Body Fluid Changes & Electrolytes PDF

FUNDAMENTAL CONCEPTS 1. AMOUNT & COMPOSITION OF BODY FLUIDS 2. REGULATION OF BODY FLUID COMPARTMENT 3. SYSTEMIC ROUTES OF GAINS AND LOSES 4. LABORATORY TESTS FOR EVALUATING FLUID STATUS 5. HOMEOSTATIC MECHANISM 6. GERONTOLOGICAL CONSIDERATIONS AMOUNT & COMPOSITION OF BODY FLU...

FUNDAMENTAL CONCEPTS 1. AMOUNT & COMPOSITION OF BODY FLUIDS 2. REGULATION OF BODY FLUID COMPARTMENT 3. SYSTEMIC ROUTES OF GAINS AND LOSES 4. LABORATORY TESTS FOR EVALUATING FLUID STATUS 5. HOMEOSTATIC MECHANISM 6. GERONTOLOGICAL CONSIDERATIONS AMOUNT & COMPOSITION OF BODY FLUIDS  Approximately 60% of a typical adult’s weight consists of fluid (water and electrolytes). Factors influencing the amount of body fluid are: age, gender, body fat Younger people have a higher percentage of body fluid than older adults, and men have proportionately more body fluid than women. People who are obese have less fluid than those who are thin, because fat cells contain little water. The skeleton also has low water content. Muscle, skin, and blood contain the highest amounts of water TWO FLUID COMPARTMENTS 1. EXTRACELLULAR 2. INTRACELLULAR EXTRACELLULAR COMPARTMENTS 1. INTRAVASCULAR 2. INTERSTITIAL 3. TRANSCELLULAR  INTRAVASCULAR SPACE FLUID COMPONENTS 1. PLASMA – 3L of the average 6L of blood volume is made up of plasma 2. ERYTHROCYTES 3. LEUCOCYTES remaining 3L 4. THROMBOCYTES INTERSTITIAL SPACE Approximately 11-12L of fluids in adult Lymph is an interstitial fluid TRANSCELLULAR SPACE Smallest division of the ECF compartment – contains approx. 1L Cerebrospinal fluids, pericardial, synovial, intraocular, pleural fluids, and digestive secretions. “THIRD SPACE FLUID SHIFT” PHASES OF THIRD SPACING Etiology Increased capillary permeability Increased capillary hydrostatic pressure Na depletion Increased fluid volume Albumin losses Lymphatic system obstruction 2 PHASES LOSS PHASE - This phase lasts 24 to 72 hours after the initial insult that led to the increased capillary permeability REABSORPTION PHASE - tissues begin to heal and fluid is transported back into the intravascular space. Signs of hypovolemia resolve, urine output increases, the patient's weight stabilizes, and signs of shock (if any) begin to reverse. “THIRD SPACE FLUID SHIFT” Loss of ECF into a space that does not contribute to equilibrium between the ICF and the ECF Example of third-space fluid shift: decrease in urine output despite adequate fluid intake. Urine output decreases because fluid shifts out of the intravascular space; the kidneys then receive less blood and attempt to compensate by decreasing urine output. Other signs and symptoms of third spacing that indicate an intravascular fluid volume deficit include: increased heart rate, decreased blood pressure, decreased central venous pressure, edema, increased body weight, imbalances in fluid intake and output (I&O). Third-space shifts occur in patients who have hypocalcemia, decreased iron intake, severe liver diseases, alcoholism, hypothyroidism, malabsorption, immobility,, burns, and cancer  Other signs and symptoms of third spacing that indicate an intravascular fluid volume deficit include: increased heart rate, decreased blood pressure, decreased central venous pressure, edema, increased body weight, imbalances in fluid intake and output (I&O). Third-space shifts occur in patients who have hypocalcemia, decreased iron intake, severe liver diseases, alcoholism, hypothyroidism, malabsorption, immobility,, ELECTROLYTES Electrolytes in body fluids are active chemicals (cations that carry positive charges and anions that carry negative charges). CATATIONS sodium, potassium, calcium, magnesium, and hydrogen ions. ANIONS chloride, bicarbonate, phosphate, sulfate, and proteinate ions. CATION & ANION chemicals bind together , and it is expressed in terms of milliequivalents (mEq)/L , a measure of chemical activity, rather than in terms of milligrams (mg), a unit of weight. Defined as being equivalent to the electrochemical activity of 1 mg of hydrogen. Therefore, they are equal to milliequivalent/liter  Electrolyte concentrations in the ICF differ from those in the ECF. Sodium ions, which are positively charged, far outnumber the other cations in the ECF. Because sodium concentration affects the overall concentration of the ECF, sodium is important in regulating the volume of body fluid. Retention of sodium is associated with fluid retention, and excessive loss of sodium is usually associated with decreased volume of body fluid.  The ECF has a low concentration of potassium and can tolerate only small changes in potassium concentrations. The major electrolytes in the ICF are potassium and phosphate. Therefore, release of large stores of intracellular potassium, typically caused by trauma to the cells and tissues, can be extremely dangerous. HYDROSTATIC VS OSMOTIC PRESSURE Normal movement of fluids through the capillary wall into the tissues depends on hydrostatic pressure (the pressure exerted by the fluid on the walls of the blood vessel) at both the arterial and the venous ends of the vessel and the osmotic pressure exerted by the protein of plasma. The direction of fluid movement depends on the differences in these two opposing forces (hydrostatic versus osmotic pressure). REGULATION OF BODY FLUID COMPARTMENT OSMOSIS & OSMOLALITY OSMOSIS - This diffusion of water caused by a fluid concentration gradient. Tonicity is the ability of all the solutes to cause an osmotic driving force that promotes water movement from one compartment to another. Other terms associated with OSMOSIS: Osmotic pressure Oncotic pressure Osmotic diuresis Osmotic pressure Is the amount of hydrostatic pressure needed to stop the flow of water by osmosis. It is primarily determined by the concentration of solutes. Oncotic pressure Is the osmotic pressure exerted by proteins (eg, albumin). Osmotic diuresis Is the increase in urine output caused by the excretion of substances such as glucose, mannitol, or contrast agents in the urine. DIFFUSION Diffusion is the natural tendency of a substance to move from an area of higher concentration to one of lower concentration. It occurs through the random movement of ions and molecules Example: The exchange of oxygen and carbon dioxide between the pulmonary capillaries and alveoli. The tendency of sodium to move from the ECF compartment, where the sodium concentration is high, to the ICF, where its concentration is low. Filtration Hydrostatic pressure in the capillaries tends to filter fluid out of the intravascular compartment into the interstitial fluid. The movement of water and solutes occurs from an area of high hydrostatic pressure to an area of low hydrostatic pressure. Example: The kidneys filter approximately 180 L of plasma per day. The passage of water and electrolytes from the arterial capillary bed to the interstitial fluid; in this instance, the hydrostatic pressure results from the pumping action of the heart. Sodium–Potassium Pump Since sodium concentration is greater in the ECF than in the ICF, sodium then tends to enter the cell by diffusion. This tendency is offset by the sodium–potassium pump that is maintained by the cell membrane and actively moves sodium from the cell into the ECF. The high intracellular potassium concentration is maintained by pumping potassium into the cell. By definition, active transport implies that energy must be expended for the movement to occur against a concentration gradient. Systemic Routes of Gains and Losses When fluid balance is critical, all routes of systemic gain and loss must be recorded and all volumes compared. Organs of fluid loss include the kidneys, skin, lungs, and gastrointestinal (GI) tract. Kidneys The usual daily urine volume in the adult is 1 to 2 L. A general rule is that the output is approximately 1 mL of urine per kilogram of body weight per hour (1 mL/kg/h) in all age groups. Or 25-30 mls/hr. Observe UOP closely especially for patients with medical problems. Such as: Cardiac patients Renal patients Post-op patients Skin Sensible perspiration refers to visible water and electrolyte loss through the skin (sweating). The chief solutes in sweat are sodium, chloride, and potassium. Actual sweat losses can vary from 0 to 1000 mL or more every hour, depending on factors such as the environmental temperature. Continuous water loss by evaporation (approximately 600 mL/day) occurs through the skin as insensible perspiration, a nonvisible form of water loss. Fever greatly increases insensible water loss through the lungs and the skin, as does loss of the natural skin barrier (eg, through major burns). Lungs The lungs normally eliminate water vapor (insensible loss) at a rate of approximately 300 mL every day. The loss is much greater with increased respiratory rate or depth, or in a dry climate. Gastrointestinal Tract The usual loss through the GI tract is 100 to 200 mL daily, even though approximately 8 L of fluid circulates through the GI system every 24 hours. Because the bulk of fluid is normally reabsorbed in the small intestine, diarrhea and fistulas cause large losses. Laboratory Tests for Evaluating Fluid Status Osmolality is the concentration of fluid that affects the movement of water between fluid compartments by osmosis. Osmolality measures the solute concentration per kilogram in blood and urine. It is also a measure of a solution’s ability to create osmotic pressure and affect the movement of water. Serum osmolality primarily reflects the concentration of sodium, although blood urea nitrogen (BUN) and glucose also play a major role in determining serum osmolality. Urine osmolality is determined by urea, creatinine, and uric acid. When measured with serum osmolality, urine osmolality is the most reliable indicator of urine concentration. Osmolality is reported as milliosmoles per kilogram of water (mOsm/kg) In healthy adults, Normal serum osmolality is 280 to 300 mOsm/kg, Normal urine osmolality is 200 to 800 mOsm/kg Sodium predominates in ECF osmolality and holds water in this compartment. Serum osmolality may be measured directly through laboratory tests or estimated at the bedside by doubling the serum sodium level or by using the following formula: Urine specific gravity Normal range: 1.010 – 1.025 Urine specific gravity measures the kidneys’ ability to excrete or conserve water. The specific gravity of urine is compared to the weight of distilled water, which has a specific gravity of 1.000. Urine specific gravity can be measured at the bedside by placing a calibrated hydrometer or urinometer in a cylinder of approximately 20 mL of urine. It can also be assessed with a refractometer or dipstick with a reagent for this purpose. Specific gravity varies inversely with urine volume; normally, the larger the volume of urine, the lower the specific gravity is. Specific gravity is a less reliable indicator of concentration than urine osmolality; increased glucose or protein in urine can cause a falsely elevated specific gravity. Factors that increase or decrease urine osmolality are the same as those for urine specific gravity.  Specific gravity varies inversely with urine volume; normally, the larger the volume of urine, the lower the specific gravity is. Specific gravity is a less reliable indicator of concentration than urine osmolality; increased glucose or protein in urine can cause a falsely elevated specific gravity. Factors that increase or decrease urine osmolality are the same as those for urine specific gravity. BUN BUN is made up of urea, which is an end product of the metabolism of protein (from both muscle and dietary intake) by the liver. Amino acid breakdown produces large amounts of ammonia molecules, which are absorbed into the bloodstream. Ammonia molecules are converted to urea and excreted in the urine. The normal BUN is 10 to 20 mg/dL (3.6 to 7.2 mmol/L). The BUN level varies with urine output. Factors that increase BUN include: decreased renal function, GI bleeding, dehydration, increased protein intake, fever, and sepsis. Those that decrease BUN include end-stage liver disease, a low-protein diet, starvation, and any condition that results in expanded fluid volume (eg, pregnancy). Creatinine Creatinine is the end product of muscle metabolism. It is a better indicator of renal function than BUN because it does not vary with protein intake and metabolic state. The normal serum creatinine is approximately 0.7 to 1.4 mg/dL (62 to 124 mmol/L); however, its concentration depends on lean body mass and varies from person to person. Serum creatinine levels increase when renal function decreases. Hematocrit Hematocrit measures the volume percentage of red blood cells (erythrocytes) in whole blood and normally ranges from 42% to 52% for males and 35% to 47% for females. Conditions that increase the hematocrit value are dehydration and polycythemia, and those that decrease hematocrit are overhydration and anemia. Urine sodium values change with sodium intake and the status of fluid volume: As sodium intake increases, excretion increases; As the circulating fluid volume decreases, sodium is conserved. Normal urine sodium levels range from 75 to 200 mEq/24 hours (75 to 200 mmol/24 hours). A random specimen usually contains more than 40 mEq/L of sodium. Urine sodium levels are used to assess volume status and are useful in the diagnosis of hyponatremia and acute renal failure. Homeostatic Mechanisms The body is equipped with remarkable homeostatic mechanisms to keep the composition and volume of body fluid within narrow limits of normal. Organs involved in homeostasis include: the kidneys, lungs, heart, adrenal glands, parathyroid glands, and pituitary gland Kidney Functions Vital to the regulation of fluid and electrolyte balance, the kidneys normally filter 180 L of plasma every day in the adult and excrete 1 to 2 L of urine. They act both autonomously and in response to bloodborne messengers, such as aldosterone and antidiuretic hormone (ADH) Major functions of the kidneys in maintaining normal fluid balance include the following: Regulation of ECF volume and osmolality by selective retention and excretion of body fluids Regulation of normal electrolyte levels in the ECF by selective electrolyte retention and excretion Regulation of pH of the ECF by retention of hydrogen ions Excretion of metabolic wastes and toxic substances Failure of the kidneys results in multiple fluid and electrolyte abnormalities. Heart and Blood Vessel Functions The pumping action of the heart circulates blood through the kidneys under sufficient pressure to allow for urine formation. Failure of this pumping action interferes with renal perfusion and thus with water and electrolyte regulation. Lung Functions The lungs are also vital in maintaining homeostasis. Through exhalation, the lungs remove approximately 300 mL of water daily in the normal adult. Abnormal conditions, such as hyperpnea (abnormally deep respiration) or continuous coughing, increase this loss; mechanical ventilation with excessive moisture decreases it. The lungs also play a major role in maintaining acid–base balance. Pituitary Functions The hypothalamus manufactures ADH, which is stored in the posterior pituitary gland and released as needed to conserve water. Functions of ADH include maintaining the osmotic pressure of the cells by controlling the retention or excretion of water by the kidneys and by regulating blood volume Adrenal Functions Aldosterone, a mineralocorticoid secreted by the zona glomerulosa (outer zone) of the adrenal cortex, has a profound effect on fluid balance. Increased secretion of aldosterone causes sodium retention (and thus water retention) and potassium loss. Decreased secretion of aldosterone causes sodium and water loss and potassium retention. Cortisol, another adrenocortical hormone, has less mineralocorticoid action. However, when secreted in large quantities (or administered as corticosteroid). Parathyroid Functions The parathyroid glands, embedded in the thyroid gland, regulate calcium and phosphate balance by means of parathyroid hormone (PTH). PTH influences bone resorption, calcium absorption from the intestines, and calcium reabsorption from the renal tubules. Other Mechanisms Changes in the volume of the interstitial compartment within the ECF can occur without affecting body function. However, the vascular compartment cannot tolerate change as readily and must be carefully maintained to ensure that tissues receive adequate nutrients. Baroreceptors Renin–Angiotensin–Aldosterone System Antidiuretic Hormone and Thirst Osmoreceptors Release of Atrial Natriuretic Peptide BARORECEPTORS Located in the left atrium and the carotid and aortic arches. These receptors respond to changes in the circulating blood volume and regulate sympathetic and parasympathetic neural activity as well as endocrine activities. BARORECEPTORS As arterial pressure decreases, baroreceptors transmit fewer impulses from the carotid and the aortic arches to the vasomotor center. A decrease in impulses stimulates the sympathetic nervous system and inhibits the parasympathetic nervous system. The outcome is an increase in cardiac rate, conduction, and contractility and an increase in circulating blood volume. Sympathetic stimulation constricts renal arterioles; this increases the release of aldosterone, decreases glomerular filtration, and increases sodium and water reabsorption. Renin–Angiotensin–Aldosterone System Renin is an enzyme that converts angiotensinogen, a substance formed by the liver, into angiotensin I. Renin is released by the juxtaglomerular cells of the kidneys in response to decreased renal perfusion. Angiotensin-converting enzyme (ACE) converts angiotensin I to angiotensin II. Angiotensin II, with its vasoconstrictor properties, increases arterial perfusion pressure and stimulates thirst.  As the sympathetic nervous system is stimulated, aldosterone is released in response to an increased release of renin. Aldosterone is a volume regulator and is also released as serum potassium increases, serum sodium decreases, or adrenocorticotropic hormone (ACTH) increases. Antidiuretic Hormone and Thirst ADH and the thirst mechanism have important roles in maintaining sodium concentration and oral intake of fluids. Oral intake is controlled by the thirst center located in the hypothalamus. As serum concentration or osmolality increases or blood volume decreases, neurons in the hypothalamus are stimulated by intracellular dehydration; thirst then occurs, and the person increases his or her intake of oral fluids. Water excretion is controlled by ADH, aldosterone, and baroreceptors, as mentioned previously. The presence or absence of ADH is the most significant factor in determining whether the urine that is excreted is concentrated or dilute. Osmoreceptors Located on the surface of the hypothalamus, osmoreceptors sense changes in sodium concentration. As osmotic pressure increases, the neurons become dehydrated and quickly release impulses to the posterior pituitary, which increases the release of ADH, which then travels in the blood to the kidneys, where it alters permeability to water, causing increased reabsorption of water and decreased urine output. The retained water dilutes the ECF and returns its concentration to normal. Restoration of normal osmotic pressure provides feedback to the osmoreceptors to inhibit further ADH release Release of Atrial Natriuretic Peptide Atrial natriuretic peptide (ANP), also called atrial natriuretic factor, is a peptide that is synthesized, stored, and released by muscle cells of the atria of the heart in response to several factors. These factors include: increased atrial pressure, angiotensin II stimulation, endothelin (a powerful vasoconstrictor of vascular smooth muscle peptide released from damaged endothelial cells in the kidneys or other tissues), and sympathetic stimulation.  Any condition that results in volume expansion (exercise, pregnancy), hypoxia, or increased cardiac filling pressures (e.g., high sodium intake, heart failure, chronic renal failure, atrial tachycardia, or use of vasoconstrictor agents such as epinephrine) increases the release of ANP. The action of ANP is the direct opposite of the renin– angiotensin– aldosterone system. ANP decreases blood pressure and volume.  The ANP measured in plasma is normally 20 to 77 pg./mL (20 to 77 ng/L). This level increases in conditions like: acute heart failure, paroxysmal supraventricular tachycardia, hyperthyroidism, subarachnoid hemorrhage, small cell lung cancer. The level decreases in chronic heart failure and with the use of medications such as urea (Ureaphil) and prazosin (Minipress). Gerontologic Considerations Normal physiologic changes of aging, including reduced cardiac, renal, and respiratory function and reserve and alterations in the ratio of body fluids to muscle mass, may alter the responses of elderly people to fluid and electrolyte changes and acid–base disturbances. Decreased respiratory function can cause impaired pH regulation in older adults with major illness or trauma. Renal function declines with age, as do muscle mass and daily exogenous creatinine production. Therefore, high-normal and minimally elevated serum creatinine values may indicate substantially reduced renal function in older adults  multiple medications by older adults can affect renal and cardiac function, thereby increasing the likelihood of fluid and electrolyte disturbances. Routine procedures, such as the vigorous administration of laxatives or enemas before colon x-ray studies, may produce a serious fluid volume deficit, necessitating the use of intravenous (IV) fluids to prevent hypotension and other effects of hypovolemia. Alterations in fluid and electrolyte balance that may produce profound changes in older adults. The clinical manifestations of fluid and electrolyte disturbances may be subtle or atypical. For example, fluid deficit may cause delirium in the elderly person Rapid infusion of an excessive volume of IV fluids may produce fluid overload and cardiac failure in elderly patients. Dehydration in the elderly is common as a result of decreased kidney mass, decreased glomerular filtration rate, decreased renal blood flow, decreased ability to concentrate urine, inability to conserve sodium, decreased excretion of potassium, and a decrease of total body water END OF TOPIC