Electrolytes

• Fluid compartments and fluid balance

  The water and dissolved solutes in the body constitute the body fluids

  
• Intracellular fluid (ICF)


  About two-thirds of the body's fluid is located within cells
• Extracellular fluid (ECF)


  The other 1/3 includes all other body fluids
• Interstitial fluid


  Occupies the microscopic spaces between tissue cells, is about 80% of the ECF

  
• Blood plasma


  The liquid portion of the blood, is about 20% of the ECF
• Fluid balance


  The various body compartments contain the normal amount of water and solutes
• Total body mass


  About 45–70% of lean adults is water
• Electrolyte


  An inorganic substance that dissociates into ions in solution
• Daily water gain and loss are each about 2500ml
• Sources of water gain


  • Ingested liquids and foods
  • Water produced by metabolic reactions (metabolic water)
• Ways water is lost from the body

  • Urine
  • Feces
  • Sweat
  • Respiration
  • Insensible water loss

  
• Thirst center


  In the hypothalamus, governs the urge to drink
• Aldosterone


  Reduces urinary loss of Na+ and Cl- and thereby increases the volume of body fluids
• Atrial natriuretic peptide (ANP)


  Promotes natriuresis, elevated excretion of Na+ (and Cl-) and water, which decreases blood volume

  
• Plasma membrane


  Separates intracellular fluid from the surrounding interstitial fluid, is a selectively permeable barrier
• Blood vessel walls


  Separate the interstitial fluid from blood plasma, only in capillaries are the walls thin enough and leaky enough to permit the exchange of water and solutes between blood plasma and interstitial fluid
• Dehydration stimulates thirst


  Regulation occurs via negative feedback
• Excessive aldosterone secretion


  Causes edema
• Factors that maintain body water balance

  • Thirst
  • Antidiuretic hormone
  • Aldosterone
  • Atrial natriuretic peptide

  
• Electrolytes

  Control osmosis of water between fluid compartments, help maintain acid base balance, carry electrical current, act as enzyme cofactors
• Major electrolytes


  Na+ Cl- K+
• Sodium ions (Na+)


  Most abundant extracellular ions, involved in action potentials, muscle contraction, fluid electrolyte balance

  
• Chloride ions (Cl-)


  Major extracellular anions, play a role in regulating osmotic pressure and forming HCl in gastric juice

  
• Potassium ions (K+)


  Most abundant cations in intracellular fluid, play a key role in establishing the resting membrane potential in neurons and muscle fibers, and contribute to regulation of pH
• Calcium (Ca2+)


  Most abundant mineral in the body, calcium salts are structural components of bones and teeth, function in blood clotting, neurotransmitter release, and contraction of muscle
• Calcium regulation


  Controlled mainly by parathyroid hormone, calcitriol, and calcitonin

  
• Acidosis


  Systemic arterial blood pH below 7.35, principal effect is depression of the central nervous system (CNS)
• Alkalosis


  Systemic arterial blood pH above 7.45, principal effect is overexciteability of the CNS

  
• Holding breath for 30 seconds

  Likely to increase blood pH (cause alkalosis)
• Carbonic acid-bicarbonate buffer system


  Helps maintain pH outside cells
• Phosphate buffer system


  Helps maintain pH inside cells
• Kidney function


  Declines with increasing age, adversely affects fluid and electrolyte balance
• Calcium


  An important electrolyte found in the blood
• Low blood calcium


  Causes the body to move calcium from bones into the bloodstream

  
• Small calcium losses over the years may result in low bone density in midlife and old age
• Excess dietary sodium


  Causes calcium loss from bones
• A diet high in sodium is associated with low bone density in later life