Fluid Balance

Question 1

Different fluid compartments and composition

Answer 1

• intracellular fluid: high potassium and magnesium
• extracellular fluid: high sodium and chloride and protein (albumin)
• interstitial fluid: high sodium and chloride but low protein

Question 2

Normal blood electrolyte levels

Answer 2

• sodium: 135-145 mEq/L
• potassium: 3.5-5.5 mmol/L
• chloride: 96-106 mEq/L

Question 3

Basis of fluid balance (obligatory water loss and water gain)

Answer 3

• water losses from urine (at least 500 mL per day), sweat (100+mL/day), faecal loss (~200 mL/day), insensible water loss (i.e on breath, no electrolytes, 800 mL/day)
• water replaced from ingested fluids and food (external), and metabolic sources from CHO metabolism (internal)

Question 4

Oral rehydration therapy

Answer 4

• fluid needs to be hypo-osmolar so electrolytes actually absorbed
• needs to have source of sodium and glucose (CHO aid with absorption)
• need to be ingested in a 3-4 hour window
• needs to palatable, especially for children

Question 5

Sweating: losses and adaptation

Answer 5

• loss of electrolytes, fluid and heat
• max rate is 50 mL per min, but this cannot be sustained
• 0.2-1% solute concentration, which decreases over time (adaptation)

Question 6

Hypothalamic control of water balance

Answer 6

• thirst centre: hypertonicity of cells detected by osmoreceptors feedback to hypothalamus, hypovolemia (lack of water) detected by baroreceptors in great veins and right atrium and feedback to hypothalamus, hypotension detected by baroreceptors in carotid sinus and aorta and feedback to hypothalamus, angiotensin II is produced in response to renin by kidneys and causes vasoconstriction
• once thirst is quenched, mechanoreceptors in pharynx and mouth feedback to hypothalamus
• antidiuretic hormone is synthesised by hypothalamus and reduces fluid loss by kidneys

Question 7

General rules of osmolality

Answer 7

1) water crosses membranes easily
2) ECF regulates ICF volume
3) ECF concentration is determined by Na
4) intracellular osmolality = extracellular osmolality
5) [Na] kept constant due to tight osmoreceptor control
6) total body solute relatively constant

Question 8

Effects of caffeine and alcohol

Answer 8

• caffeine only diuretic (inhibits ADH at 300 mg or more)

- alcohol inhibits ADH but does not dehydrate you