 In this video, I will describe the processes of reabsorption and secretion by the proximal convoluted tubule, descending limb of the loop of Henle, ascending limb of the loop of Henle, distal convoluted tubule, and collecting duct. And then I will explain the role of the loop of Henle, the vasorecta, and the countercurrent multiplication mechanism in the concentration of urine. There are three major processes that contribute to urine formation. The first is filtration. And filtration occurs at the renal corpuscle, where fluid is forced out of the blood in the glomerular capillaries into Bowman's capsule. After filtration, the liquid in the nephron is known as filtrate. Filtrate is modified through reabsorption and secretion. Reabsorption is movement from the filtrate to the blood. In contrast, secretion is the opposite. Secretion is movement from the blood to filtrate. So reabsorption is important to reclaim the majority of water and essential nutrients that we don't want to lose in the urine. In contrast, secretion is a mechanism to help remove waste from the body and secrete that waste from the blood into the urine. This illustration shows us the locations of secretion and reabsorption in the nephron in order to regulate the composition of urine. Reabsorption, I'll highlight with green, is the movement of substances from filtrate back to the blood. Here at the proximal convoluted tubule, we have reabsorption of a large amount of water. 67% of the water that is filtered gets reabsorbed at the proximal convoluted tubule. So a large amount of water as well as a large amount of the solutes in that water are reabsorbed. For example, sodium chloride is a major solute that's reabsorbed in large amounts by the proximal convoluted tubule. And then some essential nutrients like glucose, amino acids, proteins, vitamins, and lactate are reabsorbed in very large amounts. Essentially 100% of the glucose is reabsorbed at the proximal convoluted tubule. For many of the vitamins, essentially 100% will be reabsorbed at the proximal convoluted tubule. So in contrast, secretion is movement from the blood into the filtrate. Secretion is a mechanism to remove waste materials from the blood to get them out of the body. And so here we can see the arrow that's pointing into the nephron representing secretion at the proximal convoluted tubule. Secretion removes the nitrogenous waste from the breakdown of proteins as polypeptides are broken down by hydrolysis to produce amino acids. And then amino acids are deaminated. The nitrogen from deamination is removed as urea as well as ammonia and uric acid is also a nitrogen containing waste. And so urea, uric acid, and ammonia are all secreted at the proximal convoluted tubule to help remove these waste from the body. Also creatinine and some drugs are secreted as well as hydrogen ions are secreted. Of course, secretion of hydrogen ions is a mechanism to help maintain the pH balance of the body. If the blood pH is too low, hydrogen ions can be secreted in order to help elevate the blood pH as a long-term pH balance mechanism. So now moving into the loop of Henle, at the descending limb of the loop of Henle, we see water reabsorption. Then in the loop of Henle, we also see secretion of urea, the nitrogenous waste. And as we're moving up the ascending limb of the loop of Henle, we'll see reabsorption of sodium chloride as well as potassium. Then now in the distal convoluted tubule, we see there is reabsorption again of water, sodium chloride, and bicarbonate. So the reabsorption that occurs at the distal convoluted tubule is variable. Whereas reabsorption by the proximal convoluted tubule and the loop of Henle is not regulated by the endocrine system, we will see that there is hormonal regulation of the reabsorption of water at the distal convoluted tubule. There's also hormonal regulation of the reabsorption of sodium at the distal convoluted tubule. Then the reabsorption of bicarbonate is coupled to the secretion of hydrogen ions at the distal convoluted tubule. And likewise, the reabsorption of sodium at the distal convoluted tubule is coupled to the secretion of potassium ions. Here at the distal convoluted tubule, we see more secretion occurring, secretion of hydrogen ions, potassium ions, and ammonia. Then as we move through the collecting duct, the collecting duct provides the final opportunity to concentrate the filtrate to produce the final urine volume. So there is more reabsorption that can occur at the collecting duct, and this is also variable. So just like in the distal convoluted tubule, there was variable reabsorption of water in the collecting duct. There's also variable reabsorption of water and urea, and this enables us to regulate the volume and concentration of the urine produced. Here we can see the mechanisms that are driving reabsorption and secretion at the proximal convoluted tubule. The driving force for most of the solute transport at the proximal convoluted tubule will involve primary active transport of sodium. The sodium potassium pump that uses energy released by hydrolysis VATP to fuel the active transport of sodium out of the proximal convoluted tubule cell into the interstitial space across the basal lateral membrane creates a low concentration of sodium inside of the proximal convoluted tubule cell that enables sodium to then move across the apical surface down its concentration gradient. And as sodium moves down its concentration gradient, this will fuel the secondary active transport of other solutes across the apical surface of the proximal convoluted tubule epithelial cells. So here we can see the sodium dependent chloride transport, sodium dependent calcium transport, sodium dependent amino acid transport, sodium dependent glucose transport, sodium dependent phosphate transport. So all of those are secondary active transport mechanisms that are driving reabsorption. So I'll change the color to green because we've been using green to represent reabsorption. All these substances are being reabsorbed and it's a secondary active transport mechanism that drives reabsorption of chloride, calcium, amino acids, glucose and phosphate for the transport across the apical surface of the proximal convoluted tubule epithelial cell. In order to exit the basal lateral surface of the proximal convoluted tubule epithelial cell, these substances will move through a facilitated diffusion mechanism. So there are channels and transporter proteins on the basal lateral surface that enable facilitated diffusion. Similarly, water is moving by facilitated diffusion through aquaporin proteins, proteins that provide water channels for water to move by osmosis. The high colloidal osmotic pressure of the blood creates a osmotic driving force for water to move into the blood and as we're actively transporting solutes from the filtrate into the interstitial space, this will create an osmotic driving force to add to that osmotic driving force of the blood colloidal osmotic pressure in order to continue driving water reabsorption. So a large amount of water reabsorption occurs at the proximal convoluted tubule. The majority of water that was filtered gets reabsorbed and that's being reabsorbed by facilitated diffusion through aquaporin proteins. There's also secretions shown in this illustration and that's the secretion of hydrogen ions. And so the sodium proton exchange pump as well as the transporters for calcium, amino acids, glucose, and phosphate are all providing a secondary active transport mechanism to move hydrogen ions into the filtrate. So hydrogen ions are secreted at the proximal convoluted tubule while water and numerous other valuable nutrients are reabsorbed. Here we can see the mechanism for the reabsorption of bicarbonate ions at the proximal convoluted tubule which is coupled to the secretion of hydrogen ions. Bicarbonate ions that are filtered into the filtrate can react with hydrogen ions to form carbonic acid. Then the enzyme carbonic anhydrase converts carbonic acid into water and carbon dioxide. Carbon dioxide then moves into the proximal convoluted tubule cell by simple diffusion. Within the proximal convoluted tubule cell, carbon dioxide and water are converted back to carbonic acid by the enzyme carbonic anhydrase. Carbonic acid then dissociates from a hydrogen ion to be converted to bicarbonate. The resulting hydrogen ion can be actively transported back into the filtrate whereas the bicarbonate ion can be actively transported across the basal lateral surface to enable reabsorption into the bloodstream. The nitrogenous waste are nitrogen containing molecules that are produced as we break down proteins and nucleic acids. When proteins are broken down, the hydrolysis of polypeptides produces amino acids. Then amino acids can be deaminated to enable the use of the resulting carbon skeleton for an energy source. But the nitrogen then is released from amino acids in the form of ammonia. Ammonia is one of the nitrogenous waste produced in the deamination of amino acids but ammonia is relatively toxic so the liver will convert ammonia to urea and then urea can be transported to the kidneys through the blood. Then urea will be secreted in order to remove that nitrogenous waste from the body. So uric acid is also a nitrogen containing waste product and uric acid is primarily a waste product from the breakdown of nucleic acids. Here we can see the mechanisms of reabsorption and secretion in the loop of Henle. In the descending limb of the loop of Henle, water is reabsorbed while urea is secreted. Then in the ascending limb of the loop of Henle, sodium chloride and potassium are reabsorbed. The ascending limb of the loop of Henle is impermeable to water and so water reabsorption does not occur in the ascending limb of the loop of Henle. As sodium chloride reabsorption by active transport forces sodium chloride out of the filtrate, the concentration of solute in the filtrate within the ascending limb of the loop of Henle becomes more and more dilute. Meanwhile, the solute concentration of the extracellular fluid in the medulla of the kidney becomes more and more concentrated. The descending limb of the loop of Henle is permeable to water and so water moves out through the descending limb by osmosis and urea moves in across the descending limb by diffusion. The concentration of solute is deep, the concentration of solute is greater as we move deeper into the medulla of the kidney. And this high concentration of solute is a factor that will enable the concentration of urine at the collecting duct. A countercurrent mechanism in the loop of Henle helps to keep the concentration of solute very high deep in the medulla of the kidney. The countercurrent multiplier system results from the structure of the loop of Henle where the ascending and descending limbs are in close proximity to one another and filtrate within is moving in opposite directions. Then the vasorecta, the blood vessels, the capillaries that are surrounding the loop of Henle contain blood that's also flowing in the opposite direction to the filtrate. In this way, as sodium chloride is actively transported out of the filtrate in the ascending limb of the loop of Henle, that sodium chloride can be taken up by blood in the vasorecta as that blood is moving deeper into the medulla of the kidney. This will help to concentrate sodium chloride deep within the kidney. Then as water reabsorption occurs in the descending limb of the loop of Henle, blood of the vasorecta is moving up towards the cortex and will be able to carry that water away out of the kidney preventing dilution of the solutes deep within the medulla. While most of the water that entered the filtrate has been reabsorbed by the time we arrive at the ascending limb of the loop of Henle, the concentration of solutes in the filtrate at the ascending limb of the loop of Henle becomes very dilute. The filtrate exiting the ascending limb of the loop of Henle into the distal convoluted tubule is around 100 milliosmol per kilogram, which is about one third of the osmolarity of the extracellular fluid in the cortex of the kidney or the osmolarity of the blood plasma, which is around 300 milliosmol per kilogram. This low solute concentration of filtrate in the distal convoluted tubule enables an osmotic driving force for water reabsorption. However, the reabsorption of water at the distal convoluted tubule and collecting duct is variable. We will see that hormones regulate reabsorption at the distal convoluted tubule and collecting duct. However, having an osmotic driving force enabling water reabsorption makes it possible to create a very small volume of concentrated urine if we're dehydrated. And in contrast, if we're over hydrated, if we've been drinking a large amount of water, we can produce urine that's very dilute in this way regulating the permeability of the distal convoluted tubule and collecting duct enables us to regulate the volume and concentration of urine as a mechanism to help maintain water balance of the blood plasma.