Practice question

Plasma osmolarity tightly regulated in mammals at approximately two hundred eighty five to two hundred ninety five milliosmoles per liter by neuroendocrine circuit involving hypothalamic osmoreceptors magnocellular neurons in supraoptic nucleus sensing shrinkage, releasing antidiuretic hormone vasopressin from posterior pituitary promoting aquaporin-2 insertion in collecting duct increasing water reabsorption, ensuring stable cell volume via aquaporin water channels bidirectional passive transport. Basal culture media attempt to mimic plasma tonicity to avoid osmotic stress altering cytoskeletal organization and signaling. Contributions: sodium chloride one hundred to one hundred twenty millimolar major driver because Na+ and Cl- dominant extracellular ions, potassium chloride four to five millimolar, sodium bicarbonate twenty four to forty four millimolar, glucose five to twenty five millimolar energy source, amino acids total ten to fifteen millimolar, other salts calcium magnesium. Total osmolarity measured via freezing point depression osmometer where freezing point depression proportional to solute concentration colligative property, calibrated with three hundred milliosm standard.