Acid-Base Balance

• Newborn baby's body weight

  About 75% water
• Young men's average body water

  55% to 60%
• Women's average body water

  Slightly less than young men
• Obese and elderly people's body water

  As little as 45% by weight
• Total body water (TBW) of a 70 kg (150 lb) young male

  About 40 L
• Homeostasis


  Cellular function requires a fluid medium with a carefully controlled composition
• Three types of homeostatic balance

  • Fluid Balance
  • Electrolyte balance
  • Acid–base balance
• Fluid compartments of the body

  • 65% intracellular fluid (ICF)
  • 35% extracellular fluid (ECF)
  • 25% tissue (interstitial) fluid
  • 8% blood plasma and lymphatic fluid
  • 2% transcellular fluid

  
• Fluid continually exchanged between compartments; H20 moves by osmosis
• H20 moves easily through membranes, osmotic gradients never last long
• Osmosis restores balance within seconds, so ICF & ECF osmolarity are equal
• If osm of tissue fluid increases

  H2O moves out of cell
• If osm of tissue fluid decreases

  H2O moves into cell
• Electrolytes


  Most abundant solute particles
• Sodium salts


  Most abundant in ECF
• Potassium salts


  Most abundant in ICF
• Electrolytes


  Principal role in controlling H2O distribution and total H2O content
• Net filtration occurs at arterial end of capillary since capillary hydrostatic pressure (CHP) > blood colloidal osmotic pressure (BCOP)

  
• No net movement of fluid near midpoint where CHP = BCOP
• Net reabsorption occurs at venous end of capillary since BCOP > CHP
• Balance in tissue drains to lymphatics
• Units (U)


  Used in insulin, heparin, and pitocin dosages
• International Units (IU)


  Used for vitamins, such as Vitamin D 600 IU
• Milliequivalents (mEq)


  Used for electrolyte replacement, such as Potassium 40 mEq
• Percentages


  Used for (IV) fluids, such as 0.9% Normal Saline IV fluid
• Ratios


  Used in medications such as Epinephrine 1:1000
• Milliequivalent (mEq)


  Unit of measure for an electrolyte, describes electrolytes' ability to combine and form other compounds
• 1 mEq of any cation will react with 1 mEq of an anion
• Total [cations] always equals total [anions]
• Dissolving 40 mEq of KCL in water

  Results in a solution containing 40 mEq of K+/L and 40 mEq of Cl-/L
• Osmole (osm)


  1 mole of dissolved particles, takes into account whether solute ionizes in water
• Osmolality


  Solution's ability to create osmotic pressure & affect movement of water, # of osmoles/L of solution
• Plasma osmolality


  280-300 mOsm/kg
• ECF osmolality


  Determined by Na+
• Serum osmolality


  Can be estimated by doubling serum Na+
• Urine specific gravity


  Measures kidney's ability to secrete or conserve water, normal range 1.003 - 1.030
• Tonicity


  Ability of a surrounding solution to affect fluid volume & pressure in a cell

  
• Hypotonic solution


  Causes cell to absorb water and swell, has a lower concentration of nonpermeating solutes than intracellular fluid (ICF)

  
• Hypertonic solution


  Causes cell to lose water and shrivel (crenate), has a higher concentration of nonpermeating solutes than ICF
• Isotonic solution


  Causes no change in cell volume, concentrations of nonpermeating solutes in bath and ICF are the same