaporins in the collecting duct is controlled also by a hormone and this hormone is called antidiuretic hormone also known as vasopressin this hormone targets the collecting ducts and essentially tells the cells here to make more aquaporins which which means with more aquaporins it means more water will be retained in circulation after all the reabsorption and secretion that takes place along the tubules of the nephron urine is produced so what is in urine then well it's a mainly water but it also contains a nitrogenous waste which is toxic if it stays in the body it also contains lots of metabolites and also can contain red blood cells and white blood cells usually in extremely lower numbers however red blood cells and white blood cell numbers in urine can obviously be high but this is of course when there are when there's an infection or some form of pathology that is occurring along the urinary tract you