water, electrolyte and acid-base balance

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Water, Electrolyte and Acid-Base Balance. Chapter 21. Balance – a state of equilibrium – substances are maintained in the right amounts and in the right place in the body. Water Balance. Osmosis is the primary method of water movement into and out of body fluid compartments. - PowerPoint PPT Presentation


  • Water, Electrolyte and Acid-Base BalanceChapter 21

  • Balance a state of equilibrium substances are maintained in the right amounts and in the right place in the body

  • Water BalanceOsmosis is the primary method of water movement into and out of body fluid compartments.Osmosis is the net movement of water molecules through a selectively permeable membrane from an area of high water concentration to an area of lower water concentration.

  • The concentration of solutes determines the direction of water movement.Most solutes in the body are electrolytes inorganic compounds which dissociate into ions in solution.Where sodium goes, water follows.

  • About 40 Liters (10.56 gallons) of body waterBabies 75% waterMen 63 %Women 52%

  • Fluid compartmentsSeparated by selectively permeable membranesIntracellular 2/3 (63%) of total body waterExtracellular 1/3 (37%)Interstitial fluid 80 % of extracellular waterBlood plasma 20 % of extracellular water

  • Composition of compartments Extracellular fluids:High in Na+, Cl-, Ca++, HCO3-Blood plasma has more protein than interstitial fluid and lymphIntracellular fluids:High in K+, phosphate, Mg++, and more protein than plasma

  • Movement of waterHydrostatic pressure pressure of fluids

    Osmotic pressure solute concentration (often Na+)In blood referred to as colloid osmotic pressure (COP)

  • Water intake = Water lossAverage adult takes in about 2,500 ml/daySources of water:Preformed water: 2,300 mlDrinking water: 1,500 ml (60%)Moist food : 750 ml (30%)Water of metabolism: 250 ml (10%)Cellular respirationDehydration synthesis

  • Regulation of water intakeMain regulator is thirst.Dehydration (output>intake) as little as 1% decrease in body water causes:Decreased production of salivaIncreased blood osmotic pressure stimulates osmoreceptors in the hypothalamusDecreased blood volume renin is produced

  • The thirst center in hypothalamus is stimulated ( or mistakenly, the hunger center) and person feels thirstyWetting of the mouth and stretching of stomach or intestines decrease thirst before we take in too much water.Water is absorbed, and blood osmotic pressure decreases.

  • Sources of water lossThrough kidneys in urine 1500 ml (60%)Through intestines - 150 ml (6%)Can be significant in vomiting and diarhheaFrom skin (sweat) - 150 ml (6%)From lungs and skin 700 ml (28%)Last is called insensible loss(menstruation)

  • Regulation of Water OutputThrough regulating urine formationADH production stimulated by blood tonicity of decrease in volume.Acts on distal convoluted tubules and collecting ducts of kidney permits reabsorption of water

  • Aldosterone production is stimulated by angiotensin II through renin productionCauses sodium ( and water) to be reabsorbedANP causes sodium (and water) loss when pressure in right atrium is too high

  • Water imbalancesDehydration is the imbalance seen most often.Prolonged diarrhea or vomitingExcessive sweating

  • Water toxicityIf lose water by sweating, we also lose sodium.Rapidly drinking large quantities of water decreases plasma sodium concentration initially, then see decrease in ISF as well.Water is drawn into cellsThis increases ISF tonicity, and water is drawn from bloodAdd salt when replacing fluids like this!

  • OverhydrationCan occur if I.V. fluids are given too rapidly or in too large amounts.Extra fluid puts strain on heart

  • Water that moves back into capillaries depends on concentration of plasma proteins.Decrease in blood proteins caused by:Dietary deficiency in proteinsLiver failureBlockage of lymphatic systemIncreased capillary permeabilityBurns, infection

  • Fluid moves from the blood to the interstitial fluid.Get large amounts of fluid in the intercellular spaces Edema

  • Of the three main compartments (IVF, ICF and ISF) the interstitial fluid varies the most.

  • EdemaCan be caused by:Decrease in plasma proteinsRetention of electrolytes, esp. Na+Increase in capillary blood pressure

  • Electrolyte BalanceCations positively charged ionsAnions negatively charged ionsBody fluids also contain charged organic moleculesOnly a small percentage of molecules in fluids are non-electrolytes: glucose, urea, creatinine

  • Functions of electrolytesCertain ions control the osmosis of water between body compartmentsIons help maintain the acid-base balance necessary for cellular activityIons carry electric current, which allows for action potentials and secretion of neurotransmittersSeveral ions are cofactors needed for the optimal activity of enzymes

  • Electrolyte intakeFood and waterProduced by metabolismSalt craving

  • Electrolyte lossSweatFecesUrine

  • OsmolarityThe total concentration of dissolved particles determines osmolarity.Glucose one dissolved particleNaCl dissolves into two particlesOne mole of NaCl = 2 osmoles Osmoles/L = osmolarity of solution

  • Sodium (Na+)90 % of extracellular cations and half the osmolarity of extracellular solutionsNecessary for action potentials in nerve & muscle cellsAldosterone increases reabsorption from DCT and collecting ducts blood volume, extracellular Na+ , extracellular K+ANP causes loss of Na+

  • Potassium (K+)Most numerous intracellular cationMembrane potential and repolarizationControlled by aldosterone causes loss of K+ in urine

  • Calcium (Ca++)Part of bone, most abundant mineral in body. 98% of Ca is in boneExtracellular cationNeeded for blood clotting, nerve and muscle functionPTH causes reabsorption of bone and increases reabsorption from G.I tract and glomerular filtrateCalcitonin inhibits osteoclasts and stimulates osteoblast, so calcium is removed from blood

  • Chloride (Cl-)Most common extracellular anionsCl- diffuses easily between compartments can help balance charges (RBCs)Parietal cells in stomach secrete Cl- & H+Aldosterone indirectly adjusts Cl- when it increases the reabsorption of Na+ - Cl- follows the Na+

  • Bicarbonate (HCO3-)Part of the bodys chief buffer and transports CO2 in blood stream.CO2 + H2O H2CO3 H+ + HCO3-The kidneys are the main regulators of bicarbonate: they form bicarb when levels are low and excrete it when levels are high.

  • Phosphate (HPO42-)Like calcium, most of the phosphate is found in the bones. 15% is ionizedFound in combination with lipids, proteins, carbohydrates, nucleic acids and ATP.Three different formsPart of the phosphate buffer systemPTH causes phosphate to be released from bones and to be excreted by the kidneys. Calcitonin removes phosphate by encouraging bone formation.

  • Acid-Base BalancepH negative log of H+ concentrationAffects functioning of proteins (enzymes)Can affect concentrations of other ionsModify hormone actions (proteins)

  • Acid intakeFoodsProduced by cellular metabolism

  • Strengths of Acids and BasesAcids and bases that ionize (break apart) completely are strong acids and bases. (HCl; NaOH)Acids and bases that do not completely dissociate in solution are weak acids and bases. (lactic acid, carbonic acid)

  • Remember, blood needs to stay between 7.35 and 7.45 for the body to function properly.Since more acids than bases are formed, pH balance is mainly a matter of controlling excess H+.

  • Control of Acid-Base BalanceBuffer systemsExhalation of carbon dioxideKidney excretion

  • BuffersAre pairs of chemical substances that prevent a sharp change in the pH of a solution.Buffers exchange strong acids for weaker acids that do not release as much H+ and thus change the pH less.

  • Bicarbonate Buffer System NaHCO3 + H2CO3 sodium bicarbonate carbonic acid

    Addition of a strong acid:

    HCl + NaHCO3 H2CO3 + NaClCarbonic acid does not dissociate completely, and pH is changed much less.

  • Addition of a strong base:NaOH + H2CO3 NaHCO3 + H2O

    Water dissociates very little, and pH remains nearly the same.

  • Usually the body is called upon to buffer weaker organic acids, such as lactic acid.Carbonic acid is formed, and amount of bicarbonate ion decreases.Blood needs to maintain a 20:1 ratio of bicarbonate ion : carbonic acid.H+ concentration increases slightlypH drops slightly

  • Carbonic acid is the most abundant acid in the body because it is constantly being formed by buffering fixed acids and by:

    H2O + CO2 H2CO3 H+ + HCO3-

  • Phosphate Buffer SystemIs present in extracellular and intracellular fluids, most important in intracellular fluids and renal tubules.H+ + HPO42- H2PO4- monohydrogen dihydrogen phosphate phosphateOH- + H2PO4- H2O + HPO42-

  • Protein Buffer SystemThe most abundant in body cells and plasma.Carboxyl group -COOH -COO- + H+

    Amino group NH2 -NH3+

  • Respiratory Mechanisms Exhalation of CO2Because carbonic acid can be eliminated by breathing out CO2 it is called a volatile acid.Body pH can be adjusted this way in about 1-3 minutespH also affects breathing ratePowerful eliminator of acid, but can only deal with carbonic acid.

  • Kidney excretion of H+Metabolic reactions produce large amounts of fixed acids.Kidneys can eliminate larger amounts of acids than the lungsCan also excrete basesCan excrete acids while conserving bicarbonate ionCan produce more bicarbonate ionKidneys are the most effective regulators of pH; if kidneys fail, pH balance fails

  • The regulators work at different ratesBuffers are the first line of defense because they work almost instantaneously.


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