Fluid, Electrolyte and Acid-Base Balance Dr. Carlene Graham

Introduction The maintenance of normal volume and normal composition of the extracellular fluid is vital to life. Three types of homeostasis are involved in this maintenance:fluid balance, electrolyte balance, and acid-base balance. The ICF contains nearly2/3rd of total body water;the ECF contains the rest. Exchange occurs between the ICF and ECF.

Fluid compartments Body fluids includes water and solutes. About 2/3rd of the body’s fluid is located within cells and is called intracellular fluid (ICF). The other 1/3rd called extracellular fluid (ECF) includes interstitial fluid, plasma and lymph, GI tract fluids, synovial fluid, fluids of the eyes and ears, CSF, pleural, pericardial and peritoneal fluids and glomerular filtrate. The term fluid balance means that various body compartments contain the normal amount of water.

Fluid Balance An inorganic substance that dissociates into ions is called an electrolyte. Fluid balance and electrolyte balance are interrelated. Water is the largest single constituent in the body-45-75% of total body mass. Daily water gain and loss are about 2.5 L. sources of water gain are ingested liquids and foods, and water produced by cellular respiration and dehydration synthesis reactions. Water is lost by the process of urination, evaporation from the skin surface, exhalation of water vapor, and defecation.in women also menstrual flow.

Fluid Balance The main way to regulate body water gain is by adjusting the volume of water intake, mainly by drinking more or less fluid. The thirst center in the hypothalamus governs the urge to drink. Although increased amounts of water and solutes are lost through sweating and exhalation during exercise, loss of excess water or excess solutes depends mainly on regulating excretion in the urine. The extent of urinary NaCl loss is the main determinant of body fluid volume, whereas the extent of urinary water loss is the main determinant of body fluid osmolarity.

Fluid Balance Angiotensin II and aldosterone reduce urinary loss of Na+ and Cl- and thereby increase the volume of body fluids. ANP promotes natriuresis, elevated excretion of Na+ (and Cl- ), which decreases blood volume. The major hormone that regulates water loss and thus body fluid osmolarity is ADH. An increase in the osmolarity of interstitial fluid draws water out of cells and they shrink slightly. A decrease in the osmolarity of interstitial fluid also causes cells to swell. When a person consumes water faster than the kidneys excrete it or renal fn. Is poor-water intoxication,cells swell.

Electrolytes in Body Fluids Ions formed when electrolytes dissolve in body fluids control the osmosis of water between fluid compartments, help maintain acid-base balance, and carry electrical current. Plasma, interstitial fluid and ICF contain varying kinds and amounts of ions. The conc. Of ions is expressed as mEq/L. sodium ions are the most abundant ions in ECF: involved in impulse transmission, muscle contraction, and fluid and electrolyte balance. The level of sodium is controlled by aldosterone, ADH and ANP.

Electrolytes in Body Fluids Chloride ions are the major anions of the ECF. They play a role in regulating osmotic pressure and forming HCl in gastric juice. Cl- level is controlled indirectly by ADH and by process that increase or decrease renal absorption of Na+. Potassium ions are the most abundant cations of ICF. They play a key role in the resting membrane potential and action potential of neurons and muscle fibers. Help maintain ICF volume and contribute to regulation of pH. K+ level is controlled by aldosterone. Bicarbonate ions (HCO3- )are the second most abundant anions in the ECF. Most important buffer in the plasma.

Electrolytes in Body Fluids Calcium is the most abundant mineral in the body. Calcium salts are structural components of bones and teeth. Ca+2 which are primarily extracellular cations, function in blood clotting, neurotransmitter release, and contraction of muscle. Ca+2 level is controlled by parathyroid and calcitrol. Phosphate ions are principally intracellular anions and their salts are structural components of bones and teeth, also required for the synthesis of nucleic acids and ATP and participate in buffer reactions. Level is controlled by PTH and calcitrol. Mg+2 ions -intracelluar cations that act as cofactors for enzymes.

Acid-Base Balance The normal pH of systemic arterial blood is 7.35-7.45. pH is maintained by buffers. The important buffer systems include proteins, carbonic acid-bicarbonate buffers and phosphates. Homeostasis of pH is maintained by buffer systems, via exhalation of carbon dioxide, and via kidney excretion of H+ and reabsorption of HCO3- . The overall acid-base balance is maintained by controlling the H+ conc. Of body fluids, especially ECF.

Mechanisms that maintain pH of body fluids Buffer systems-made of a weak acid or base and its salt. Prevent drastic changes in body fluid pH. Proteins:the most abundant buffers in body and cells. His and Cys are the two main acids and Hb in RBC’s. Carbonic acid-bicarbonate: important regulator of blood pH. The most abundant buffers in the ECF. Phosphates:important buffer in ICF and in urine. Exhalation of CO2:increased exhalation-inc pH and decreased exhalation dec. pH. Kidneys: renal tubules secrete H+ into the urine and reabsorb HCO3- , so it is not lost in the urine.

Acid-Base Balance An increase in exhalation of carbon dioxide results in inc. blood pH and a decrease in exhalation causes dec. in blood pH. The kidneys excrete H+ and reabsorb HCO3- . Acidosis is a systemic arterial blood pH of below 7.35; its principal effect is depression of CNS. Alkalosis is a systemic arterial blood pH over 7.45 and its principal effect is overexcitability of CNS. Respiratory acidosis and alkalosis are disorders due to changes in blood Pco2 ,(compensated by renal mechanisms) whereas metabolic acidosis and alkalosis are disorders associated with changes in blood HCO3- concentration. Can be compensated by respiratory mechanisms.