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Fluid, Electrolyte, and Acid-Base Balance

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Charlotte Youngson

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26 Fluid, Electrolyte, and Acid-Base Balance Pre-lecture questions 1. What mechanisms does the body have to ensure body pH is regulated? 2. What happens when there is a drop in blood pressure and blood volume? Composition of Body Fluids • Water: the universal solvent • Solutes: nonelectrolytes and electrolytes • Nonelectrolytes: most are organic • Do not dissociate in water: e.g., glucose, lipids, creatinine, and urea • Electrolytes • Dissociate into ions in water; e.g., inorganic salts, all acids and bases, and some proteins • The most abundant solutes • Have greater osmotic power than nonelectrolytes, contribute to fluid shifts Extracellular and Intracellular Fluids • Each fluid compartment has a distinctive pattern of electrolytes • ECF • Major cation: Na + • Major anion: Cl – • ICF: + – • Low Na and Cl • Major cation: K + • Major anion HPO 2– 4 Fluid Movement Among Compartments • Regulated by osmotic and hydrostatic pressures • Water moves freely by osmosis • Two-way osmotic flow is substantial • Ion fluxes require active transport or channels • Change in solute concentration of any compartment leads to net water flow Regulation of Water Intake • Thirst mechanism is the driving force for water intake • The hypothalamic thirst center osmoreceptors are stimulated by • increased Plasma osmolality • Angiotensin II or baroreceptor input • Dry mouth • Substantial decrease in blood volume or pressure Regulation of Water Output: Influence of ADH • Water reabsorption in collecting ducts is proportional to ADH release •  ADH  dilute urine and  volume of body fluids •  ADH  concentrated urine Disorders of Water Balance: Dehydration • Negative fluid balance • ECF water loss due to: hemorrhage, severe burns, prolonged vomiting or diarrhea, profuse sweating, water deprivation • Signs and symptoms: thirst, dry flushed skin, oliguria • May lead to weight loss, fever, mental confusion, hypovolemic shock, and loss of electrolytes Disorders of Water Balance: Hypotonic Hydration • Cellular overhydration, or water intoxication • Occurs with renal insufficiency or rapid excess water ingestion • ECF is diluted  hyponatremia  net osmosis into tissue cells  swelling of cells  severe metabolic disturbances (nausea, vomiting, muscular cramping, cerebral edema)  possible death Disorders of Water Balance: Edema • Atypical accumulation of IF fluid  tissue swelling • Due to anything that increases flow of fluid out of the blood or hinders its return •  Blood pressure •  Capillary permeability (usually due to inflammatory chemicals) • Incompetent venous valves, localized blood vessel blockage • Congestive heart failure, hypertension,  blood volume • Hindered fluid return occurs with an imbalance in colloid osmotic pressures, e.g., hypoproteinemia ( plasma proteins) • Fluids fail to return at the venous ends of capillary beds • Results from protein malnutrition, liver disease, or glomerulonephritis • Blocked (or surgically removed) lymph vessels • Cause leaked proteins to accumulate in IF Electrolyte Balance • Electrolytes are salts, acids, and bases • Electrolyte balance usually refers only to salt balance • Salts enter the body by ingestion and are lost via perspiration, feces, and urine • Importance of salts • Controlling fluid movements • Excitability • Secretory activity • Membrane permeability Central Role of Sodium • Most abundant cation in the ECF • Sodium salts in the ECF contribute to most of the solute concentration Regulation o+ Sodium Balance: Aldosterone • Na reabsorption • 65% is reabsorbed in the proximal tubules • 25% is reclaimed in the loops of Henle + • Aldosterone  activ+ reabsorption of remaining Na • Water follows Na if ADH is present Regulation of Sodium Balance: ANP • Released by atrial cells in response to stretch ( blood pressure) • Effects • Decreases blood pressure and blood volume: •  ADH, renin and aldosterone production •  Excretion of Na and water • Promotes vasodilation directly and also by decreasing production of angiotensin II Regulation of Potassium Balance • Importance of potassium: • Affects RMP in neurons and muscle cells (especially cardiac muscle) •  ECF [K ]  RMP  depolarization  reduced excitability + •  ECF [K ] hyperpolarization and nonresponsiveness • Influence of aldosterone •
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