InterviewSolution
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InterviewSolution
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Question : Describe the physiology or mechnismof urine formation. |
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Answer» Solution :Physiology of urineformation /Mechnism of urine formation : Urineformationtakes place in three stages. These areultrafiltration , selective reabsorption and tubularsecretion. (I) Ultrafiltration : (1) This is a pysical process taking place in the Malpighian body. The diameter of afferentrenal arterioleis largerthan thediameterof efferentrenal arteriole and hencethere is hydrostaticpressuredevelopedon the contents inside the blood vessels in the GLOMERULUS. This pressure is alled effectivefiltration pressure or EFP. (2) `99%` of the filtrate is reabsorbed and only 1 to 1.5 litre of urine is formed per day . (3) Outer parietal layer and inner visceral layer of Bowman'scapsule enclose aspace called urinaryspace. The innr visceral layer contains podocytes which are specialized CELLS. (4) Due to semiprmeable glomerular endothelial MEMBRANE, low molecular weight substances are pulled out of the blood and enter into urinary space. Thisfiltrate is blood plasma exceptproteinsand blood corpuscles. (II) Selective reabsorption. (1) In this process 99% of the filtrate is reabsorbed. (2) There aretwo processes involvedin this step :Passive transport for osmosisand active transport. Active transport uses ATP molecules since it is against the concentratioin gradient. (3)When the filtrate passes through the renal TUBULE there is exchange of substances betweenthe surroundingblood and renal tubular fluid. (4) Highthreshold substances e.g.,glucose, amino acids, potassium and calciumions, etc. are completely reabsorbed. Urea anduric acid are low threshold substances. They are reabsorbednegligibly. (5) Waterus reabsorbed everywhere except in the ascendingloopof Henle. This absorption occurs by osmosisand is known as obligatoryabsorptionof water. (6) Amino acids and ions are pumped out from PCT. Glucoseis maximally reabsorbedin healthy persons sothat no glucosemolecules remainin the urine. (III) Tubular secretion : (1) Someabnormal and unwanted substanceswhich have passed into peritubular capillaries are secreted back in DCT,duringthis process. (2) Nitrogenous substances like creatinine and ions like potassium and hydrogenare also takenbackin the lumenof uriniferous tubule through for peritubular capillaries. (3) Secretion of hydrogen ions is in important homeostatic mechanism to keep the balance of the acidityof the blood. (4) Sodium ion concentrationis maintainedby aldosterone while calcium ion concentration is maintaindby calcitonin and parathor-mone. (5) If any toxicand abnormalconstituent is present in the body it is secreted back into urine, in this stage. 
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