Introduction:

The kidneys which are the main organ in the urinary system are very important to the body. Many people in the world suffer from diabetes, which involve the kidneys. Everybody has to urinate many times throughout the day, and the process is more complicated then people think. Without the proper function of the kidneys our bodies would not function. The main function of the kidneys is to regulate the extracellular fluid throughout the body. Blood pressure regulation through blood plasma, pH of plasma, electrolyte concentration, and the concentration of waste products in the plasma are also functions that are done by the kidneys. These functions are performed at a microscopic level, but play huge roles in the body.


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Concept #1: Structure of the kidney

There are two kidneys located in the abdominal cavity of the body. Attatched to each kidney are ureters which lead to the urinary bladder that holds the urine. Inside each kidney are nephrons, which are the functional unit of the kidney. There are millions of nephrons in each kidney, and they are responsible for the production of urine. Inside each nephron are tubules and blood vessels. Blood enters the kidneys through the renal arteries and the into the interlobar arteries. Interlobar arteries branch into arcuate arteries at the outside of the cortex and the medulla. Afferent arterioles are extremely small and they deliver blood to the glomeruli. The glomeruli is a network of capillaries that produce a blood filtrate which then enters the urinary tubules. There will be left over blood left in the glomerulus and the blood leaves through the efferent arteriole. Each efferent arteriole sends the blood to the peritubular capillaries which surround each renal tubule. After each peritubular capillary the blood then travels to the interlobar veins and leave the kidney through the renal vein which delivers the blood back into the heart through the inferior vena cava. All of these arrangements of blood vessels and circulation are very unique to the body. Each nephron tubule contains many parts. The glomerulus is a bundle of capillaries that produces the filtrate from the blood. The glomerular (Bowman's) capsule which wraps around each glomerulus, and the glomerulus together make up the renal corpuscle. Glomeruluar filtrate that enter the glomerular capsule then passes into the proximal convoluted tubule. The walls of the proximal convoluted tubule are composed of cuboidal cells which contain millions of microvilli. The microvilli increase the surface area of the tubule for reabsoption. Each glomerulus, glomerulus capsule, and the convoluted tubule are located in the renal cortex. The nephron loop, or the loop of Henle, receives the fluid from the proximal convoluted tubule. Fluid then travels into the medulla in the descending limb of the loop, and then back into the cortex in the ascending limb of the loop. Once the fluid is back in the cortex it goes into the distal convoluted tubule, which contains less microvilli and is shorter then the proximal convoluted tubule. The distal convoluted tubule ends when it empties into a collecting duct. There are two different categories of nephrons which is determined by their location in the kidneys and the lengths of their loops of Henle. Juxtamedullary nephrons start in the inner one-third of the cortex and have long loops, while the cortical nephrons have shorter loops and are located in the outer two-thirds of the cortex. The production of concentrated urine is an important role for the juxtamedullary nephrons. Fluid that comes from the distal convoluted tubule empties into a collecting duct. After the draining of the fluid by the collecting duct from the cortex to the medulla as the collecting duct passes through a renal pyramid. Urine is the new name of the fluid, and then the urine passes into a minor calyx. Each kidney funnels the urine through each renal pelvis and out of the kidneys into each ureter. The ureter takes the urine into the urinary bladder where it is stored until micturition, or urination.



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Concept #2: Glomerular filtration

The glomeruli are a network of capillaries with large pores that produce the a blood filtrate that enters the urinary tubules. Endothelial cells line each capillary in the glomerulus and have large pores which are called fenestrae. Because there are such large pores larger molecules are capable of entering easier. Not all molecules are passed through into the filtrate. Red blood cells, white blood cells, and platelets are not capable of passing through the pores. Before any of the blood can be filitered in the glomerulus it has to pass through three layers. Filtrate enters the glomerular capsule, becomes modified to leave as urine. The capillary fenestrae is the first filtration barrier, and allows proteins to pass through. Glomerular basement membrane is the second barrier, and most restricts the rate of fluid that flows into the capsule lumen. The third potential barrier is through the inner layer made of cells called podocytes, called the slit diaphragms. Podocytes are shaped like an octopus, with a circular like body and many arm like extensions. Off of each arm are pedicels, or foot processes, and come together like hands in a prayer with fingers interlaced. Each of these interlaced fingers wrap around the glomeruluar capillaries, and there are tiny slits where the filtrate must pass to enter the interior of the glomeruluar filtrate. Plasma dissolved solutes can pass through all three layers of the filtration barriers to enter the glomerulus capsule, plasma proteins can not pass through because of their large size. Filtrate, or ultrafiltrate, is the fluid that enters the glomerular capsule. The capillaries in the glomerulus have a large surface area and are very permeable so therefore produce an extremely large amount of filtrate. Glomerular filtration rate (GFR) is the volume of filtration produced by both kidneys per minute. The total volume of blood averages 5.5L, and there is about 7.5L of blood filtered every hour, so that means that the total amount of blood is filtered every 40 minutes. The dilation and constriction of the afferent arterioles affect the rate of blood flow to the glomerulus and the glomeruluar filtration rate. When the sympathetic nervous system is stimulated is causes constriction on the afferent arterioles which then will preserve blood volume and then direct the blood to the heart and muscles. If the blood pressure starts to fluctuate, the kidneys have an ability to maintain a constant GFR, which is called renal autoregulation. When the blood pressure becomes too low, the afferent arterioles dilate, and when the blood pressure becomes too high, the afferent arterioles constrict.



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Concept #3: Reabsorption of salt and water

Water is reabsorbed after the glomerulus filtrate through osmosis. Because of this transport of water back into the blood, it causes the transport of sodium ions, Na+, and chloride ions ,Cl-,across the tubule wall. The promiximal tubule ends up reabsorbing most of the filtered water and salt, and the rest of the water and salt is reabsorbed in the collecting duct through the stimluation of the anti-diuretic hormone (ADH). Even though the kidneys produce an extremely large amount of glomeruluar ultrafiltrate, the body is only able to urinate a small amount. Since only a small amount can be excreted, but such a large amount is filtered, the rest of the filtrate has to be reabsorbed back into the vascular system. When the filtered molecules are returned from the tubules to the blood, it's called reabsorption. Everyone knows that humans need to drink alot of water everyday. Wastes are excreted in the urine, and the minimum amount of urine needed to excrete the metabolic wastes that are produced by the body is called the obligatory water loss. The transportation of molecules back into the blood happens by osmosis, and thus a concentration gradient must be in place in order for this to happen. The concentration of sodium ions that are in the glomerular ultrafiltrate is the same concentration as in the plasma. Epithelial cells compose the proximal tubule and in the cytoplasma of those cells contains a much lower concentration of sodium ions. Sodium/potassium pumps, Na+/K+, are located in the basal and lateral sides of the plasma membrane of the proximal tubule. There is no Na+/K+ pump located in the apical membrane of the proximal tubule, and because of these transport pumps creates a concentration gradient. The concentration gradient is in the benefit for Na+ from the tubular fluid across the apical plasma membranes and then into the epithelial cells of the proximal tubule. Na+ are then placed into the surrounding tissue fluid by the Na+/K+ pumps. When the Na+ move across these membranes it causes the formation of an electrical gradient, which then favors the passive transport of Chloride ions, Cl-. Chloride ions passively follow the Na+ into the interstial fluid, which causes the accumulation of NaCl. The cells of the proximal tubule are very permeable to water, so water moves by osmosis from the tubular fluid into the epithelial cells and then into the interstial fluid. Salt and water are reabsorbed from the tubular fluid can be passively transported into the surrounding peritubular capillaries. Once the salt and water are back into the capillaries, they are then recirculated back into the blood.



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Application:

The kidneys play such an important role in our bodies. Without the proper function of the kidneys, we would not be able to survive. I have worked with a few people who have renal failure, and now that I understand what was happening in their bodies, makes sense to me now. The kidneys filter the blood, and because of the their failure in their kidneys, they had to go to the hospital every other day for dialysis. If these women didn't go to receive their dialysis treatments they would die, because there is nothing in their body that is filtering their blood properly. As a nurse understanding how large of an impact the kidneys play on the body is important. In the healthcare field, urine analysis tests are done constantly. Now I understand how the body can show that something is wrong in the urine analysis test. For example if there are proteins or blood in the urine, that is not normal. Even in my personal life, I understand why I would feel thirsty, or why my urine would be pale yellow. It amazes me to think of all the drugs, and toxins that a person ingests, and then the kidneys take a huge hit, because they have to filter to clean out the toxins. Being educated on the physiology of the kidneys, makes me a better nurse someday.



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Essential question:

Filtration and reabsorption are some of the main functions of the kidneys. The filtration of the kidneys happens in the glomerulus which is a large bed of capillaries. The fluid that enters the glomerular capsule is called filtrate. The three barriers that the filtrate must pass through include the capillary fenestrae, the glomeruluar basement membrane, and the slit diaphragms. After the filtrate passes through three barriers, it is then made into urine. Reabsorption is the process in which filtered molecules are transfered from the tubules back into the blood. There is a concentration gradient which is present and causes the movement of the tubular filtrate and plasma in the surrounding capillaries. Because of this gradient, water is able to move back into the vascular system where it originated. The concentrations of Na+ in the glomeruluar ultrafiltrate must be the same as that in the plasma. The Na+ are followed by the Cl- by the Na+/K+ pumps. Salt, NaCl, and water are reabsorbed from the tubular fluid into the peritubular capillaries, and then back into the blood. When there is not enough water in the blood, antidiuretic hormone, ADH, levels rise and then decreases the excretion of water in the urine. The secretion of ADH is stimulated by the osmorecptors in the hypothalamus. An increase in blood osmolality above the normal range causes the ADH to be released. When the plasma becomes more concentrated, increased secretion of ADH promotes the increased permeability of the collecting ducts to water. There are many different types of electrolytes, sodium, potassium, chloride, phosphate, and bicarbonate. Controlling sodium ions are important for the regulation of blood pressure and volume. Potassium ions are important to maintain the proper functions for the skeletal and cardiac muscle. Most of the sodium ions are reabsorbed in the early part of the nephron. If aldosterone levels are absent, most of the filtrate will be reabsorbed through the wall of the tubule into the peritubular blood. There is only a small amount of sodium excreted without aldosterone. Like sodium, most of potassium is reaborbed at the early regions of the nephron. In order for the potassium to be in the urine, it must be secreted later in the nephron regions.



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References from top to bottom:

http://www.trialsightmedia.com/exhibit_store/images/urinary-system.jpg
http://www.youtube.com/watch?v=chhNaLi9P3E
http://www.youtube.com/watch?v=glu0dzK4dbU
http://www.physioweb.org/IMAGES/kidney.jpg
http://www.colorado.edu/intphys/Class/IPHY3430-200/image/19-1j.jpg
http://www2.estrellamountain.edu/faculty/farabee/biobk/glomerulus.gif
http://academic.kellogg.edu/herbrandsonc/bio201_mckinley/f27-6a-b_renal_corpuscl_c.jpg
http://people.eku.edu/ritchisong/554images/proximal_tubule.jpg
http://www.dls.ym.edu.tw/ol_biology2/ultranet/nephron.gif
http://www1.davita.com/UploadedImages/NursesArticle_th.gif
http://www.colorado.edu/intphys/Class/IPHY3430-200/image/19-1.jpg
http://www.youtube.com/watch?v=ytjSE9xEK28&feature=related