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Unformatted text preview: Chapter 5 Regulation of Body Fluid Osmolality: Regulation of Water Balance Introduction Water constitutes ~ what % of a healthy adult human body? [ ~ 60% ] What are the two components of body water? [ ICF and ECF ] Are these compartments in equilibrium? [ Yes ] Water intake into the body by two methods: - Orally (normal) - By IV (clinical) Water enters ECF ICF Excretion of H 2 0 or Water Loss: 1. Renal Which organ is responsible for: -regulating water balance & -are major routes for elimination of water? . . . Kidneys (for both) 2. Other Water loss from the body by other routes: 1. Insensible water loss Evaporation from skin cells & lungs 2. Sweat production in hot environment dramatically Exercise Fever 3. G.I. Tract Fecal water loss is small Increase with diarrhea Vomiting -Water loss by 1, 2 & 3 cannot be regulated. Renal excretion of water is tightly regulated. Maintenance of water balance depends on precise matching of: - water intake - water loss If intake > losses : Positive water balance If intake < losses: Negative water balance Body fluid osmolality is maintained at a value of ~ 300 mOsm/Kg H 2 O When urine osmolality is equal to blood osmolality, it is called isosmotic urine. When urine osmolality is higher than blood osmolality, it is called hyperosmotic urine. When urine osmolality is lower than blood osmolality, it is called hyposmotic urine. What would prompt kidneys to conserve water and produce a small volume of urine that is hyperosmotic with respect to plasma? - water intake - water losses Produce large volume of hyposmotic urine? - water intake Urine osmolality varies ~ 50 1200 mOsm/Kg H 2 O- corresponding urine volume can vary - ~ 18 L/day to 0.5 L/day Kidneys control water excretion independently of the excretion of other substances (e.g., Na + , K + , H + , urea). - Necessary for survival Regulation of Body Fluid Osmolality Bodys Response to Water Deprivation Person lost in the desert with no source of drinking water 1. Water continuously lost from the body in - sweat and water vapor (insensible loss) - plasma osmolality 2. Osmoreceptors in anterior hypothalamus stimulated - very sensitive - stimulated by in osmolality of < 1mOsm/ Kg H 2 O 3. Receptor stimulation has two effects: - stimulates thirst * drives person to drink H 2 O - secretion of ADH from posterior pituitary gland 4. ADH blood kidneys - water permeability of late distal tubule and collecting duct (DC) 5. water permeability water reabsorption in late distal tubule & CD urine osmolality urine volume 6. H 2 O reabsorption H 2 O returned to body fluids - thirst and drinking behavior plasma osmolality back to normal - elegant example of negative feedback _____________________________________________________________________ * When body fluid osmolality is increased or blood volume or pressure is reduced, the...
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- Winter '11