 This video will cover the following objective from the urinary system. Describe the structure and function of a nephron and collecting doubt. The nephron is the structural and functional unit of the kidney that's responsible for filtering the blood and producing urine. The most proximal region of the nephron where filtration occurs is known as the renal corpuscle. The renal corpuscle includes the glomerular capillary bed where filtration is occurring. During filtration, liquid is forced out of the glomerular capillary and into the nephron. The part of the nephron that surrounds the glomerulus and forms the other portion of the renal corpuscle is called Bowman's capsule. So liquid will flow into Bowman's capsule from the blood and that liquid is called filtrate. The filtrate then flows into the proximal convoluted tubule. Then from the proximal convoluted tubule liquid will flow deep into the medulla down the descending limb of the loop of Henle. And then there's a hairpin turn in the loop of Henle before the filtrate will flow. Back towards the cortex in the ascending limb of the loop of Henle. Then the liquid from the ascending limb of the loop of Henle flows into the distal convoluted tubule. And now the distal convoluted tubules of all of the nephrons drain into collecting ducts. So there will be several nephrons draining into each collecting duct. And then the collecting duct provides the final step of processing the filtrate to produce urine. So when the fluid flows out of the collecting duct through the renal papilla, that liquid is the finished urine. The renal corpuscle is Bowman's capsule and the glomerulus. The illustration on the left here shows us the renal corpuscle and the adjacent structures of the proximal and distal convoluted tubule as well as the afferent and efferent arterioles. So filtration occurs at the renal corpuscle when liquid is forced out of the blood across the glomerulus into the lumen of Bowman's capsule. Bowman's capsule has two layers, an outer parietal layer consisting of a simple squamous epithelium and an inner visceral layer consisting of special epithelial cells called podocytes that have extensions called pedicles that form filtration slits in order to prevent large particles from exiting the blood and flowing into the lumen of Bowman's capsule. So the liquid that enters into the lumen of Bowman's capsule will flow into the proximal convoluted tubule, then down the descending limb of the loop of Henley, up the ascending limb of the loop of Henley, and then into the distal convoluted tubule. But the distal convoluted tubule folds back to contact the renal corpuscle at the afferent arterioles where the distal convoluted tubule and the afferent arterioles touch each other. A special structure known as the juxtaglomerular apparatus is located to help regulate the function of the kidneys. The juxtaglomerular apparatus consists of two regions, a part of the distal convoluted tubule called macula densa and a part of the afferent arterioles called the juxtaglomerular cells. So you'll notice that afferent arterioles has a thick smooth muscle layer, and that's important to contract in order to regulate the flow of blood into the glomerulus to regulate the glomerular filtration rate, how fast liquid enters into the nephron. If the afferent arterioles were to relax and dilate, more blood would flow into the glomerulus, and more liquid would be filtered, whereas if the afferent arterioles contracts, this decreases the diameter, this vasoconstriction will then decrease the blood flow into the glomerulus and decrease the glomerular filtration rate. The macula densa is a sensory region of the distal convoluted tubule that can detect the flow rate of liquid through the distal convoluted tubule and also monitor the sodium concentration of the liquid flowing through the distal convoluted tubule. In response to a high flow rate, the macula densa will stimulate the afferent arterioles to contract in order to decrease the glomerular filtration rate as a tubuloglomerular feedback mechanism helping to maintain a stable flow rate through the nephron. This illustration shows us the structure of the podocytes that have numerous small extensions called pedicoles that come together to form small filtration slits in between the pedicoles. The filtration slits allow liquid with small dissolved solutes to flow into Bowman's capsule, and larger particles like the proteins in the plasma and the formed elements of blood are not able to pass through the filtration slits and are held back inside of the blood. The glomerulus is a fenestrated capillary that has numerous small pores called fenestrations where liquid is forced out of the blood through the fenestrations and then that liquid will flow into Bowman's capsule. After filtration, the liquid known as filtrate is modified as it's flowing through the nephron and collecting duct in order to produce urine. The process of reabsorption is the transport of substances from the filtrate back into the blood and the opposite secretion is the transport of substances from the blood into the filtrate to be removed from the body in the urine. The proximal convoluted tubule is responsible for reabsorbing the majority of the liquid that is filtered. So after filtration, immediately the proximal convoluted tubule reabsorbs the majority of the water and essentially all of the glucose and amino acids and other vitamins and essential nutrients that we want to return to the blood. Also a large amount of the sodium is reabsorbed along with a large amount of the water. However some water will continue to flow from the proximal convoluted tubule down into the descending limb of the loop of Henlein where some water reabsorption will occur before the liquid flows up through the ascending limb of the loop of Henlein where a large amount of sodium chloride and potassium reabsorption will occur. And then finally as we move through the distal convoluted tubule and collecting duct is the final step of reabsorption that regulates the composition of the urine. The distal convoluted tubule and collecting duct are reabsorbing sodium chloride and water in variable amounts in response to hormonal controls. So while the proximal convoluted tubule is constantly reabsorbing large amounts the distal convoluted tubule and the collecting duct can reabsorb large amounts of water if we're dehydrated in response to the hormone antidiuretic hormone or they can reabsorb large amounts of sodium in response to the hormone aldosterone if there's a low sodium level in the blood or a low blood pressure and then that reabsorption of sodium will also drive the reabsorption of water by osmosis leading to increased blood volume and increased blood pressure. So several substances are secreted into the filtrate to be excreted from the body in the urine so the nitrogenous waste like urea and ammonia that come from the breakdown of protein are secreted into the filtrate or they're secreted into the filtrate and then excreted from the body in the urine.