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Fluid, Electrolyte & Ph Balance

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Fluid, Electrolyte & Ph Balance Fluid, Electrolyte & pH Balance Fluid / Electrolyte / Acid-Base Balance Body Fluids: Cell function depends not only on continuous nutrient supply / waste removal, but also on the physical / chemical homeostasis of surrounding fluids 1) Water: (universal solvent) Body water varies based on of age, sex, mass, and body composition H2O ~ 73% body weight Low body fat Low bone mass H2O (♂) ~ 60% body weight H2O (♀) ~ 50% body weight ♀ = body fat / muscle mass H2O ~ 45% body weight 1 Fluid / Electrolyte / Acid-Base Balance Body Fluids: Cell function depends not only on continuous nutrient supply / waste removal, but also on the physical / chemical homeostasis of surrounding fluids 1) Water: (universal solvent) Total Body Water Volume = 40 L (60% body weight) Plasma Intracellular Fluid (ICF) Interstitial Volume = 3= Volume Volume = 25 L Fluid (40% body weight) Volume = 12 L L Extracellular Fluid (ECF) Volume = 15 L (20% body weight) Fluid / Electrolyte / Acid-Base Balance Clinical Application: The volumes of the body fluid compartments are measured by the dilution method Step 1: Step 2: Step 4: Identify appropriate marker Inject known volume of Calculate volume of body substance marker into individual fluid compartment Total Body Water: Amount Volume = A marker is placed in (L) Concentration the system that is distributed (mg) wherever water is found Amount: Marker: D2O Step 3: Amount of marker injected (mg) – Amount excreted (mg) Extracellular Fluid Volume: Let marker equilibrate and A marker is placed in measure marker Concentration: the system that can not cross • Plasma concentration Concentration in plasma (mg / L) cell membranes • Urine concentration Marker: Mannitol Note: ICF Volume = TBW – ECF Volume 2 Fluid / Electrolyte / Acid-Base Balance Electrolytes have great osmotic power: Body Fluids: 2+ - - MgCl2 Mg + Cl + Cl Cell function depends not only on continuous nutrient supply / waste removal, but also on the physical / chemical homeostasis of surrounding fluids 2) Solutes: A) Non-electrolytes (do not dissociate in solution – neutral) • Mostly organic molecules (e.g., glucose, lipids, urea) B) Electrolytes (dissociate into ions in solution – charged) • Inorganic salts • Inorganic / organic acids • Proteins Although individual [solute] are different between compartments, the osmotic concentrations of the ICF and ECF are usually identical… Marieb & Hoehn (Human Anatomy and Physiology, 8th ed.) – Figures 26.2 Fluid / Electrolyte / Acid-Base Balance Fluid Movement Among Compartments: The continuous exchange and mixing of body fluids are regulated by osmotic and hydrostatic pressures Plasma 300 mosm Hydrostatic Pressure Interstitial 300 mosm Fluid Osmotic Pressure Intracellular 300 mosm Fluid Osmotic Osmotic Pressure Pressure In a steady state, The volume of a particular The shift of fluids between intracellular osmolarity compartment depends on compartments depends on is equal to amount of solute present osmolarity of solute present extracellular osmolarity 3 Fluid / Electrolyte / Acid-Base Balance Water Balance: ICF functions as a reservoir For proper hydration: Waterintake = Wateroutput Water Output Water Intake Feces (2%) < 0 Metabolism (10%) Sweat (8%) Osmolarity rises: • Thirst Solid foods (30%) Skin / lungs (30%) • ADH release > 0 Osmolarity lowers: Ingested liquids (60%) Urine (60%) • Thirst • ADH release 2500 ml/day 2500 ml/day = 0 Fluid / Electrolyte / Acid-Base Balance Water Balance: Thirst Mechanism: osmolarity / volume (-) of extra. fluid Osmoreceptors stimulated (Hypothalamus) volume / osmolarity of Sensation Drink extracellular fluid of thirst saliva Dry mouth secretion (-) 4 Fluid / Electrolyte / Acid-Base Balance Pathophysiology: Disturbances that alter solute or water balance in the body can cause a shift of water between fluid compartments Volume Contraction: A decrease in ECF volume 300 mOsm < 300 mOsm NaCl not reabsorbed from kidney filtrate Water Aldosterone Diarrhea deficiency insufficiency 300 mOsm 300 mOsm > 300 mOsm < 300 mOsm Extracellular Extracellular Extracellular Extracellular Fluid Fluid Fluid Fluid 300 mOsm 300 mOsm > 300 mOsm < 300300 mOsm Intracellular Intracellular Intracellular Intracellular Fluid Fluid Fluid Fluid Isomotic Contraction Hyperosmotic Contraction Hyposmotic Contraction ECF volume = Decrease ECF volume = Decrease ECF volume = Decrease ECF osmolarity = No change ECF osmolarity = Increase ECF osmolarity = Decrease ICF volume = No change ICF volume = Decrease ICF volume = Increase ICF osmolarity = No change ICF osmolarity = Increase ICF osmolarity = Decrease Fluid / Electrolyte / Acid-Base Balance Pathophysiology: Disturbances that alter solute or water balance in the body can cause a shift of water between fluid compartments Water intoxication Volume Expansion: An increase in ECF volume Greater than normal 300 mOsm NaCl water reabsorption IV bag Osmotic IV Yang’s lunch SIADH 300 mOsm 300 mOsm > 300 mOsm < 300 mOsm Extracellular Extracellular Extracellular Extracellular Fluid Fluid Fluid Fluid 300 mOsm 300 mOsm > 300 mOsm < 300 mOsm Intracellular Intracellular Intracellular Intracellular Fluid Fluid Fluid Fluid Isomotic Expansion Hyperosmotic Expansion Hyposmotic Expansion IV bags of varying solutes ECF volume = Increase ECF volume = Increase ECF volume = Increase allow for manipulation of ECF osmolarity = No change ECF osmolarity = Increase ECF osmolarity = Decrease ECF / ICF levels… ICF volume = No change ICF volume = Decrease ICF volume = Increase ICF osmolarity = No change ICF osmolarity = Increase ICF osmolarity = Decrease 5 Fluid / Electrolyte / Acid-Base Balance Acid-Base Balance: Acid-base balance is concerned with maintaining a normal hydrogen ion concentration in the body fluids Compatible range for life + pH = -log10 [H ] -9 pH = -log10 [40 x 10 ] Normal [H+] pH = 7.4 Note: 1) [H+] and pH are inversely related 2) Relationship is logarithmic, not linear [H+] = 40 x 10-9 Eq / L Normal range of arterial pH = 7.37 – 7.42 Problems Encountered: 1) Disruption of cell membrane stability 2) Alteration of protein structure 3) Enzymatic activity change Costanzo (Physiology, 4th ed.) – Figure 7.1 Fluid / Electrolyte / Acid-Base Balance Acid-Base Balance: Acid production in the human body has two forms: volatile acids and fixed acids 1) Volatile Acids: Acids that leave solution and enter the atmosphere - + CO2 + H2O H2CO3 HCO3 + H End product of carbonic aerobic metabolism acid (13,000 – 20,000 mmol / day) Eliminated via the lungs 2) Fixed Acids: Acids that do not leave solution (~ 50 mmol / day) Products of normal Products of extreme Ingested (harmful) catabolism: catabolism: products: Diabetes • Sulfuric acid (amino acids) • Acetoacetic acid mellitus • Salicylic acid (e.g., aspirin) (ketobodies) • Phosphoric acid (phospholipids) • B-hydroxybutyric acid • Formic acid (e.g., methanol) • Lactic acid (anaerobic respiration) • Oxalic acids (e.g., ethylene glycol) Eliminated via the kidneys 6 Fluid / Electrolyte / Acid-Base Balance Acid-Base Balance: H+ Gain: Distant from H+ Loss: one another • Across digestive epithelium • Release at lungs • Cell metabolic activities • Secretion into urine Chemical Buffer: A mixture of a weak acid and its conjugate base or a weak base and its conjugate acid that resist a change in pH Robert Pitt: + 150 mEq H 150 mEq H+ Brønsted-Lowry Nomenclature: Weak acid: Acid form = HA = H+ donor Base form = A- = H+ acceptor 11.4 L Weak base: 11.4 L Acid form = BH+ = H+ donor Base form = B = H+ acceptor 7.44 7.14 7.00 1.84 Fluid / Electrolyte / Acid-Base Balance For the human body, the Acid-Base Balance: most effective physiologic buffers will have a pK at 7.4 1.0 The Henderson-Hasselbalch equation is used to calculate the pH of a buffered solution Henderson-Hasselbalch Equation: Most effective -1 +1 [A-] buffering pH = pK + log [HA] + pH = -log10 [H ] pK = -log10 K (equilibrium constant) [A-] = Concentration of base form of buffer (mEq / L) [HA] = Concentration of acid form of buffer (mEq / L) pK is a characteristic value for a buffer pair (strong acid = pK; weak acid = pK) Costanzo (Physiology, 4th ed.) – Figure 7.2 7 Fluid / Electrolyte / Acid-Base Balance pK = 6.1 - [HCO3 ] = 24 mmol / L Acid-Base Balance: Solubility = 0.03 mmol / L / mm Hg PCO2 = 40 mm Hg The major buffers of the ECF are bicarbonate and phosphate - 1) HCO3 / CO2 Buffer: - + CO2 + H2O H2CO3 HCO3 + H (HA) (A-) Utilized as first line of defense when H+ enters / lost from system: - (1) Concentration of HCO3 normally high at 24 mEq / L - (2) The pK of HCO3 / CO2 buffer is 6.1 (near pH of ECF) (3) CO2 is volatile; it can be expired by the lungs The pH of arterial blood can - HCO3 24 be calculated with the pH = pK + log pH = 6.1 + log Henderson-Hasselbalch equation 0.03 x PCO2 0.03 x 40 pH = 7.4 Fluid / Electrolyte / Acid-Base Balance Acid-Base Balance: The major buffers of the ECF are bicarbonate and phosphate - 1) HCO3 / CO2 Buffer: Acid-base map: Shows relationship between the acid and base forms of a buffer and the pH of the solution Ellipse: Normal values for arterial blood Note: Abnormal combinations of PCO2 - Isohydric line and HCO3 concentration can yield normal values of pH (‘same pH’) (compensatory mechanisms) Costanzo (Physiology, 4th ed.) – Figure 7.4 8 Fluid / Electrolyte / Acid-Base Balance Acid-Base Balance: The major buffers of the ECF are bicarbonate and phosphate - 2- 2) H2PO4 / HPO4 Buffer: - 2- + H2PO4 HPO4 + H (HA) (A-) Why isn’t inorganic phosphate the primary ECF buffer? (1) Lower concentration than bicarbonate (~ 1.5 mmol / L) (2) Is not volatile; can not be
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