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. 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 water Babies – 75% water Men – 63 % Women – 52%

Fluid compartments Separated by selectively permeable membranes Intracellular – 2/3 (63%) of total body water Extracellular – 1/3 (37%) Interstitial fluid – 80 % of extracellular water Blood 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 lymph Intracellular fluids: High in K+, phosphate, Mg++, and more protein than plasma

Movement of water Hydrostatic pressure – pressure of fluids Osmotic pressure – solute concentration (often Na+) In blood referred to as colloid osmotic pressure (COP)

Water intake = Water loss Average adult takes in about 2,500 ml/day Sources of water: Preformed water: 2,300 ml Drinking water: 1,500 ml (60%) Moist food : 750 ml (30%) Water of metabolism: 250 ml (10%) Cellular respiration Dehydration synthesis

Regulation of water intake Main regulator is thirst. Dehydration (output>intake) as little as 1% decrease in body water causes: Decreased production of saliva Increased blood osmotic pressure – stimulates osmoreceptors in the hypothalamus Decreased blood volume – renin is produced

The thirst center in hypothalamus is stimulated ( or mistakenly, the hunger center) and person feels thirsty Wetting 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 loss Through kidneys in urine – 1500 ml (60%) Through intestines - 150 ml (6%) Can be significant in vomiting and diarhhea From skin (sweat) - 150 ml (6%) From lungs and skin 700 ml (28%) Last is called insensible loss (menstruation)

Regulation of Water Output Through regulating urine formation ADH – 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 production Causes sodium ( and water) to be reabsorbed ANP – causes sodium (and water) loss when pressure in right atrium is too high

Water imbalances Dehydration is the imbalance seen most often. Prolonged diarrhea or vomiting Excessive sweating

Water toxicity If 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 cells This increases ISF tonicity, and water is drawn from blood Add salt when replacing fluids like this!

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

Decrease in blood proteins caused by: Dietary deficiency in proteins Water that moves back into capillaries depends on concentration of plasma proteins. Decrease in blood proteins caused by: Dietary deficiency in proteins Liver failure Blockage of lymphatic system Increased capillary permeability Burns, 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.

Edema Can be caused by: Decrease in plasma proteins Retention of electrolytes, esp. Na+ Increase in capillary blood pressure

Electrolyte Balance Cations – positively charged ions Anions – negatively charged ions Body fluids also contain charged organic molecules Only a small percentage of molecules in fluids are non-electrolytes: glucose, urea, creatinine

Functions of electrolytes Certain ions control the osmosis of water between body compartments Ions help maintain the acid-base balance necessary for cellular activity Ions carry electric current, which allows for action potentials and secretion of neurotransmitters Several ions are cofactors needed for the optimal activity of enzymes

Electrolyte intake Food and water Produced by metabolism Salt craving

Electrolyte loss Sweat Feces Urine

Osmolarity The total concentration of dissolved particles determines osmolarity. Glucose – one dissolved particle NaCl – dissolves into two particles One mole of NaCl = 2 osmoles Osmoles/L = osmolarity of solution

Sodium (Na+) 90 % of extracellular cations and half the osmolarity of extracellular solutions Necessary for action potentials in nerve & muscle cells Aldosterone increases reabsorption from DCT and collecting ducts ↓ blood volume, ↓ extracellular Na+ ,↑ extracellular K+ ANP causes loss of Na+

Potassium (K+) Most numerous intracellular cation Membrane potential and repolarization Controlled by aldosterone – causes loss of K+ in urine

Calcium (Ca++) Part of bone, most abundant mineral in body. 98% of Ca is in bone Extracellular cation Needed for blood clotting, nerve and muscle function PTH causes reabsorption of bone and increases reabsorption from G.I tract and glomerular filtrate Calcitonin inhibits osteoclasts and stimulates osteoblast, so calcium is removed from blood

Chloride (Cl-) Most common extracellular anions Cl- diffuses easily between compartments – can help balance charges (RBC’s) 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 body’s 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 ionized Found in combination with lipids, proteins, carbohydrates, nucleic acids and ATP. Three different forms Part of the phosphate buffer system PTH causes phosphate to be released from bones and to be excreted by the kidneys. Calcitonin removes phosphate by encouraging bone formation.

Acid-Base Balance pH – negative log of H+ concentration Affects functioning of proteins (enzymes) Can affect concentrations of other ions Modify hormone actions (proteins)

Acid intake Foods Produced by cellular metabolism

Strengths of Acids and Bases Acids 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 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 Balance Buffer systems Exhalation of carbon dioxide Kidney excretion

Buffers Are 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 + NaCl Carbonic 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 slightly pH 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 System Is present in extracellular and intracellular fluids, most important in intracellular fluids and renal tubules. H+ + HPO42- → H2PO4- monohydrogen dihydrogen phosphate phosphate OH- + H2PO4- → H2O + HPO42-

Protein Buffer System The most abundant in body cells and plasma. Carboxyl group -COOH ↔ -COO- + H+ Amino group –NH2 ↔ -NH3+

Respiratory Mechanisms – Exhalation of CO2 Because 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 minutes pH also affects breathing rate Powerful 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 lungs Can also excrete bases Can excrete acids while conserving bicarbonate ion Can produce more bicarbonate ion Kidneys are the most effective regulators of pH; if kidneys fail, pH balance fails

The regulators work at different rates Buffers are the first line of defense because they work almost instantaneously. Secondary defenses take longer to work: Respiratory mechanisms take several minutes to hours Renal mechanisms may take several days

pH imbalances The normal blood pH range is 7.35 – 7.45 Any pH below this range is considered to be a condition of acidosis Any pH above this range is considered to be a condition of alkalosis The body response to acid-base imbalance is called compensation: Compensation may be complete if the blood pH is brought back to normal, or partial if it is still outside the norms.

Respiratory problems Respiratory acidosis is a carbonic acid excess (blood CO2 is too high) Respiratory alkalosis is a carbonic acid deficit (blood CO2 is too low) Compensation would occur through the kidneys

Metabolic problems Metabolic acidosis is a bicarbonate deficit Metabolic alkalosis is a bicarbonate excess Compensation would occur through changes in the depth and rate of respiration.