Osmolality: Importance, Regulation, and Measurement in Clinical Settings
What is Osmolality?
Osmolarity is a measure of the weight of solutes in a liter of solution [liter = volume]
Osmolality is a measure of the weight of solutes in a kilogram of water (solution) [kilogram =
weight]
Osmolarity and osmolality are influenced by:
- The weight of substances in a solution
- The number of particles of a substance (ions)
Sodium contributes ≈ 50% of plasma osmolality
Importance Of Osmolality
- Water homeostasis: the movement of water is balanced by the solutes in the ICF and ECF
- Osmotic pressure: the amount of pressure or force applied to a flux to prevent osmosis.
Increase in osmotic pressure in the ECF will cause fluid to move from the ICF to the ECF and vice-versa. It helps to maintain the blood volume or the water constituent of the body.
Regulation
Osmolarity of the ECF is stimulated by:
- Kidney Renin-angiotensin-aldosterone system
- Hypothalamus (thirst and ADH)
- Thirst sensation: decreases water content of ECF (and increases Sodium)
- Antidiuretic hormone: acts on distal convoluted tubule (DCCT) and collecting ducts in kidney to reabsorb water
- Aldosterone: stimulates sodium (and water) retention by kidney (at the expense of K+)
- Lymphatic system: removes excess water from tissue and returns to bloodstream
- Plasma proteins (especially albumin): maintain osmotic pressure keeping correct amount of water in bloodstream
- Electrolytes: maintain osmotic balance between intracellular and extracellular fluids
Protein and/or electrolyte imbalance lead to oedema.
Measurement
It is measured using an Osmometer. Types of Osmometer:
- Freezing point depression Osmometer: (mostly used in clinical laboratories)
- Vapour pressure depression osmometer: (used for research and also in clinical settings)
Osmolality Measurement
Specimen collections for measurement include:
- Serum
- Plasma
- Urine
- Body fluids
- Stool
Serum Osmolality (275-295Mosm/Kg)
Consequences of serum osmolality:
- Increased serum osmolality: indicates increase solute in the blood stream or lack of plasma water. CAUSES:
- Diabetes mellitus: they have high glucose and high ketone bodies
- Renal disease
- Poisoning such as ethylene glycol
- Decrease serum osmolality: indicates decrease in solute in the blood stream or excess plasma water. CAUSES:
- Shock
- Lymphomas (inability of the lymph to return water to the blood)
- Myocardial infarction
- Hyponatremia (decrease in Na+in the blood)
Urine Osmolality (300-900Mosm/L)
- High osmolality in the urine: indicates increase in solutes or lack of water in urine. CAUSES:
- Dehydration
- Diabetes mellitus: glucose and ketones in urine
- Excess release of ADH
- Low osmolality in the urine: indicates decrease in solute concentration in urine or excess water in urine. CAUSES:
- Inadequate secretion of ADH
- Excessive water intake (polydipsia)
- Diabetes insipidus
- Renal disease e.g. glomerulonephritis
Stool Osmolality (270-290mosm/L)
Normal stool osmolality is approximately the osmolality of the serum. Useful in identifying diarrhoea related disorders.
- High stool osmolality: some malabsorption and/or maldigestion disorders cause secretory diarrhoea
