ch 26

• Body fluid compartments

  •Intracellular fluid (ICF)

  •~66% of total body water found here

  
  

  •Extracellular fluid (ECF)

  •~33% of remaining body water found here
• Extracellular fluid (ECF) sub-compartments

  • Plasma
  • Interstitial fluid (IF)
• Examples of extracellular fluids

  • Lymph
  • Cerebrospinal fluid
  • Humors
  • Serous fluid
  • Synovial fluid
• Electrolytes

  Anything that dissociate into ions in water, with (+) or (-) charge

  •Most abundant solutes

  •More responsible for fluid shifts/movement of water

  •Ex: inorganic salts, acids & bases, some proteins
• Non-electrolytes

  Do not dissociate in water, no charge, make up the bulk of body fluids
• Examples of non-electrolytes

  • Glucose
  • Urea
  • Lipids
• Fluid movement

  Water moves freely, solutes do not
• Changing the osmolality of one compartment


  Leads to net water flow
• Optimal body water content

  Depends on age, body mass, sex, and body fat %
• Sources of water intake

  • Ingested food & liquid
  • Metabolic water
• Sources of water output

  • Insensible water loss (lungs, skin)
  • Sensible water loss (sweat, urine, feces)
• When properly hydrated, water intake = water output
• Hypothalamic thirst center


  Controls the thirst mechanism
• Factors that activate the thirst mechanism


  • Osmoreceptors detecting changing ECF osmolality
  • Dry mouth/salivary glands cannot draw water from blood
  • Decreasing blood volume/pressure (~5-10% drop)
• Feelings of thirst stop almost as soon as we drink water
• The body will always lose water, even if we never drink water (obligatory water loss)

  •Urine output depends on fluid intake, diet, other sources of water loss

  •Excess water is eliminated in urine
• ADH


  Causes aquaporins to be inserted in collecting ducts
• Osmoreceptors of hypothalamus


  Monitor osmolality of ECF to inhibit or stimulate ADH release
• Baroreceptors


  Monitor blood pressure to inhibit or stimulate ADH release
• Central diabetes insipidus


  Decrease in ADH produced by hypothalamus or released by posterior pituitary
• Nephrogenic diabetes insipidus


  ADH is produced and released in normal amounts, but the kidneys are unresponsive to it
• Electrolytes


  Influence water movement in body, essential for excitability, membrane permeability
• Salt intake comes mostly from diet, with small amount coming from metabolic processes
• Sources of salt loss

  • Urine & feces
  • Sweat
  • Vomit
• Renal processes help body retain what electrolytes are needed
• Sodium

  NaHCO3 and NaCl account for ~280 mOsm of total ECF solute, key player in maintaining ECF volume
• Changing Na+ levels


  Affects blood plasma volume and blood pressure
• Aldosterone


  Release causes increased reabsorption of Na+ in DCT & collecting ducts, side effect is increase in ECF volume
• Atrial Natriuretic peptide (ANP)


  Release causes decreased reabsorption of Na+, is diuretic and natriuretic
• Hormones regulating Na+ balance

  • Aldosterone
  • Atrial Natriuretic peptide (ANP)
  • Sex hormones (estrogen, progesterone)
  • Glucocorticoids
• Hypernatremia


  Na+ serum value is >145 mEq/L, caused by dehydration or excessive IV NaCl, effects include thirst, twitching, confusion, coma, death if untreated
• Hyponatremia


  Na+ serum value is <135 mEq/L, caused by overhydration, excessive solute loss, excessive water loss, effects include mental confusion, giddiness, muscular twitching, irritability, convulsions, coma, decreased blood volume/pressure
• Potassium


  Heavy regulation due to affect on resting membrane potential, hypokalemia and hyperkalemia can disrupt neuromuscular electrical conduction, also acts as a buffer
• Potassium secretion


  Depends on plasma concentration and aldosterone
• The kidneys are very limited in their ability to reabsorb potassium
• Acidosis


  pH 7.35 or lower
• Alkalosis


  pH 7.45 or higher
• Sources of H+ in the body


  • Ingested food
  • Metabolic processes (lactic acid, CO2 loading, phosphoric acid, etc.)
• Chemical buffer systems


  One or more compounds that resist changes in pH when strong acids or bases are introduced
• Bicarbonate buffer system


  Mixture of carbonic acid (weak acid) and bicarbonate salt (weak base), important for ECF