The Roles of the Nephron of the 120 ml of blood that is ... - Mrs Stovel

The Roles of the Nephronof the 120 ml of blood that is filtered by the kidneys each minute, only I ml(that's less than I%) turns into urine that will eventually leave the body(after approximately 300 - 400 mis of it accumulates to fill the bladder!)That leaves 119 ml of fluid called filtrate to be returned back to the bloodstream. Good thing, otherwise you would have to micturate (pee, urinate)once every 3 minutes and drink 1 L of fluid every 10 minutes in order tomaintain Homeostasis!!!The one million nephrons in each human kidney are amazingly efficient atselectively removing wastes from the blood while at the same timeconserving water, salt ions, glucose and other needed materials. Thenephrons accomplish this task in 3 main steps; these 3 steps are also calledthe 3 main roles of the nephron: Filtration, Reabsorption and Secretion.FiftratfflnFiltration isaecomplished by themovement of fluids fromthe blood into theBowman's capsule.BeabsorpttortReatuorptias wolves theselective transfer ofessential solutes andwater back into the blood.SecretionSecretion Involves themovement of wastes fromthe blood into themphron.1. FiltrationThe renal artery carries blood into the kidney (approximately 600 mis ofblood enters a kidney each minute). The renal artery then branches intoarterioles which then branch intoaspecialized capillaries called theglomerulus. Because of the great difference in diameter between the renalartery and the glomerul us, blood entering the glomerulus is under very highpressure. This pressure forces about 20% of the blood plasma (about 120m1of the 600 ml) out of the glomerulus and across the membrane of Bowman's

capsule. Bowman's capsule acts to "filter" or separate some of thesubstances that are located in blood plasma from others. This is becausesome substances are small enough to fit through the pores of the membraneof Bowman's capsule and some are too large and thus do not enterBowman's capsule with the rest of the blood plasma. Water, salt ions(sodium, potassium and choride), glucose molecules, amino acids and ureamolecules are all small enough to go through the membrane pores intoBowman's capsule. Blood cells (rbc, wbc and platelets) and proteins on theother hand are too large to leave the capillaries or enter Bowman's capsule.The fluid inside of Bowman's capsule gets a name change; it is now called"filtrate" because it is blood plasma that has been filtered, This filtrate isidentical to blood plasma minus the blood cells and proteins. Filtrate is saidto be isotonic to blood plasma with respect to its concentration of water, saltions, glucose, amino acids and urea. The filtrate will then proceed fromBowman's capsule through the rest of the nephron in the following order:proximal convoluted tubule, loop of henle, distal convoluted tubule andfinally the collecting tubule. From the collecting tubule, the filtrate will enterthe pelvis of the kidney and be called urine.2. ReabsorptionUseful materials such as sugars and salt ions are reabsorbed back into theblood stream. That is, materials that could still be used by the body are sentback to the blood. Reabsorption happens as filtrate passes sequentiallythrough the nephron. Materials re-entering the blood stream do so throughthe capillary network surrounding the nephrons. In short, "good" stuff is sentfrom the nephron back into the blood.A. Proximal Convoluted Tubule (PCn:As the filtrate enters the PCT approximately 80% of the salt ions (sodiumand potassium), glucose and amino acids are ACTIVELYTRANSPORTED out of the PCT and back into the blood stream byspecial "pumping" cells located in the walls of the PCT. The process ofactive transport requires energy. Energy in the form of ATP is suppliedby the numerous mitochondria that are embedded in the walls of thePCT. Because of ionic attraction, negatively charged chloride ions (CI-)will flow passively out of the PCT as they are attracted by the positivelycharged sodium and potassium ions (Na+, K+). As the concentration ofthe above mentioned solute molecules drops inside of the PCT, water

then diffuses out of the PCT and into the capillary network passively bythe process of OSMOSIS. The lining of the PCT contains microvilli toincrease the surface area over which this reabsorption can occur.B. Descending Loop of Henle:As the filtrate travels into the descending Loop of Henle, both sodiumand potassium ions passively diffuse from the salty tissues of thesurrounding medulla BACK INTO the Loop of Henle. (Although this isreabsorption of materials, the materials are not going back into the bloodstream at this point). At the same time, water continues to move out ofthe Loop of Henle and into the capillary network by osmosis. The filtrateat this point is more concentrated (hypertonic) with respect to salt ionsthan it was in the PCT, both because water has been removed from it, andbecause salt ions have been again added to it.C. AscendineLoop of HenleAs the filtrate proceeds up into the ascending Loop of Henle, the chorideions are actively pumped back out of the nephron. Because of ionicattraction, sodium ions then passively follow the chloride ions out of thetubule and into the tissues of the medulla. These ions only move into themedulla and not back into the blood stream. Since the ascending Loop ofHenle is impermeable to water, water cannot leave this part of thenephron. Because of this, the filtrate gets more dilute again.Because the opposite happens in the ascending and descending Loops ofHenle, the process is called the COUNTER CURRENTMECHANISM. The process is also known as the CHLORIDE SfUFT.D. Distal Convoluted Tubule (DCT)As the filtrate passes through this part of the nephron, water continues topassively diffuse out of the nephron and back into the blood. Watercontinues to diffuse out of the DCT because the surrounding tissues ofthe medulla are now very salty due to so much sodium and chloride ionsaccumulating there. The salty tissues attract the water out of the DCTbecause the medulla is hypertonic with respect to salt concentration whencompared with the salt concentration of the filtrate (now hypotonic).Water that enters the medulla will then diffuse back into the blood

stream. Because of the continual re-absorption of water, the filtratebecomes more and more concentrated with wastes, mainly urea. Theamount of water that diffuses can be regulated by a hormone calledADH. The amount of water that diffuses from the DCT back into theblood depends on the needs of the body; if the body is dehydrated, morewater will go back into the blood, and less will be left in the nephron tomake less urine. The opposite occurs if the body is over hydrated.E. Collecting Tubule CThe same thing that occurs in the DCT also occurs in the CT3. SecretionOccurring at the same time as reabsorption is a process called secretion.Secretion is when a cell releases a substance to its outside... in this case,non-useful and toxic substances are ACTIVELY TRANSPORTED from theblood into the nephron - usually in the regions of the distal and proximalconvoluted tubules. Substances which are secreted include excess acid (H+)or base (OH-) ions, excess glucose (high glucose levels are found in diabeticurine or urine of someone who has recently consumed a large amount ofsugar-this is the kidney's way of helping to ensure that the blood sugarlevel doesn't get too high), ammonia, and drugs (this is why urine is usedfrom many drug tests - the breakdown of many drugs including marijuana,cocaine, heroin, sleeping pills, codeine and many other medications can bedetected even in minute amounts in the urine). The process of secretionensures that materials that are potentially harmful to the body are quicklydisposed of by being "dumped" into the fluid that is about to become urine.Secretion happens mainly in the regions of the DCT and CT but some alsooccurs in the PCT. In short, secretion involves "bad" stuff being removedfrom the blood being sent to the urine.

The Counter Current Mechanism of theNephronThe loop of Henle works toward thegoal of water conservation. Animals that livein a terrestrial environment need to be carefulnot to waste water. It is clearly a waste,if wateris in short supply, to release too much waterwith the urine. As a result there needs to be amechanism to encourage water out of the urineand back into the blood. The loop of Henlecreates that mechanism in terrestrialanimals.There is no way of actively capturingwater in the urine that is passing through thecollecting ducts. It would almost seem too lateto capture the water that is already on its wayout of the body. However, the nifty nephroncreates a trick with its loop of lienle to get thewater out of the collecting duct before it leavesthe kidney, It does so by creating a saltyenvironment in the medulla area of thekidney.The ascending loop of Henle activelytransports chlorine ions out of the filtrate withcarrier proteins. Chlorine builds up in thefluids of the medulla by active transport.Because it is a negative ion, it creates a causefor the sodium ion, which is positive, to rush outof the loop after chlorine. The sodium rushesout by diffusion because of its 'fatal attraction'to chlorine. The chlorine and the sodium ionscollect and dominate the fluids outside the loopof I-fenle creating a salty environment. Thissalty environment catches the attention of thewater that is passing through the nearbycollecting duct. The collecting duct ispermeable to water but not permeable to thesalt. Water can. not resist moving into thesalty medulla. The salt creates an osmoticpressure that pulls the water out of the collectingduct by osmosis. (Water has a 'fatal attraction'to salty solutions.) Once the water is out of theduct it is no longer destined for elimination butcan' now be picked up by the nearby. ,Moodcapillaries and returned to be used by;.bodysystems.Meanwhile, back at the loop of`-•Henle,trouble is starting. The ascending loop isrunning out of salt. There is no need to worry.The salt trick can continue because thedescending loop in its wisdom is stealing backthe salt that the ascending loop is so generouslyreleasing. This helps to keep a constant flow ofsalt inside the loop for the ascending lope topump out . Because of the generosity gftheascending loop and the stinginess of thedescending loop a salt trade or salt current isestablished as the salt moves ouc of theascen ding loop and into the descending loop,This salt current established by the loop oflienle maintains an environment that attractswater out of the ducts containing urine andback into the blood. This process is called thecounter current mechanism.

Cl (ER 25 EXCRETION AND WE BALANCING OF WATER ANTubularruti"n i secretionNctwp-i►Urea _,Al. How Materials Move Into and Out of the Nephron As Urine Forms. As anephron extends through the kidney's cortex and medulla and dumps urine into the collecting duct,various substances enter and leave the filtrate. Broken lines represent segments of the nephron wallthat are permeable to water, while solid lines represent wall segments impermeable to water. Narrowarrows represent passive diffusion of materials into or out of the nephron tubule, while wide arrowsrepresent active transport against concentration gradients. Filtration activities are shown in blue,tubu.l 'eabsorption activities in green, and tubular secretion in yellow. U ne is shown as yellow.The t traces nephron function and material movements step by step.

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Use your notes to complete the following Chart :Kegzon of the Material(s) Keabsorbed Method used to transportNephron (Back into the blood) the material (osmosis,Diffusion, etc....)Proximal ConvolutedTubuleDescending LoopOf HenleAscending LoopOf Henle.Distal ConvolutedTubuleCollecting Tubule

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How are PLASMA and FILTRATE alike?.• Both are clear, slightly yellow liquids•:• Both can be found in the kidney•:• Both contain water, salts, nutrients (ex. sugars)and nitrogenous (N) wastesv Plasma that is forced from the glomerulus through bowman'scapsule becomes filtratev Filtrate used to be plasmaNephron contains filtrateBloodvesselscontainplasmaHow are PLASMA and FILTRATE different?PLASMAv Found in blood vesselsv Not found in nephrons.• Contains blood cellsv Contains large proteinsv Has a lower concentration ofN. wastes (such as urea)-:• Usually has more salts &nutrientsFILTRATE- Found in nephrons- Not found in blood vesselsDoes not contain blood cellsDoes not contain large proteins- Has a higher concentration of N.waste (except in bowman's capsule)- Usually has less salts & nutrients

How are FILTRATE and URINE alike?v Both are clear, liquidsBoth have a yellow color-.• Both can be found in the kidney- Both contain water, salts and nitrogenous (N) wastesv Both pertain to the excretion of N. wastesOnce filtrate drips out of the collecting tubule of the nephronand into the renal pelvis, it is now called urine-;• Urine used to be filtrateHow are FILTRATE and URINE different?FILTRATEURINEvvv-:•Found in nephronsNot found in urinary pelvis,ureters & bladderLower concentration [ of wasteLighter yellow colorAlways contains useful substancesin thePCT(ex. Nutrients suchas sugars)- Not found in nephrons- Found in urinary pelvis,ureters & bladder- Higher [ of waste- Darker yellow color- Only contains usefulsubstances if they are inexcess in the blood