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| Formation And Composition Of Urine |
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| Composition |
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| The urine is a pale yellow coloured fluid. The yellow colour is due to the presence of urochrome pigment formed from the haemoglobin of dead RBC's in the liver cells. It is acidic in nature and has a pH of 6.0. It has a faint anamalic odour due to the presence of urinod. It soon gets a strong smell of ammonia which forms as a result of degradation of urea. |
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| Daily urine output is 1.5 to 1.8 lts. The volume of urine output is directly proportional to the fluid intake. |
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| Chemically it is formed of water 95 - 96%, urea 2%, other wastes like uric acid, hip puric acid, createnine, phosphates and oxalates 2-3%. It also contains non-nitrogenous organic compounds like vitamin C, oxalic acid and phenolic substances. |
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| Formation of urine |
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| The formation of urine takes place by 3 processes. |
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| Filtration, reabsorption and secretion at different parts of the nephron |
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| (a) Ultra filtration or glomerular filtration. |
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| (b) Selective reabsorption |
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| (c) Tubular secretion |
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| Ultra filtration |
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| Due to the efferent arterioles being narrower than the afferent arteriole, the blood pressure is twice that in the capillary network. Blood enters the glomerulus at a hydrostatic pressure of 60 mm Hg., to filter out some fine components of blood. The glomerular filteration pressure is about 10 mm Hg and is responsible for filtering a large amount of water, glucose, amino acids, vitamins, Na+ and a number of harmful substances like urea, uric acid, creatinine, ammonium salts. Pigments, K+. The proteins, fats and carbohydrates are not filtered out. |
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| The direction and magnitude of pressures influencing the filtration pressure in the human glomerulus |
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| The glomerular filtrate is same as blood minus cells and proteins. It resembles the protein free plasma in composition and Osmotic pressure. |
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| About 650 ml of blood passes through the glomerular capillaries in all nephrons of both kidneys, but in one minute about 125 ml of filtrate is filtered out. So about 150 - 180 lts of nephric filtrate is formed per day but only 1.5 - 1.8 lts of urine is excreted out per day and this is called urine output. |
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| Glomerular filtration rate is controlled by two intrinsic mechanisms, which provide autoregulation. They are: |
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| 1) Myogenic mechanism |
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| 2) Justaglomerular apparatus. |
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| 1) Myogenic mechanism |
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| Myogenic mechanism helps to control variations of the blood flow to the glomerulus due to fluctuations in blood pressure. An increase in blood pressure makes the wall of the afferent arteriole contract thus reducing the diameter of the arteriole and increasing the resistance to flow. |
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| 2) Juxtaglomerular apparatus |
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| When there is a decrease in blood pressure or blood volume in the afferent arteriole of the glomerulus, the Justaglomerular Apparatus (TGA) that is situated between the afferent and efferent arterioles release an enzyme 'renin' into the blood stream. A plasma protein angiotensinogen is converted to apeptide angiotensin II by renin. |
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| (a) |
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| (b) |
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| Regulationofrenal function by feedback circuits: |
| (a) control by ADH; (b) control by RAAS |
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| Selective reabsorption |
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| As the nephric filtrate moves in the nephron towards the collecting tubule, about 99% of water, whole of glucose and amino acids, most of the Na+ and Cl, some urea and uric acid are reabsorbed back into the blood. This process by which only useful substances are reabsorbed from the nephric filtrate is called selective reabsorption. |
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| Water conservation and production of concentrated urine in renal tubule |
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| The PCT actively absorbs almost whole of glucose, 75% of amino acids and Vitamin C, about 70% Na+, about 75% of K+ and a large amount of Ca++ from the glomerular filtrate. The ascending limb of Henle's loop actively reabsorbs about 25% of K+ and some Cl- ions. DCT and the collecting tubules actively reabsorb some Na+ from nephric filtrate in exchange with K+ of the interstitial fluid. |
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| The main function of reabsorption is to maintain homeostasis so that volume, electrolytic balance composition, pH and osmotic pressure of blood are kept constant. |
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| Tubular secretion |
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| It is a process by which the glandular cells of nephron especially of DCT, extract the wastes like uric acid, creatinine, hippuric acid, K+, H+, ammonia from the blood in the capillaries by active transport and secrete them in the nephric filtrate. Most of K+ eliminated in urine is secreted by DCT and collecting tubules in exchange with reabsorbed Na+. It is the only mode of excretion in marine fishes and desert amphibians whose nephrons are without glomeruli and Bowman's capsules. |
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| As a result of these 3 processes the wastes are changed into urine. |
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