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L31 Renal turnover 2011

L31 Renal turnover 2011 - COPYRIGHT Prof Beyenbach...

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- 1/ 8 - COPYRIGHT Prof. Beyenbach Mammalian Physiology BIOAP 4580 Spring 2011 RENAL TURNOVER OF THE EXTRACELLULAR FLUID COMPARTMENT A. Functional résumé: the kidney, the organ of extracellular fluid homeostasis. Maintaining the volume and composition of the extracellular fluid compartment is the primary concern of the kidneys; the formation of urine is only the consequence of this regulation. What is regulated to remain „normal‟ is the extracellular fluid compartment, not the urine. Accordingly, the volume and composition of the urine can vary widely. However, it is considered abnormal if the urine contains protein and/or glucose. The presence of these two solutes in the urine signals problems that must be followed up. 1. Extracellular fluid homeostasis: the selective excretion or conservation of: a) Extracellular ions K + , Na + , Cl - , etc., glucose, amino acids, urea, etc., all of which are kept essentially at constant plasma concentrations over the lifetime of the animal, b) H 2 O; a change by only 1 mOsm (from 295 to 296 mOsm) is sufficient stimulus for the release of the antidiuretic hormone and the initiation of antidiuresis, c) Nitrogenous wastes of protein metabolism (urea, uric acid, creatinine, ammonia), d) H + ; pH is regulated within narrow limits, e) Removal of natural (endogenous) organic acids and bases and foreign organic compounds (including drugs) from the circulation. 2. Endocrine functions: a) Synthesis of erythropoeitin (EPO) by the renal erythropoeitic factor (REF) which stimulates hematopoiesis, b) Renin (regulation of extracellular fluid volume and hence blood pressure), c) Synthesis of the active form of vitamin D: 1, 25 dihydroxycholecalciferol. B. Renal regulation of the extracellular fluid compartment: Matching output to input. 1. Regulation of ion balance in the extracellular fluid a) Intake of ions Na + , K + , Cl - , H + , HCO 3 - , Mg 2+ , Ca 2+ , PO4 2- etc. The intake is unpredictable and at the whim of the diet and social habits, b) Output of ions. The output is also unpredictable since the loss of salts (ions) in feces, sweat, lactation from the extracellular fluid compartment serves other physiological interests, c) Constant extracellular fluid concentrations are maintained through the precise regulation of conservation/excretion via the kidney. The regulation is mostly under endocrine control involving the hormones aldosterone, calcitonin, parathyroid hormone, 1,25-dihydroxycholecalciferol, the renin angiotensin system, and natriuretic peptides. There is also some neural innervation/regulation of the kidney. However, kidneys function quite well without neural input, which is one of the reasons why they were the first organ to be successfully transplanted. 2. Regulation of water balance in the extracellular fluid
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- 2/ 8 - a) Intake of water. The intake of water is unpredictable and influenced by thirst, habit and the generation of metabolic water, b) Output of water. The output of water is unpredictable as i. insensible water loss; evaporation, breathing, ii. sensible water loss; sweat, lactation, c)
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