Definition of Osmolarity
Osmolarity refers to the concentration of osmotically active particles per liter of solution, expressed in milliosmoles per liter (mOsm/L). It determines the movement of water between compartments and is calculated as:
Mechanisms of Regulation of Plasma Osmolality (8 marks)
Plasma osmolality is tightly regulated by mechanisms involving osmoreceptors, antidiuretic hormone (ADH), and thirst.
- Sensors: Osmoreceptors
- Located in the hypothalamus (e.g., organum vasculosum of the lamina terminalis).
- Detect changes in plasma osmolality (threshold: ~280–290 mOsm/L).
- Effectors
- ADH Secretion:
- Increased osmolality (>280 mOsm/L) stimulates ADH release from the posterior pituitary.
- ADH acts on V2 receptors in renal collecting ducts, promoting water reabsorption via aquaporin-2 insertion.
- Thirst Mechanism:
- Stimulated when plasma osmolality exceeds ~290 mOsm/L.
- Results in increased water intake, reducing plasma osmolality.
- Kidney’s Role
- Kidneys regulate water excretion based on ADH levels.
- Maximal urine concentration (~1200 mOsm/L) in dehydration; dilute urine (~50 mOsm/L) in overhydration.
- Feedback Regulation
- Normalization of plasma osmolality inhibits ADH secretion and suppresses thirst.
Calculation, Determinants, and Diagnostic Criteria of SIADH
Calculation
Plasma osmolality:
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Urine osmolality and sodium concentration are critical in assessing SIADH.
Determinants
- ADH Secretion: Inappropriate secretion despite normal or low osmolality.
- Renal Response: Persistent water retention leading to dilutional hyponatremia.
- Plasma Findings: Low plasma sodium (<135 mEq/L) and osmolality (<275 mOsm/L).
- Urine Findings: Elevated urine osmolality (>100 mOsm/L) and sodium (>40 mEq/L).
Diagnostic Criteria (3 marks)
- Hyponatremia with low plasma osmolality.
- Inappropriately concentrated urine.
- Euvolemia (normal extracellular fluid status).
- Absence of adrenal, thyroid, or renal dysfunction.
Regulation of Plasma Osmolality (5 marks)
- Primary Determinants: Sodium and water balance.
- Mechanisms: Coordination between hypothalamus, pituitary gland, and kidneys.
- Pathophysiological States:
- Dehydration: Increased osmolality triggers ADH secretion and thirst.
- Overhydration: Suppressed ADH secretion reduces water reabsorption.
- SIADH: Disruption in ADH regulation leading to hyponatremia and low plasma osmolality.
