Distribution of Body Fluids
- Body fluids are distributed amount functional compartments and are classified as intracellular fluid (ICF) and extracellular fluid (ECF).
- The sum of all fluids is the total body water (TBW), which varies with age and amount of body fat.
- Water moves between the ICF and ECF compartments principally by osmosis.
- Water moves between the plasma and interstitial fluid by osmosis (pulling of water) and hydrostatic pressure (pushing of water), which occur across the capillary membrane.
- Movement across the capillary wall is called net filtration and is described according to Starlings Law (the balance between hydrostatic and osmotic forces).
Alterations in Water Movement
- Edema is a problem of fluid distribution that results in accumulation of fluid within the interstitial spaces.
- The pathophysiologic process that leads to edema is related to an increase in forces favoring fluid filtration from the capillaries or lymphatic channels into the tissues.
- Edema is caused by arterial dilation, venous or lymphatic obstruction, increased vascular volume, or increased capillary permeability.
- Edema may be localized or generalized and usually is associated with weight gain, swelling and puffiness, tighter-fitting clothes and shoes, and limited movement of the affected area.
Sodium, Chloride, and Water Balance
- Sodium and water balance are intimately related; chloride levels are generally proportional to change in sodium levels.
- Water balance is regulated by the sensation of thirst and by antidiuretic hormone (ADH), which is secreted in response to by an increase in plasma osmolality or a decrease in circulation blood volume.
- Sodium balance is regulated by aldosterone, which increases reabsorption of sodium from the urine into the blood by the distal tubule of the kidney.
- Renin and angiotensin are enzymes that promote secretion of aldosterone and thus regulate sodium and water balance.
- Atrial natriuretic hormone also is involved in decreasing tubular reabsorption and promoting urinary excretion of sodium.
Alterations in Sodium, Chloride and Water Balance
- Alterations in water balance may be classified as isotonic, hypertonic, or hypotonic.
- Isotonic alterations occur when changes in TBW are accompanied by proportional changes in electrolytes.
- Hypertonic alterations develop when the osmolality of the ECF is elevated above normal, usually because of an increased concentration of ECF sodium or a deficit of ECF water.
- Hypernatremia (sodium levels more than 147 mEq/L) may be caused by an acute increase in sodium or a loss of water.
- Water deficit, or hypertonic dehydration, is rare but can be caused by lack of access to water, pure water losses, hyperventilation, arid climates, and increased renal elimination of water.
- Hyperchloremia is caused by an excess of sodium or a deficit of bicarbonate.
- Hypotonic alterations occur when the osmolality of the ECF is less than normal.
- Hyponatremia (serum sodium concentration less than 135 mEq/L) usually causes movement of water into cells.
- Hyponatremia may be caused by sodium loss, inadequate sodium intake, or dilution of the body’s sodium level with excess water.
- Water excess is rare but can be caused by compulsive water drinking, decreased urine formation, or the syndrome of inappropriate secretion of ADH (SIADH).
- Hypochloremia usually is the result of hyponatremia or elevated bicarbonate concentrations.
Alterations in Potassium and Other Electrolytes
- Potassium is the predominant ICF ion; it regulates ICF osmolality, maintains the resting membrane potential, and is required for deposition of glycogen in liver and skeletal muscle cells.
- Potassium balance is regulated by the kidney, by aldosterone and insulin secretion, and by changes in pH.
- The mechanism of potassium tolerance or adaptation allows the body to accommodate slowly to increased levels of potassium intake.
- Hypokalemia (serum potassium concentration less than 3.5 mEq/L) indicates loss of total body potassium, although ECF hypokalemia can develop without losses of total body potassium, and plasma potassium levels may be normal or elevated when total body potassium is depleted.
- Hypokalemia may be caused by reduced potassium intake, a shift from ECF to ICF potassium, increased aldosterone and increased renal excretion.
- Hyperkalemia (potassium levels that are more than 5.5 mEq/L) may be caused by increased potassium intake, a shift from ICF to ECF potassium, or decreased renal excretion.
- Calcium is a necessary ion in the structure of bones and teeth, in blood clotting, in hormone secretion and the function of cell receptors, and in membrane stability.
- Phosphate acts as a buffer in acid-base regulation and provides energy for muscle contraction.
- Calcium and phosphate concentrations are rigidly controlled by parathyroid hormone (PTH), vitamin D, and calcitonin.
- Hypocalcemia (serum calcium concentration less than 8.5 mg/dl) is related to inadequate intestinal absorption, deposition of calcium into bone or soft tissue, blood administration, or decreased PTH and vitamin D levels.
- Hypercalcemia (serum calcium concentration more than 12 mg/dl) can be caused by a number of diseases, including hyperparathyroidism, bone metastases, sarcoidosis, and excess vitamin D.
- Hypophosphatemia is usually caused by intestinal malabsorption and increased renal excretion of phosphate.
- Hyperphosphatemia develops with acute or chronic renal failure when there is significant loss of glomerular filtration.
- Magnesium is a major intracellular cation and is regulated principally by PTH.
- Magnesium functions in enzymatic reactions and often interacts with calcium at the cellular level.
- Hypomagnesemia (serum magnesium concentrations less than 1.5 mEq/L) may be caused by malabsorption syndromes.
- Hypermagnesemia (serum magnesium concentrations more than 2.5 mEq/L) is rare is usually is caused by renal failure.
- Hydrogen ions, which maintain membrane integrity and the speed of enzymatic reactions, must be concentrated within a narrow range if the body is to function normally.
- Hydrogen ion concentration [H+] is expressed as pH, which represent the negative logarithm (i.e. 10-7) of hydrogen ions in solution (i.e. .0000001).
- Different body fluids have different pH values.
- The renal and respiratory systems, together with the body’s buffer systems, are the principal regulators of acid-base balance.
- Buffers are substances that can absorb excessive acid or base without a significant change in pH.
- Buffers exist as acid-base pairs; the principal plasma buffers are carbonic acid-bicarbonate, protein (hemoglobin), and phosphate.
- The lungs and kidneys act to compensate for changes in pH by increasing or decreasing ventilation and by producing more acidic or more alkaline urine.
- Correction is a process different from compensation’ correction occurs when the values for both components of the buffer pair return to normal.
- Acid-base imbalances are caused by changes in the concentration of hydrogen in the blood’ an increase causes acidosis, and a decrease causes alkalosis.
- An abnormal increase or decrease in bicarbonate concentration causes metabolic alkalosis or metabolic acidosis; changes in the rate of alveolar ventilation and removal of carbon dioxide produce respiratory acidosis or respiratory alkalosis.
- Metabolic acidosis is caused by an increase in non-carbonic acids or loss of bicarbonate from the extracellular fluid.
- Metabolic alkalosis occurs with an increase in bicarbonate usually caused by loss of metabolic acids from conditions such as vomiting or gastrointestinal suctioning from excessive bicarbonate intake, hyperaldosteronism, and diuretic therapy, which increase plasma bicarbonate.
- Respiratory acidosis occurs with a decrease in alveolar ventilation, which in turn causes hypercapnia (an increase in carbon dioxide) and increases in carbonic acid concentration.
- Respiratory alkalosis occurs with alveolar hyperventilation and excessive reduction of carbon dioxide, or hypocapnia with decreases in carbonic acid.