Electrolytes, Osmolality, Hydration

Question 1

What is a healthy animals Total Body water?

Answer 1

• ~60% of body weight
  
  • ⅔ intracellular fluid (ICF)
  • ⅓ extracellular fluid (ECF)
    
    • ¼ vasculature
    • ¾ interstitial
      
      • interstitial fluid
      • Third space (peritoneal, pericardia, pleural fluid)
      • GI fluid
• TBW volume = hydration status
  
  • controlled by water intake and renal output

Question 2

What does the Proximal tubule of a nephron do?

Answer 2

• Isosmotic reabsorption of 65-70% of filtered water and NaCl
• Reabsorption of 90% of filtered HCO₃^-
• Major site of NH₃ production
• Reabsorption of almost all filtered glucose and AA
• Resorption of K, P, Ca, Mg, Urea and urate
• Secretion of organic anions and cations

Question 3

What does the Loop of Henle do?

Answer 3

• Countercurrent multiplier
• Reabsorption of 15-25% of filtered NaCl
• Active regulation of magnesium secretion

Question 4

What does the distal tubule do?

Answer 4

• small amount of NaCl reabsorbed
• Active regulation of calcium excretion

Question 5

What does the Connecting segment and Cortical collecting duct do?

Answer 5

• Aldosterone-mediated potassium secretion by principal cells
• H⁺ ion secretion by a-intercalated cells
• Potassium reabsorption by a-intercalated cells
• ADH-mediated water reabsorption

Question 6

What happens in the Medullary collecting duct

Answer 6

• Potassium reabsorption or secretion
• Final NaCl reabsorption
• ADH-mediated water and urea reabsorption
• H⁺ ion and NH₃ secretion

Question 7

Where do the different electrolytes concentrate? intracellular or extracellular?

Answer 7

• Sodium - predominantly extracellular
• Potassium - predominantly intracellular
• Chloride - predominantly extracellular

Question 8

What causes abnormal electrolyte concentrations in plasma/serum?

Answer 8

• Increased or decreased intake
• Shifts between the ECF and ICF
• increased renal retention
• Increased loss through GI, renal, skin, or airway

Question 9

What are the general concepts of electrolytes?

Answer 9

• Loss or gian of H₂O or electrolytes from one compartment will alter volume and solute concentration of other compartments
• Plasma/serum [electrolyte] reflects ECF concentration, NOT ICF
• Loss of hypertonic ECF results in hypotonic ECF
• loss of hypotonic ECF results in hypertonic ECF
• Loss of isotonic fluid reslts in loss of ECF volume
  
  • Does not by itself cause osmotic shifts/electrolyte changes
  • Compensatory H₂O intake/renal H₂O retention will dilute ECF
• Most changes occur in pairs
  
  • Maintain electrical neutrality
  • ⇡Na⁺ & ⇡Cl^-
  • ⇡Cl^- & ⇣HCO₃^-

Question 10

What is dehydration?

Answer 10

• Decreased total body water
  
  • Loss of weight - best way to determine - not always feasible
  • Clinical evidence of dehydration (most used)
    
    • Loss of skin elasticity, dry mucous membranes, retraction of eyes, prolonged CRT, inceased HR, shock
    • Specific lab values - proteins Hct, UN(BUN), USG
      
      • Values may be influenced by other coniitions - must be used in conjuction with physiccal examination

Question 11

What are the pathophysiologic mechanisms & Na²⁺ changes?

Answer 11

• Loss or sequestration of hypotonic fluid
  
  • Results in hypertonic dehydration ⇡Na+
  • Los of water > loss of salt
• Loss of isotonic fluids
  
  • Results in isotonic dehydration - WRI Na+
  • loss of water = loss of salt
• Loss of hypertonic fluids
  
  • Results in hypotonic dehydration - ⇣ Na+
  • loss of water < loss of salt