Osmolarity

Introduction – What is osmolarity?

Osmolarity describes the sum of all osmotically active particles per volume of a given liquid. Osmotically active particles in the blood are for example electrolytes such as sodium, chloride or potassium, but also other substances such as urea or glucose. However, sodium has the greatest osmotic importance in the human body.

The osmolarity is expressed in osmol per liter. The determination of the osmolarity serves to control the water and electrolyte balance. It can be determined in blood or urine. In general, a distinction is made between hyperosmolarity, where there are more osmotically active particles per liter of fluid than in the reference fluid, iso-osmolarity, where the same number of osmotically active particles are present in both fluids, and hypo-osmolarity, where there are fewer osmotically active particles per liter in the examined fluid than in the reference fluid.

Definition Osmolality

Osmolality describes the sum of all osmotically active particles per kilogram of a given liquid. The unit of osmolality is osmol per kilogram. Osmolality is also used to determine the water and electrolyte balance and can be determined in blood or urine.

The term osmolality is preferred to the term osmolality in medicine. A distinction is also made here between hyperosmolal – there are more osmotically active particles in the examined fluid than in the reference fluid, isoosmolal – there are the same number of osmotically active particles in both fluids, and hypoosmolal – there are fewer osmotically active particles in the examined fluid than in the reference fluid. Here, too, sodium has the greatest importance for the osmotic processes in the human body.

Osmolarity and osmolality of the blood

The osmolarity or osmolality of the blood is determined by the electrolytes in the blood, i.e. sodium, potassium, calcium and magnesium, and by osmotically active substances such as glucose and urea, but above all by sodium. The osmolarity of the blood in healthy people is about 290-300 milliosmol/liter. The regulation of osmolarity is controlled by the renin-angiotensin-aldosterone system (RAAS) and by the antidiuretic hormone (ADH).

If there are elevated osmolar or osmolar values (the sodium concentration in the blood is elevated), this can be an indication of a fluid deficiency (dehydration). The cause of dehydration can be a small amount of drinking, but also fluid loss through heavy vomiting, diarrhoea or sweating. However, drinking salty water, as well as reduced kidney function or hormonal disorders (e.g. Conn syndrome) can also lead to increased sodium concentrations in the blood and thus to increased osmolarity in the blood.

Another cause of hyperosmolarity in the blood is poorly controlled diabetes mellitus (diabetes). Due to the increased sugar levels, more glucose is excreted through the urine, which can also lead to a severe loss of fluid because the glucose osmotically draws water. Hyperosmolarity in the blood can be accompanied by numerous neurological deficits, such as confusion and seizures, and even coma.

Reduced osmolarity or osmolality in the blood is caused, for example, by an overdose of diuretics, hormonal disorders or metabolic acidosis. Metabolic acidosis is an accumulation of acidic substances in the blood (e.g. lactate or hydrogen ions), which leads to hyperacidity of the blood. This is usually caused by a kidney dysfunction. A reduced osmolarity in the blood can also be accompanied by neurological deficits such as seizures, disorientation and coma.