Water & Electrolyte Balans

Water & Electrolyte BalanceWater is essential to life50% or more of bodyweight, age & sex differencesnewborn to 70 years: 73% to 45% wateryoung adult male: 60%young adult female: 50%key is muscle to fat ratio; total muscle mass

need to regulate fluid balance inside cells & outsideblood pressureblood volumeelectrolyte balance

Body fluidsWater40 liters; 60% of body weight

Soluteselectrolytes: ionic compoundssalts: NaCl, NaHCO3acids, bases: H2CO3some proteins: most negatively charged

Nonelectrolytesusually covalentno electrical chargeorganicglucoselipidscreatinineurea

Fluids fill 2 compartmentsIntracellularwithin cells25/40 liters or 63%Extracellular15 liters or 46%two partsplasma: 3 litersinterstitial fluid: 12 literswater moves between compartments by osmosis

Fluid composition differs between compartmentsDue primarily to membrane activityNa / K balance

Fluid is lost continuallyObligate losseslungsskinG.I. Tractminimal urine flow (500 ml/day)Facultativeexcess urine production

Balance of gain and lossapproximately 2,500 ml / dayIntake:drinking - 1500 ml, 60%eating - 750 ml, 30%metabolism (water of oxidation) - 250 ml, 10%Loss:lungs & skin - insensible - 700 ml, 28%perspiration - 200 ml, 8%feces - 100 ml, 4%kidneys - 1500 ml, 60%

Primary mechanism for water replacement is thirst reflexDecrease ECF volumeIncrease concentration of ECFHypothalamicthirst centerDecrease salivaDry mouththirstdrinkRaise volumelower concentration

Regulating water loss / electrolytesObligate losses cannot be controlledSolute concentrations & urine production are mechanisms for regulating facultative lossNa+ is principal electrolyte90-95% of all soluted in ECFSodium & water regulation tightly linked

Sodium BalanceNo specific receptorsregulated throughblood volumeblood pressureAldosterone is primary regulatorsodium uptakerelease triggered mostly by AngiotensinII

Renin from JG cellsSympathetic n.s.low solutesless stretch (low pressure)angiotensin IIvasoconstrictstimulate aldosterone release

Effect of aldosterone depends on ADH levelsHypothalamushigh sodiumrelease ADH

ANP also affects water balanceStretch atria (high BP)Collecting ductsJG apparatusposteriorpituitaryadrenalcortexvasodilatekidney tubules Na+ & water reabsorption

Other factors that affect water balanceestrogensaldosterone-like: enhance Na+ uptakeglucocorticoidsenhance tubular reabsorption of Na+especially at high concentrationsenhance glomerular filtration

Calcium regulationPTH: from parathyroidactivate osteoclastsenhance intestinal absorptionincrease kidney reabsorption

most calcium in bones as calcium phosphatePO4- reabsorbed in proximal tubulesregulated by PTH

pH (acid-base) balancebuffering systems in blood & tissuesbicarbonate

phosphate

protein

Why do we need buffering capacity?Metabolism produces wastesCO2aerobic & anaerobic respirationlactic acidanaerobic respiration of glucosephosphoric acidprotein metabolismketone bodiesfat metabolism

pH changes that followalter enzymeschange metabolism

* Blood & tissues never really acidic; only relatively more or less so

Different buffering systems operate in different fluidsPLASMA

proteinsHCO3-EXTRA-CELLULAR FLUID

HCO3-INTRA-CELLULARFLUID

proteinsHCO3-HPO4---

How is pH regulated?RespiratoryCO2 effects on breathing rate & depthmedullary centershigh capacity0 - 15 fold change in ventilation2 fold change moves pH from 7.2 to 7.6beyond normal limits

RenalHCO3-by regulating H+ secretion: CO2 H+ secretedPCT, collecting ductfrom CO2 , H20 reaction in filtratecarbonic anhydrase in PCT

HPO4-protects against dangerous pH in filtrate binds excess H+