PET3323 Fall 2011 Study Guide Exam 4 Body Fluid and Electrolyte Balance 1 Know the main fluid compartments within the body Water occupies two main fluid compartments w I the body total body water volume 40 L 60 body weight Little less than 2 3 by volume is in intracellular fluid ICF compartment consists of trillions of tiny individual compartments the cells in adult male of average size ICF accounts for about 25 L of 40 L of body water 40 body weight Remaining 1 3 of body water is outside cells in the extracellular fluid ECF compartment body s internal environment is the external environment of each cell volume 15 L 20 body weight divisible into 2 subcomponents 1 plasma fluid portion of blood volume 3 L 20 of ECF 2 interstitial fluid IF fluid in the microscopic spaces b tissue cells volume 12 L 80 of ECF other examples of ECF lymph CSF humors of eye synovial fluid serous fluid secretions of GI tract usually considered part of IF 2 Know the major components of body fluid and distinguish between the extracellular and intracellular compartments know what causes fluid movement between the two compartments COMPOSITION OF BODY FLUIDS Nonelectrolytes do no dissociate in solution glucose lipids Electrolytes DO dissociate in solution ions conduct electricity greater ability to change osmolarity than nonelectrolytes increased osmolarity increased solutes examples inorganic salts some proteins Electrolyte dissociation NaCl Na Cl Amounts of electrolytes in ICF ECF are measured in milliequivalents per liter mEq L the of electrical charges in 1 liter of a solution ECF vs ICF ECF Na is primary cation positive charge Cl is primary anion negative charge ICF K is primary cation HPO4 is primary anion The continuous exchange and mixing of body fluids are regulated by osmotic and hydrostatic pressures Although water moves freely between the compartments along osmotic gradients solutes are unequally distributed bc of their size electrical charge or dependence on transport proteins Anything that changes the solute concentration in any compartment leads to net water flows exchange of gases nutrients water wastes b 3 fluid compartments w I body blood plasma interstitial fluid intracellular fluid in tissue cells Many factors can change ECF ICF volumes Bc water moves freely b compartments however the osmolarities of all body fluids are equal Increasing ECF solute content mainly NaCl concentration can be expected to cause osmotic and volume change sin ICF namely a shift of water out of cells Conversely decreasing ECF osmolarity causes water to move into the cells Thus the ICF volume is determined by the ECF solute concentration These concepts underlie all events that control fluid balance in the body Osmolarity measure of solutes in a solution Na Cl enters a solution that is 2 osmolarity 3 Understand what components will regulate water intake e g the thirst mechanism and water output e g obligatory and insensible water losses WATER BALANCE Thirst Mechanism Regulation of Water Intake Increase in plasma osmolarity triggers thirst drink fluids release of antidiuretic hormone ADH causes reabsorption of water into plasma excrete concentrated urine Decrease in plasma osmolarity inhibits thirst inhibits ADH large outputs of dilute urine Regulation of Water Output Obligatory water losses output of certain amounts of water that is unavoidable explains why we cannot survive for long w o drinking includes the insensible water losses water that accompanies undigested food residue in feces a minimum daily sensible water loss of 500 ml in urine obligatory water loss in urine reflects fact that human kidneys but normally flush 600 mmol day of urine solutes out of body in water The body s water volume is closely tides to a powerful water magnet ionic sodium Our ability to maintain water balance thru urinary output is really a problem of sodium and water balance bc the 2 are always regulated in tandem by mechanisms that serve cardiovascular function blood pressure We get water each day from metabolism least food middle liquids most 2500 mL total Excretion of water GI tract lungs kidneys skin 2500 mL Water in water out 4 Where is Anti diuretic Hormone ADH vasopressin produced what is its primary role in fluid balance and what will influence its secretion Osmoreceptors of the hypothalamus sense the ECF solute concentration trigger or inhibit ADH release from the posterior pituitary accordingly see fig 26 6 pg 1000 The amount of water reabsorbed in renal collecting ducts is proportional to ADH release When ADH levels low most water reaching collecting ducts not reabsorbed but simply pass thru bc the lack of aquaporins in luminal membranes of the principal cells prevents movement of water Result dilute urine reduced volume of body fluids When ADH levels high aquaporins are inserted in principal cell luminal membranes nearly all filtered water reabsorbed small volume of concentrated urine is excreted A decrease in ECF osmolality inhibits ADH release and allows more water to be excreted in urine restring normal blood osmolarity In contrast an increase in ECF osmolality stimulates ADH release by stimulating the hypothalamic receptors ADH secretion also influenced by large changes in blood volume or blood pressure Decrease in BP triggers increase in ADH secretion from posterior pituitary both directly via baroreceptors in atria various blood vessels indirectly via the renin angiotensin mechanism Factors that trigger ADH release by reducing blood volume include prolonged fever excessive sweating vomiting or diarrhea severe blood loss traumatic burns under these conditions ADH also acts to constrict arterioles directly increasing blood pressure hence its other name vasopressin Acid Base Balance 5 Know the three primary buffer systems i e chemical respiratory renal and characteristics of each i e when are they recruited how quickly do they respond how quickly do they bring about change etc How is H concentration regulated to maintain a neutral pH 1 Chemical buffers 1st line of defense these act w I a fraction of a second in an attempt to resist changes in pH muscle carnosine a system of one or more compounds that acts to resist changes in pH when a strong acid or base is added compounds bind to H when pH drops release H when pH rises A shift in H concentration in one fluid compartment is compensated by a shift in another compartment ex hemoglobin in RBC s 2 Brain stem respiratory centers act within 1 3 minutes to counteract acidosis
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