PATHOPHYSIOLOGY Name Chapter 4: Fluids and Electrolytes, Acids and Bases I. Fluid and Electrolyte Balance A. Distribution of Body Fluids 1. Total body water (TBW) - 60% of body weight, on average, for a normal adult male. a. Fluid compartments Intracellular fluid (2/3 of total body water or about 28 L) Extracellular fluid (1/3 of total body water or about 14L) o Interstitial fluid (about 11L) o Intravascular fluid (about 3L) o Lymph, synovial, intestinal, CSF, sweat, urine, pleural, peritoneal, pericardial, and intraocular fluids (less than 1L) b. Pediatrics 75% to 80% of body weight Susceptible to significant changes in body fluids o Dehydration c. Aging The percentage of total body weight that is fluid decreases. o Increase adipose and decrease muscle mass o Renal decline o Diminished thirst perception 2. Water Movement Between the Intravascular and Interstitial Spaces Net filtration = forces favoring filtration minus forces opposing filtration (Starling’s hypothesis) o Forces favoring filtration into interstitial spaces . Capillary hydrostatic pressure (blood pressure) . Interstitial oncotic pressure (water-pulling) o Forces favoring reabsorption into blood . Plasma oncotic pressure (water-pulling) . Interstitial hydrostatic pressure (pushes into blood and cells) 3. Water Movement Between the Intracellular (ICF) and Extracellular (ECF) Fluid Compartments Caused by changes in the concentration of ECF (osmotic pressure differences). If ECF becomes less concentrated (due to fluid excess or sodium deficit) water flows into cells and they swell. If ECF becomes more concentrated (due to fluid deficit or sodium excess) water flows out of cells and they shrink. 2 ACTIVITY 1: For each of the following statements, decide which direction fluid would tend to move. Choices: B = into the blood I = into the interstitial fluid C = into the cells Lower than normal sodium levels. Low blood pressure. Higher than normal sodium levels. Lower than normal levels of plasma proteins. High blood pressure. Higher than normal levels of plasma proteins. B. Alterations in Water Movement 1. Edema Accumulation of fluid within the interstitial spaces Causes o Increase in capillary hydrostatic pressure o Decrease in plasma oncotic pressure o Increases in capillary permeability o Lymph obstruction 2. Water Balance Thirst perception Antidiuretic hormone (ADH) secretion o ADH causes increased water reabsorption. o Occurs when plasma volume drops or plasma concentration (osmolality) increases. Osmolality receptors sense increased osmolality and plasma volume depletion. C. Sodium and Chloride Balance 1. Sodium Primary ECF cation Regulates osmotic forces Roles - neuromuscular irritability, acid-base balance, and cellular chemical reactions and membrane transport 2. Chloride Primary ECF anion Provides electroneutrality Levels vary inversely with those of bicarbonate 3. Sodium and Chloride Regulation Renin-angiotensin-aldosterone (RAA) system: Trigger: A decrease in blood pressure and blood flow to the kidneys (or increased K+). Step 1: Renin is released by juxtaglomerular cells of the kidney. Step 2: Renin is an enzyme that converts angiotensinogen to angiotensin I (in the blood). 3 Step 3: Angiotensin I is converted to angiotensin II (occurs in the lungs; requires angiotensin converting enzyme [ACE]). Step 4: Angiotensin II causes vasoconstriction and release of aldosterone by the adrenal cortex. Step 5: Aldosterone causes increased sodium and water retention in the kidneys. End result: An increase in blood volume and blood pressure. Atrial natriuretic hormone (peptide) (ANH) – has the opposite effect of aldosterone o ANH is secreted by the atria in response to increased blood volume (stretching). o Function: ANH decreases sodium retention so more sodium and water are excreted in the urine. This decreases blood volume and blood pressure. ACTIVITY 2: Match the hormones with their descriptions. a. ADH b. aldosterone c. atrial natriuretic hormone Causes increased retention of sodium and water when blood pressure falls. Causes more water to be reabsorbed when the plasma becomes too concentrated. Causes excretion of sodium and water when blood volume becomes too great. D. Alterations in Sodium, Chloride and Water Balance 1. Isotonic Alterations Isotonic fluid has the same water-to-electrolyte content as normal body fluid. Total body water change with proportional electrolyte and water change. a. Isotonic fluid loss (isotonic dehydration) o Causes - hemorrhage, mild vomiting, mild diarrhea or excessive sweating. o Manifestations - weight loss, dryness of skin and mucous membranes, decreased skin turgor, decreased urine output, and symptoms of hypovolemia (rapid heart rate, flattened neck veins, and normal or decreased blood pressure, and shock). b. Isotonic fluid excess o Causes – excessive administration of intravenous fluids, hypersecretion of aldosterone, or drugs such as cortisone. o Manifestations – symptoms of hypervolemia including weight gain, decreased hematocrit, distended neck veins, increase in BP, and edema. 2. Hypertonic Alterations o Increased osmolality (fluid is more concentrated) a. Hypernatremia o Serum sodium >147 mEq/L o Water moves from the ICF to the ECF, causing intracellular dehydration including shrinkage of brain cells; but there is excess extracellular fluid. o Causes - excess administration of hypertonic IV solutions, oversecretion of aldosterone. 4 o Manifestations of hypernatremia . Increased ECF causes edema and increased blood pressure. High sodium level causes muscular weakness and hyperactive reflexes. Decreased ICF causes thirst, decreased urine output, confusion, and ultimately coma. b. Pure water deficit (hypertonic dehydration) o Loss of water alone . ECF becomes more concentrated, so water moves from ICF to ECF. Both ECF and ICF become dehydrated and hypovolemia occurs. o Causes . Inability to obtain water (ex. comatose patients) . Extended hyperventilation . Increased renal free water clearance as with decreased ADH secretion (most common cause) o Manifestations . Decreased ECF causes a weak pulse, postural hypotension, and tachycardia, elevated hematocrit and elevated serum sodium level. Decreased ICF causes thirst, fever, decreased urine output, shrinkage of brain cells, confusion and coma. 3. Hypotonic Alterations o Decreased osmolality (fluid is less concentrated) a. Hyponatremia o Serum sodium level <135 mEq/L o Sodium deficit decreases the ECF osmotic pressure, and water moves into the cells. Water movement causes symptoms related to hypovolemia and cellular swelling. o Causes – inadequate intake of Na+; hypoaldosteronism; increased loss of Na+ through diuresis, profuse sweating, or gastrointestinal losses. o Manifestations – Increased ICF causes edema, brain cell swelling, irritability, depression, confusion, weakness, muscle cramps, anorexia, nausea, and diarrhea. Pure sodium deficits cause hypotension, tachycardia, and decreased urine output. b. Water excess o Free water excess causes symptoms of hypervolemia and water intoxication. o Causes – excessive administration of hypotonic intravenous solutions, drinking water to replace isotonic fluid losses, tap water enemas, psychogenic polydipsia, renal water retention, or increased antidiuretic hormone secretion. o Manifestations . Acute excesses cause swelling of brain cells, confusion and convulsions. Long-term water accumulation causes weakness, nausea, muscle twitching, headache, and weight gain. 5 ACTIVITY 3: Place an X in the appropriate squares to indicate which fluid imbalances (on left) are accompanied by the conditions listed in the top row (there should be a total of ten Xs). Hypovolemia Hypervolemia Cells shrink Cells swell Isotonic fluid loss Isotonic fluid excess Hypernatremia Pure water deficit Hyponatremia Water excess E. Alterations in Potassium 1. Potassium Major intracellular cation Concentration maintained by Na+/K+ pump Regulates intracellular electrical neutrality in relation to Na+ and H+ Essential for transmission and conduction of nerve impulses, normal cardiac rhythms, and skeletal and smooth muscle contraction. Serum levels of K+ are regulated by kidney excretion of potassium. Potassium Levels Changes in pH affect K+ balance o Hydrogen ions accumulate in the ICF during states of acidosis. K+ shifts out of cells to maintain a balance of cations across the membrane. Aldosterone and insulin influence serum potassium levels. o Aldosterone – is secreted in response to high K+ levels; causes increased movement of K+ into urine in exchange for Na+. o Insulin – stimulates cellular uptake of K+. 2. Hypokalemia Potassium level <3.5 mEq/L Potassium balance is described by changes in plasma potassium levels Causes – reduced intake of potassium, increased entry of potassium into body cells (as during alkalosis), and increased loss of potassium in diarrhea or due to diuresis from the kidneys. Loop diuretics (like Lasix) - inhibit Na+ reabsorption in the loop of Henle, and so put an excess demand on the exchange of K+ for Na+ in the distal tubule of the nephron, thus resulting in K+ loss. Manifestations o Membrane hyperpolarization causes a decrease in neuromuscular excitability, skeletal muscle weakness, smooth muscle atony, and cardiac dysrhythmias. 6 3. Hyperkalemia Potassium level >5.5 mEq/L Hyperkalemia is rare because of efficient renal excretion Caused by increased intake, shift of K+ from ICF (as during acidosis), decreased renal excretion due to renal failure, insulin deficiency, or cell trauma Mild attacks