[Music] hi and welcome back to free size lessons by the end of this video you should be able to describe how the loop of Henley and the collecting duct work together to produce concentrated urine and if you're following the Ed XL spec you should be able to describe how the kidney of the kangaroo rat is adapted for life in a dry environment okay I'm showing you here a diagram of the nefron and remember that humans have around 1 .5 million nephrons in each kidney remember that ultra filtration takes place in the glus and Bowman's capsule and reabsorption takes place in the proximal convoluted tubule the fluid in the tubule now makes its way through the loop of Henley the distal convoluted tubule and the collecting duct and once it passes out of the collecting duct we now refer to the fluid as urine now there is a problem here at the end of the proximal conv tubule the fluid in the tubule has the same water potential as the blood in other words the fluid contains a relatively High concentration of water now if this fluid was released as urine then we would produce a very large volume of very dilute urine and that would be lethal so the loop of Henley and the collecting duct work together to reabsorb water from the fluid before it forms urine and this allows us to produce a relatively small small volume of concentrated urine now in the next video we look at how humans control the volume of urine that we produce that process is called osmo regulation and it involves the hormone ADH but in this video we're looking at how the loop of Henley and the collecting duct function I'm showing you here the loop of Henley and the collecting duct both of these extend down into the medulla of the kidney now I'm going to start by looking at what happens in very simple terms the job of the loop of Henley is to lower the water potential of the tissue in the medulla as the fluid moves down the collecting duct water now moves from the fluid into the medulla by osmosis this water is then reabsorbed back into the blood and this allows us to produce concentrated urine so you need to remember that the job of the loop of Henley is to lower the water potential in the medulla okay now the loop of Henley has two regions first fluid moves down the descending limb the descending limb is thin walled and is very permeable to water the fluid then turns around the bottom of the loop and makes its way up the ascending limb the ascending limb is thick wall and is impermeable to water in between the limbs we have the interstitial region now in order to explain how the loop of hand works we need to look at the ascending limb first as the fluid moves up the ascending limb sodium ions and chloride ions are pumped out of the fluid by active transport and these ions are transferred into the interstitial region now active transport requires energy in the form of ATP and this ATP is provided by mitochondria in the cells of the ascending limb now the walls of the ascending limb are impermeable to water so water cannot leave the fluid in the ascending limb okay so because of the ascending limb we now have a high concentration of sodium ions and chloride ions in the interstitial space and this means that the medulla has a very low water potential now based purely on what I've just said it looks as though the loop of Handley has done its job we have created a very low water potential in the medulla which allows water to be reabsorbed from the collecting duct by osmosis however you need to bear in mind that active transport requires a great deal of energy so the loop of Henley has an adaptation which reduces the amount of energy needed to understand this we need to look at the fluid moving down the descending limb initially this fluid has the same water potential as the blood in other words a relatively High concentration of water and a relatively low concentration of ions however unlike the ascending limb the walls of the descending limb are very permeable to water so as the descending limb passes down into the medulla with its low water potential water moves out of the fluid and into the medulla by osmosis the water then moves bi osmosis into the blood and is carried away so as the fluid makes its way down the descending limb it loses water and becomes progressively more concentrated so at the bottom of the loop the fluid is now a relatively concentrated solution now this has an impact on the ascending limb when this concentrated fluid starts moving up the ascending limb the concentration is so high that sodium ions and chloride ions can move out of the fluid by diffusion then further up the ascending limb active transport takes over pumping sodium ions and chloride ions out of the fluid as we saw before so because the fluid becomes more concentrated moving down the descending limb it makes the transfer of sodium ions and chloride ions more efficient in the ascending limb and this creates an extremely low water potential in the medulla now scientists refer to this as a countercurrent multiplier countercurrent refers to the fact that the fluid is moving down the descending limb and up the ascending limb in other words in opposite directions and this is a multiplier because the efficiency of ion transfer out of the fluid in the ascending limb is Amplified by the transfer of water out of the fluid in the descending limb now as we saw before the low water potential of the medulla means that water moves out of the fluid by osmosis as it passes down the collecting duct and this allows humans to produce a small volume of concentrated urine I'm showing you here the kangaroo rat which lives in arid areas of North America water is very scarce in this environment so kangaroo rats conserve water by producing a very small volume of urine and the urine produced by kangaroo rats is around 20 times more concentrated than the urine produced by humans now the loops of Henley in kangaroo rats are extremely long this allows kangaroo rats to produce a very low water potential in the medulla of their kidneys and this allows them to reabsorb a very large amount of water now before we finish I'd like to look at what happens in the distal convoluted tubule in the distal convoluted tubule substances can also be reabsorbed from the fluid passing through for example the pH of the blood can be adjusted by absorbing hydrogen ions other ions such as potassium can also be reabsorbed and the distal convoluted tubule also plays a role in water reabsorption in the next video we look at osmo regulation [Music]