FLUID, ELECTROLYTE, AND ACID-BASE BALANCE

• 60% of the body weight is water
• Proportion of water in the body decreases with age
• Women typically have less water content than men
• Fluid


  Water that contains dissolved or suspended substances
• Body fluids compartments

  • Intracellular fluid (ICF)
  • Extracellular fluid (ECF)
• Extracellular fluid components

  • Intravascular fluid (plasma)
  • Interstitial fluid
  • Transcellular fluid (cerebrospinal, pleural, peritoneal, and synovial fluids)
• Electrolyte


  Separates into ions when it dissolves in water
• Cations

  • Sodium
  • Potassium
  • Calcium
  • Magnesium
• Anions


  • Chloride
  • Bicarbonate
• Anions and cations combine to make salt
• Unit of measurement for electrolytes


  Milliequivalents per liter (mEq/L) - millimoles per liter multiplied by the electrolyte charge
• Fluid concentration


  • Fluid that contains a large number of dissolved particles is more concentrated than the same amount of fluid that contains only a few particles
• Osmolality


  Number of particles per kilogram of water; determines the tonicity of a fluid
• Osmolarity


  Concentration of a solution expressed in osmoles/1L of water
• Tonicity categories

  • Hypertonic
  • Isotonic
  • Hypotonic
• Active Transport


  How cells maintain their high intracellular electrolyte concentration by moving electrolytes across cell membranes against the concentration gradient using ATP
• Diffusion


  Passive movement of particles from areas of higher concentration to areas of lower concentration
• Active Transport

  Requires energy in the form of ATP to move electrolytes across cell membranes against the concentration gradient from areas of lower concentration to areas of higher concentration
• Active Transport


  • How cells maintain their high intracellular electrolyte concentration
• Diffusion


  Passive movement of electrolytes or other particles down a concentration gradient from areas of higher concentration to areas of lower concentration
• Diffusion


  • Diffusion of electrolytes across cell membranes requires proteins that serve as ion channels
• Osmosis


  Water moves through a semipermeable membrane that separates fluid with different particle concentrations. Osmotic pressure causes water to move into the compartment with higher osmotic pressure until particle concentration is equal
• Filtration


  Fluid moves into and out of capillaries. Hydrostatic pressure and colloid osmotic pressure play roles in fluid movement between capillaries and interstitial areas
• Third Space Syndrome (3rd Space Fluid Volume Shift) occurs when fluid shifts from the vascular space into a space where it is not accessible as extracellular fluid
• Fluid Balance or Homeostasis


  Dynamic interplay of fluid intake and absorption, fluid distribution, and fluid output. Fluid intake must equal output
• Thirst is an important regulator of fluid intake when plasma osmolality increases or blood volume decreases
• Fluid Distribution


  Movement of fluid among its various components. Fluid distribution occurs between intracellular fluid (ICF) and extracellular fluid (ECF) by osmosis, and between the vascular and interstitial parts of ECF
• Fluid Output


  Occurs through the skin, lungs, GI tract, and kidneys. Approximately 3 to 6 L of fluid moves into the GI tract daily and returns to the ECF. An average adult normally excretes only 100mL of fluid
• ECF occurs


  By osmosis
• Fluid output

  Occurs through four organs: the skin, lungs, GI tract, and kidneys
• Fluid output

  Approximately 3 to 6 L of fluid moves into the GI tract daily and returns to the ECF
• Fluid output

  Average adult normally excretes only 100mL of fluid each day through feces
• Insensible water loss through the skin and lungs is continuous
• Insensible water loss


  Increases when a person has a fever or a recent burn
• Abnormal fluid output

  Vomiting, wound drainage, or hemorrhage
• Antidiuretic Hormone


  • Regulates the osmolality of the body fluids by influencing how much water is excreted in urine
  • Causes renal cells to reabsorb water, concentrating the urine while diluting the blood by adding water to it
  • ADH levels increase if body fluids become more concentrated; ADH levels decrease if body fluids become too dilute
• Renin-Angiotensin-Aldosterone System

  • Regulates ECF volume by influencing how much sodium and water are excreted in urine
  • Contributes to regulation of blood pressure
  • Renin converts angiotensinogen to angiotensin I, which is converted to angiotensin II by other enzymes in the lung capillaries
  • Angiotensin II causes vasoconstriction in some vascular beds and stimulates aldosterone release from the adrenal cortex
  • Aldosterone resorbs sodium and water in isotonic proportion in the distal renal tubules, increasing the volume of the ECF and increasing urinary excretion of potassium and hydrogen ions
  • Atrial Natriuretic Peptide regulates ECV by influencing how much sodium and water are excreted in urine, increases the loss of sodium and water in the urine, and opposes the effect of aldosterone
• Factors contributing to fluid, electrolyte, and acid imbalances

  • Vomiting
  • Diarrhea
  • Diaphoresis
  • Use of diuretics
  • Stress chronic illness
  • Renal failure
  • Surgery
  • Pregnancy
  • Respiratory failure
• Thirst Mechanism

  1. Primary regulator of fluid intake

  2. The kidneys produce about 1500mL of urine output/day

  3. Remaining output is excreted from the skin, lungs, and GI tract
• Fluid Output

  1. Sensible water loss: 1500mL/day of urine

  2. Insensible water loss: skin (perspiration) 400mL/day, lungs 400mL/day, GI tract 100mL/day