PHYS - Body Fluid Compartments

Question 1

DETERMINING VOLUME OF FLUID IN BODY COMPARTMENTS

Answer 1

• Utilize a tracer and V = quantity of tracer/concentration of tracer
  
  • Total body water (TBW): D₂O
  • Extracellular fluid volume (EFV): inulin
  • Plasma volume (PV): ¹²⁵I-albumin, Evan’s blue
• Equations to determine volume:
  
  • Intracellular fluid volume = TBW – EFV
  • Interstitial fluid volume = EFV – PV
  • Blood volume = PV/(1-%Hct)
• TBW is lower in obese persons and women (essential fat) because fat cells increase body volume but contain only lipids and no water
• Water volume is greater inside cells than outside cells
• 20-40-60 Rule for an average adult
  
  • 60% TBW
  • 40% intracellular fluid volume
  • 20% extracellular fluid volume

Question 2

DISTURBANCES OF OSMOLARITY AND VOLUME

Answer 2

• Simplifying assumptions
  
  • Na+ is essentially an extracellular ion
  • K+ is essentially an intracellular ion
  • The intracellular fluid compartment acts as a perfect osmometer
• The Darrow Diagram
  
  • ECF compartment = 17 L
  • ICF compartment = 25 L
  • Conditions such as alka/acid-osis and hyper/hypo-kalemia = descriptions of extracellular fluid compartment
• The body’s osmolarity is approximately 300 mOsM
• Sweat is hypotonic unless occurring profusely; then it can approach isotonic (never hypertonic) because the body does not have enough time to reabsorb salts

Question 3

OVERHYDRATION

Answer 3

• Excess fluid in the ECF compartment (increase in ECF volume)
• Isotonic Overhydration
  
  • ECF: volume increases; no change in osmolarity
  • ICF: no change osmolarity or volume
  • Ex → careless overadministration of saline
  • Because isotonic, no osmotic stimulation of ICF, ECF volume increases from additional IV fluid
• Hypotonic Overhydration
  
  • ECF: volume increases; osmolarity decreases
  • ICF: volume increases; osmolarity decreases
  • Ex → compulsive water drinking
• Hypertonic Overhydration
  
  • ECF: volume increases; osmolarity decreases
  • ICF: volume decreases, osmolarity increases
  • Ex → drinking salt water

Question 4

DEHYDRATION

Answer 4

• Loss of fluid from ECF (decrease in ECF volume)
• Isotonic Dehydration
  
  • ECF: decrease in volume; no change in osmolarity
  • ICF: no change in volume or osmolarity
  • Ex → hemorrhage
• Hypotonic Dehydration
  
  • ECF: decrease in volume; decrease in osmolarity
  • ICF: increase in volume; decrease in osmolarity
  • Ex → adrenal cortical insufficiency (Addison’s disease; lack of sufficient aldosterone production)
  • Decrease in ECF OsM causes water to shift into ICF and decrease OsM
• Hypertonic Dehydration
  
  • ECF: decrease in volume; increase in osmolarity
  • ICF: decrease in volume; increase in osmolarity
  • Ex → diabetes insipidus (lack of ADH production/response)
  • Ex → perfuse sweating with no rehydration (lost in the desert)

Question 5

NATREMIA

Answer 5

• No change in [Na]₀
  
  • Isotonic overhydration – IV overadministration
  • Isotonic dehydration – hemorrhage
• Decrease in [Na]₀
  
  • Hypotonic overhydration – water intoxication
  • Hypotonic dehydration – Addison’s disease
• Increase in [Na]₀
  
  • Hypertonic overhydration – drinking sea water
  • Hypertonic dehydration – profuse sweating
  • To distinguish between causes of hypernatremia, measure plasma protein concentration which should shift with fluid balance (remember the assumption that Na+ is essentially trapped extracellularly)
    
    • Low plasma protein concentration → overhydration
    • High plasma protein concentration → dehydration

Question 6

WATER TURNOVER

Answer 6

• Steady state → input = output
• Turnover rate = 1.5L to 3L per day
  
  • Increased output with exercise (up to 5L) or diarrhea (8L)
• Input: Food, Metabolism, Drinking
• Output:
  
  • Insensible: skin, lungs
  • Sensible: urine, feces, sweat
  • Large water loss from GI secretions makes diarrhea especially deadly without fluid replacement

Question 7

REGULATION OF PLASMA OSM BY ADH AND THIRST

Answer 7

• Increased plasma OsM sensed by hypothalamic osmoreceptors
  
  • Stimulate pituitary gland to release ADH → H₂O reabsorption in kidneys
  • Stimulates feeling of thirst → Drinking H₂O
    
    • Decreased plasma OsM → decreased osmoreceptor activity

Question 8

OSMOLAR CLEARANCE

Answer 8

• Concentration of urine is not indicative of more or less osmolar clearance, should be constant regardless of concentration/tonicity (even isotonic)
  
  • Dilute urine has a greater volume and less particles per mL
  • Concentrated urine as a smaller volume and more particles per mL
• C_osm = U_osm(V)/P_osm

Question 9

FREE WATER CLEARANCE

Answer 9

• Depends on the excess of water or lack of water in urine from that required to dissolve the particles
• C_H2O = V - C_osm
  
  • Maximally dehydrated urine C_H2O < 0 (not enough water, urine oversaturated)
  • Maximally dilute urine C_H2O > 0
  • Isotonic urine V = C_osm so C_H2O = 0
• Formation of free water occurs in the ascending limb (hypotonic urine)
• Reabsorption of free water occurs in the IMCD with ADH stimulation (hypertonic urine)
• Loop diuretics decrease free water clearance C_H2O
  
  • Block salt reabsorption in the ascending limb
  • Increases C_osm, which decreases C_H2O