Chapter 24 Water, Electrolyte, and AcidBase Balance Learning
Chapter 24 Water, Electrolyte, and AcidBase Balance Learning Objectives Explain regulation of electrolytes in the body; major ions in the intracellular and extracellular fluid and units of concentration Describe common disturbances of water balance and pathogenesis Explain physiologic mechanisms in the control of pH
Describe pathogenesis of 4 common disturbances of acid-base balance and bodys compensatory mechanisms Define role of kidneys and lungs in regulating acidbase balance Body Water and Electrolytes Body water contains dissolved mineral salts or electrolytes that dissociate in solution, yielding Cations: positively charged ions
Anions: negatively charged ions Body fluids: electrically neutral Sum of cations balanced by sum of anions In disease, ion concentrations may vary but the electrical neutrality is always maintained Intracellular and Extracellular Fluid (1 of 5) Disturbances of body water are associated with corresponding change in electrolytes If electrolyte concentration changes, there is a
corresponding change in body water and vice versa Body consists of 70% water Intracellular water (inside cells) Extracellular water (within interstitial tissues surrounding cells, blood plasma, and lymph) Rule of thirds 2/3 of body weight is H2O 2/3 of H2O is within cells 1/3 of H2O is extracellular in tissues surrounding cells
(interstitial fluid) Intracellular and Extracellular Fluid (2 of 5) Adult female: water content is 10% lower than adult male due to higher body fat than water Fluids and electrolytes diffuse freely between the intravascular and interstitial fluids Because capillaries are impermeable to protein, the interstitial fluid contains very little protein Cell membrane: separates intracellular fluid from interstitial fluid by a cell membrane
Freely permeable to water Impermeable to Na+ and K+ ions Intracellular and Extracellular Fluid (3 of 5) Chief intracellular ions K+ (potassium) PO42 (phosphate) Chief extracellular ions Na+ (sodium) Cl (chloride)
Differences in concentration of ions on different sides of the cell membrane result from metabolic activity of the cell Amount of sodium in the body determines the volume of extracellular fluid as the chief extracellular cation Intracellular and Extracellular Fluid (4 of 5) Amount of potassium in the body determines the volume of intracellular fluid as the chief
intracellular cation In electrolyte disturbances: primary concern is the concentration of various ions and the interrelation of positively and negatively charged ions with one another than the actual number Intracellular and Extracellular Fluid (5 of 5) Units of concentration of electrolytes Expressed in units that define ability to combine with other ions Equivalent weight: molecular weight of
substance in grams divided by valence 1 equivalent weight dissolved in a liter equals one equivalent per liter (1Eq/L) Units expressed in milliequivalents per liter (1000 mEq = 1Eq) Regulation of Body Fluid and Electrolyte Concentration (1 of 3) Amount of H2O and electrolytes in body: represents the balance between amounts ingested in food and fluids and amounts excreted via urine, GI tract,
perspiration, and as H2O vapor excreted by lungs Disturbances of H2O balance Dehydration: most common Inadequate intake: diarrhea or vomiting Excess H2O loss: comatose or debilitated patients Overhydration: less common Excessive fluid intake when renal function is impaired: renal disease; excessive intake of fluids; excessive administration of IV fluids
Regulation of Body Fluid and Electrolyte Concentration (2 of 3) Disturbances of electrolyte balance Conditions that produce H2O imbalance also disturb electrolyte composition Most result from depletion of body electrolytes Depletion of electrolytes Vomiting or diarrhea: sodium and potassium depletion Excessive use of diuretics
Excessive diuresis in diabetic acidosis Renal tubular disease Regulation of Body Fluid and Electrolyte Concentration (3 of 3) Diuretics promote excretion of salt and H2O by the kidneys while impairing reabsorption of these substances Patients with heart failure, liver cirrhosis, kidney disease
Uncontrolled diabetes: excessive loss of H2O in urine from the diuretic effect of glucose Renal tubular disease: regenerating renal tubules unable to conserve electrolytes and water AcidBase Balance Body produces large amounts of acid from normal metabolic processes, such as breakdown of proteins and glucose or oxidation of fat Body fluids remain slightly alkaline pH is maintained within a narrow range: 7.38 to
7.42 Regulatory mechanisms maintain pH Neutralize and eliminate the acids as soon as they are produced to maintain normal pH Blood buffers: resist pH change Lungs: control carbonic acid (H2CO3)concentration Kidneys: control bicarbonate concentration Board-and-fulcrum concept of normal bicarbonate-carbonic acid relationships
Blood Buffer System (1 of 2) Minimize change in hydrogen ion by converting strong acids and bases into weaker ones chemically Weak acid and its salt Weak base and its salt Respiratory control of carbonic acid Carbonic acid (H2CO3): dissolved as CO2 in plasma Hyperventilation: lowers CO2 and H2CO3 in plasma Decreased or inadequate ventilation: raises CO2 and
H2CO3 in plasma Blood Buffer System (2 of 2) Renal control of bicarbonate concentration Kidneys selectively reabsorb filtered bicarbonate Kidneys can manufacture bicarbonate to replace amounts lost in buffering acids from metabolic processes In any buffer system pH depends on ratio of bicarbonate to H2CO3
Normal ratio: 20 parts Na bicarbonate: 1 part H2CO3 Disturbances in AcidBase Balance Acidosis Blood pH shifts to acidic side From an excess of H2CO3 From a reduced amount of bicarbonate Alkalosis Blood pH shifts to basic side From a decrease in H2CO3
From an excess of bicarbonate Classification of AcidBase Disturbances (1 of 3) Metabolic: disturbance lies in bicarbonate member of the buffer pair Respiratory: disturbance lies in carbonic acid member of the buffer pair Metabolic acidosis: increased endogenous acid generated Amount of acid generated exceeds bodys buffering
capacity Excess acid is neutralized by bicarbonate Bicarbonate in plasma falls from being consumed in neutralizing excess acid Uremia, ketosis, lactic acidosis Compensation: by hyperventilation to lower PCO2 and increased bicarbonate production in kidneys A. Derangement of acid-base balance in metabolic acidosis. B. Compensation by reduction of carbonic
acid and formation of additional bicarbonate. Classification of AcidBase Disturbances (2 of 3) Respiratory acidosis: increased H2CO3 concentration Inefficient excretion of CO2 by lungs Leads to retention of CO2 and rise in H2CO3 Compensation: increased bicarbonate production in kidneys
Metabolic alkalosis: increased plasma bicarbonate From loss of gastric juice, chloride depletion, excess corticosteroids, excess antacids With coexisting potassium deficiency Compensation: inefficient, requires simultaneous correction of potassium deficiency A. Derangement of acid-base balance in respiratory acidosis. B. Compensation by formation of additional bicarbonate.
Classification of AcidBase Disturbances (3 of 3) Respiratory alkalosis: Reduced H2CO3 concentration Hyperventilation lowers PCO2 and H2CO3 level falls Relative excess of bicarbonate Compensation: excretion of bicarbonate by kidneys
A. Derangement of acid-base balance in respiratory alkalosis. B. Compensation by excretion of bicarbonate Diagnostic Evaluation of Acid Base Balance Clinical evaluation: determination of concentration of bicarbonate in plasma as an index of patients overall status Laboratory studies Blood pH PCO2
Bicarbonate Discussion What is the difference between intracellular and extracellular fluid? What are the differences between metabolic acidosis and respiratory acidosis as to causes and compensatory mechanisms of the body? What are the differences between metabolic alkalosis and respiratory alkalosis as to
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