W9.3_Osmolarity and Osmolality

Question 1

Describe how fluid balance is achieved. What are the usual solvent and solutes in body fluids?

Answer 1

• Achieved through semipermeable membranes in body (ex. cell/plasma membranes)
• Solvent: water
• Solutes: ions, electrolytes, urea, glucose…

Question 2

Define osmosis, osmotic pressure, osmolarity, osmolality, and osmole.

Answer 2

• Osmosis: process which fluids move into/out of cells
• Osmotic pressure: pressure required to stop water movement across membrane (more non-diffusible particles = higher osmotic pressure)
• Osmolarity: concentration of solutes per litre of solvent (mOsm/L)
• Osmolality: concentration of solutes per kilogram of solvent (mOsm/kg)
• Osmole: number of moles that a solute contributes to osmotic pressure of solution (1 mol of glucose = 1 Osm vs 1 mol of NaCl = 2 Osm due to dissociation)

Question 3

How is osmolality in urine measured? State the solutes in urine and the possible indications of results.

Answer 3

• Osmolality measurement: through osmometer -> determine kidney’s ability to concentrate urine
• Solutes: chloride, sodium, urea, potassium, and possibly glucose
• Used to indicate dehydration/overhydration, kidney problems (different changes in plasma and urine osmolality can indicate different clinical diagnosis)

Question 4

How is osmolarity calculated from osmolality (3)?

Answer 4

• Osmolarity=Osmolality x 0.995
• Osmolarity=2 x [Na+]+[glucose]/18+[BUN]/2.8
• Osmolarity=2 x ([Na+]+[K+])+[glucose]+[urea]

Question 5

Briefly describe the homeostatic process of fluid balance in body. Explain the mechanism of ADH.

Answer 5

• Decreased plasma osmolality -> stops releasing ADH -> increase water in urine
• Increased plasma osmolality -> starts release ADH -> promotes water reabsorption
• ADH: secreted by hypothalamus and stored in posterior pituitary gland, stimulated by osmoreceptors and baroreceptors -> binds to AVPR2 (V2 vasopressin receptor-2) in kidney tubules -> AVPR2 coupled to Gs -> increases cAMP production -> opens aquaporin-2 channels that allow reabsorption of water (vice versa)

Question 6

Define tonicity in solutions. How is it calculated? Briefly explain how the three types of tonicity could impact cells.

Answer 6

• Tonicity in solutions: relative concentration of solution outside of cell compared to inside of cell (X unit)
• Fluid tonicity=2 x [Na+]
• Hypertonic: tonicity is higher in outside -> cell will lose water and shrink
• Hypotonic: tonicity is lower in outside -> cell will gain water and swell
• Isotonic: tonicity is equal -> cell size will not change or be damaged