RENAL PHYSIOLOGY-OSMOLALITY

SUMMARY

1. Normal serum osmolality is usually 282 +/- 2 mOsm/kg H₂O.

2. Measured osmolality should be equal to calculated osmolality: Osm_calc = 2[Na⁺] + (glucose/20) + (BUN/3)

3. Small contributions from glucose and urea in most cases except uncontrolled DM or reduced renal function.

4. Regulation of plasma osmolality is via osmoregulators/ADH in hypothalamus which: regulates water intake/excretion, increase thirst & release of ADH. This is extremely sensitive: plasma osmolality is tightly controlled.

5. ADH: neurohypophysial hormone that acts on the late distal tubule and cortical collecting duct to increase water permeability (hence resorption) and mediate the urine concentration.

6. ADH acts via the Vasopressin 2 receptor, which causes Aquaporin channels to fuse with the cell membrane, allowing passage of water through the cell.

Reference(s)

Wilkinson, I., Furmedge, D. and Sinharay, R. (2017). Oxford handbook of clinical medicine. Oxford: Oxford University Press. Get it on Amazon.
Feather, A., Randall, D. and Waterhouse, M. (2020). Kumar And Clark’s Clinical Medicine. 10th ed. S.L.: Elsevier Health Sciences. Get it on Amazon.
Hannaman, R. A., Bullock, L., Hatchell, C. A., & Yoffe, M. (2016). Internal medicine review core curriculum, 2017-2018. CO Springs, CO: MedStudy.
Therapeutic Guidelines. Melbourne: Therapeutic Guidelines Limited. https://www.tg.org.au [Accessed 2021].