---start physio 1.6.97---- Dr Spear. Handout: WATER BALANCE re: the exam....sweeney's section we all did well, spear's section we didn't do so well, hopefully we'll get them back next week. note: next series of lecs: temperature, energy balance, etc....all utilizing same "balance" scheme. it's a matter of BALANCE. you have a system with an input and an output. if you're in balance, then input == output, and amount in reserve stays relatively constant. overall water balance scheme: water insensible(lungs,skin) 800-1200 food ------intake--->TOTAL BODY WATER----loss---> ml/day oxidation in urine: 1500 ml/day tissue: 300 ml/day stool 100-200 ml/day sweat up to 1500 mL/hr so sweating can really deplete body water. we can modify the concentration of urine via kidney mechanisms, and our intake varies via thirst mechanism, so we try to stay in balance. first of all, if we look at total body water (% of body wt), vs age in years, you can see that a newborn is about 80% water. as we age, that percent drop off. as we approach puberty, it decreases rather steeply til about 60%, and then drifts VERY slowly down to about 55% over the course of the lifetime. note that for women, water is a smaller % of total body wt - because women have sl. more total body fat. you can't dissolve water in lipid, so water goes down proportionately. but its only a 2-3% difference. this is also true between individuals...fat people have a lower percent of body water, see handout 2nd page bottom row of charts. as %body fat goes up, %body water goes down. also as body fat increases, specific gravity decreases (fat floats on water) now. we're not water balloons. water in the body is compartmentalized. major compartments: intracellular space: everything within cells, bound by plasma membrane extracellular space;(interstitial(between cells,includes lymph), plasma) transcellular space: a specialized volume. it's actively secreted. like glandular tissue secreting glandular fluid w/in lumen of gland. secretions of secretory glands == transcellular fluid eg: CSF, synovial fluid, pleural fluid, eye secretions, cochlear endolymph, pericardial fluid, digestive fluid, sweat, and any other glandular secretion. now, intracellular and extracellular fluid can exchange directly across plasma membrane. To get from plasma to to intracellular, you have to go through two cells' membranes. And to get to transcellular space, you have to be secreted by the gland cells. so you have some substance in blood, freely diffusing, you still may not see it in transcellular space. but to get into the other spaces is mainly diffusion-dependent. Plasma water == 7.5% body water lymph == 2% interstitial == 18% ( + 2% from lymph) dense CT/cartilage == 7.5% bone == 7.5% intracellular water == 55% --the largest water compartment transcellular water == 2.5% now you can't get from plasma space to intracellular space w/o going through interstitial space. interstitial space exchanges directly with everything except transcellular space, though. the transcellular space exchanges only w/intracellular space since it's actively secreted by glandular cells. body water distribution: if you look at total extracellular % total body water is 45%. If you subtract out the inaccessible bone water and the transcellular water, you get a FUNCTIONAL extracellular water of 35% of total body water. this is the amount of extracellular water you can measure with tracer techniques. Intracellular, then is 55% of total. say you feed patient (or container) one gram of tracer substance. the substance can't be cleared from the patient during the measurement period, and is homogeneously distributed throughout the body. then you take a small sample from patient and determine the concentration of substance, which is quantity over volume. C = Q/V we measure C, we know Q is one gram, so we solve for volume, which tells us how much water is in patient. or container. or whatever. V = Qadministered - Q lost ----------------------- concentration but substance has to be homogeneously distributed. if not, you may get false concentration reading. if substance is metabolized and you can't tell how much is lost, then this doesn't work either. you also want an easily analyzed substance. key to choosing tracer is it must follow these guidelines, and must stay in the compartment you want to measure. so there are different tracers. there are spaces you can't measure, so you have to calculate them. you CAN measure: total body water (by injecting IV tracer that distributes freely - antipyrene, heavy water, tridiated water) plasma water (evan's blue - dye binds to albumen; radioiodinated human alb; tagged RBCs) extracellular fluid: (non-metabolized sugars eg sucrose, inulin; sodium (inaccessible bone and transcellular NOT included, so you'll be measuring FUNCTIONAL extracellular space) often when measuring this ecf, you refer to tracer, eg, "sucrose space" "inulin space" because each tracer has different characteristics. intracellular space: how could you do that? you'd have to get a tracer to each cell in the body - well, you can't get it there w/o first having it get into interstitium/plasma. so you have to calculate the intracellular fluid volume by measuring total body water and extracellular fluid volume and subtract. realize to get to interstitial space you have to inject into plasma space, something that will diffuse throughout interstitial space. realize the substance will also remain in plasma space. so you can't measure that either. take extracellular space - plasma space and you get interstitial space. take a look at bottom of p 4 - see graphs of two ways of measuring functional ecf using inulin. infusion equilibrium method and kinetic method. both rely on C = Q/V formula. in both graphs, you have a plasma inulin concentration in mg/L. in first one, there's "time after start of infusion of inulin" on x axis, and in kinetic method it's "time after bolus" ----break--- with infusion method, you sample at start of infusion, and then hourly as infusion continues such that you can chart an equilibrium level during the infusion. note that inulin is constantly being cleared by the kidneys. but for experimental purposes, you want to provide a long time period of constant level of inulin, so there's 4 or 5 hrs for inulin to distribute through tissues. so you do this, and after 5 hrs,say, you stop the infusion. if the equilibrium concentration was 258 mg/L...then, if you take hourly plasma samples you can see that you're getting decreasing amounts of inulin,but you don't need to take these samples.During this time, inulin is being excreted in urine. patient collects all urine produced for 22 hrs.then you measure total amt inulin that came out in urine: 3328 mg. so, that's the quantity of inulin that produced the concentration of 258 mg/L so....258 mg/L = 3328/volume inulin space = 12.9 L kinetic method is easier. give single injection of known quantity of inulin. take samples q 15 min at first then hourly. you'll see that inulin is being cleared at constant rate after the first hour or so. but the first couple of points are too high,aren't on the line...why? because you inject into plasma space and take sample from there. if it hasn't had time to distribute, it's abnormally high in plasma space. so you extrapolate from the straight line, and figure out what the starting concentration is through the WHOLE body. starting concentration 280 mg/L Q was 400 mg 400/280 = 14.3 L inulin space(this isn't the same guy) this is easier...only a few hours, not five,and no urine collection. NOT FROM HANDOUT: major ions in these compartments: Na+ sodium ion - main extracellular cation. contributes major osmotic pressure of ECF. there's a concentration gradient between IC and EC space set up by Na+/K+ pump which drives other transport mechanisms eg sodium calcium pump of heart. It carries the excitatory inward current in excitable tissue eg nerve, muscle. K+ potassium ion: major cytoplasmic cation (Na+/K+ pump again) contributes osmotic pressure to cytoplasm. establishes resting membrane potential and is responsible for repolarization of excitable tissue. also activates some dnzymes. Ca++ calcium: low concentration in cells. in muscle cells it is sequestered and therefore even lower concentration. is required to stabilize cell membrane, is involved in adhesion of cells together. carries excitable inward current in smooth and cardiac muscle. regulates many enzymes. involved in regulating exocytosis Mg: intra and extracellular. antagonist to Ca++ in many ways. cofactor for many enzymes eg myosin ATPase. PO4- and HCO3- : intra and extracellular. Buffers. buffer H+ Cl- main extracellular and cytoplasmic anion. main counter ion to all cations and +charged proteins lying around those are just summary statements about these ions. back to handout. details of ion distribution: compositions of body fluids substance plasma water interstitial intracellular (all mEq/L) cations Na+ 148 141 10 K+ 4.3 4.1 Ca++ 4.3 Mg++ 3.2 --------- 159.8 anions Cl- 109 HCO3- 28 PO4--- 2.1 SO4-- 1.1 org.acid 3.2 protein 16 --------- 159.4 oh, this is silly, it's in the handout. note ratio of [Na+]ISF/[Na+]p = .953 and this is very close to Cl- ratio this is the Gibbs-Donnan relationship. note: protein higher in plasma than ISF, otherwise plasma andISF similar realize that total cations must equal total anions in all of these. exchange mechanisms: between intra and extracellular: there are three kinds of transport mechanisms. one is driven by concentration gradient, one is driven by energy (ATP) and third is specialized, where you can actually have little packets of material surrounded by plasma membrane fusing with membrane - exocytosis, eg at neuromuscular junction. you can also have endocytosis/pinocytosis in the opposite direction. in terms of concentration gradients: simple diffusion: some substances can just diffuse across membrane channel mediated diff: proteins forming pores w/in membrane through which ions can diffuse. may be voltage or ligand gated. carrier mediated diff: "co-diffusion" and exchange diffusion. follows gradients but is mediated by some kind of carrier protein in the membrane. eg, Na+ Ca++ exchanger. active transport: ATP pumps, eg Na+/K+ ATPase. these can establish concentration gradients. between plasma and interstitium: simple diffusion: driven by concentration gradients bulk flow: filtration and absorption membrane mediated exchanges: as above consider some general principles before getting into gibbs-donnan. OSMOTIC PRESSURE: that particular hydrostatic pressure which must be applied to the solu'n to prevent the entry of solvent. eg, if you have u tube, with membrane at bottom which lets water through but not salt, and you put salt water in one side, and water only in other side, so you'll establish an osmotic pressure which forces fluid from dilute side to concentrated side. so water from dilute side will move to other side, and the difference in the level of concentrated side is exerting the HYDROSTATIC PRESSURE keeping more water from coming in. osmotic pressure is osmolar pressure time universal gas constant times temperature. pi = C R T as osmotic pressure goes up, osmolar pressure goes up (?) plasma = 300 mOsmols/L = osmolar pressure; RT = 22 x 4 6.7 = osmotic pressure of ECF/plasma osmolar pressure relates to PARTICLES. one mol of sucrose = one osmol sucrose BUT one mol NaCl = 2 osmol because NaCl dissociates into solu'n into two osmotically active particles, 1 Na+ and 1 Cl- if you take salt and throw it on ice, ice melts b/c freezing point of water is lowered when salt in solu'n with it. if you throw salt into boiling water, the water temp will rise about 212 F and you can cook your potatoes faster :) if you want to measure osmotic pressure, you can do it using a modern osmometer which uses freezing point lowering effects. two things with same osmotic pressure are "isosmotic." "isotonic" solution will not shrink or swell cells - it exerts same osmotic pressure across cell membrane as cell fluid. eg, 0.9% NaCl is isotonic saline. why bother calling it that, and not isosmotic? well, if you put 0.9% NaCl across an osmometer from 1.8% urea solu'n, and use a non permeable membrane, those solu'ns will be isosmotic. if you take RBCs and throw them into 0.9% NaCl, they will not lyse or shrink. but if you take them and throw them into 1.8% urea solu'n, they will immediately lyse. why???? the reason is, the urea molecule is small enough to diffuse across the RBC membrane. so it's not providing an osmotically active particle. it's as if you threw it into diH2O. ----end----