Introduction - What is Osmolarity?

Osmolarity describes the sum of all osmotically effective particles per volume of a certain 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 given in osmol per liter.

The determination of the osmolarity is used to control the water and electrolyte balance. It can be determined in the blood or in the urine.

Generally one differentiates one Hyperosmolarity, in which there are more osmotically effective parts per liter of liquid than in the comparison liquid, one Isoosmolarity, in which the same number of osmotically effective particles is present in the two liquids, as well as one Hypoosmolarity, in which there are fewer osmotically effective particles per liter in the liquid examined than in the comparison liquid.

Definition of osmolality

The Osmolality describes the sum of all osmotically active particles per kilogram of a certain liquid.
The unit of osmolality is osmol per kilogram.

The osmolality is also used to determine the Water and electrolyte balance and can be determined in the blood or urine.

The term osmolality is preferred to the term osmolarity in medicine. Here, too, a distinction is made between hyperosmolal - there are more osmotically effective particles in the examined liquid than in the comparison liquid, isoosmolal - There are the same number of osmotically active particles in both liquids, and hypoosmolal - there are fewer osmotically effective particles in the examined liquid than in the comparison liquid.

Also here has sodium 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, so sodium, potassium, Calcium and magnesium, as well as osmotically active substances such as Glucose and urea certainly, but above all about sodium.

The osmolarity of the blood in healthy people is around 290-300 milliosmol / liter.
The regulation of osmolarity is controlled by the Renin-Angiotensin-Aldosterone System (RAAS) and about that antidiuretic hormone (ADH) controlled.

Lie increased osmolar or osmolar values before (the concentration of sodium in the blood is increased) this can be an indication for one Dehydration (Dehydration) be.
Dehydration can be a cause of dehydration small amount of drink, but also fluid loss severe vomiting, Diarrhea or sweat be. But also drinking from salty water, as well as a impaired kidney function or hormonal disorders (e.g. Conn syndrome) can lead to increased sodium concentrations in the blood and thus to increased osmolarity in the blood.

Another cause of hyperosmolarity in the blood is a poorly controlled diabetes mellitus (Diabetes). As a result of the increased sugar levels, more glucose is excreted in the urine, which can also lead to a strong loss of fluid, because the glucose osmotically draws water.

Hyperosmolarity in the blood can be associated with numerous neurological deficits, such as States of confusion and Seizures up to coma.

A decreased osmolarity or osmolality for example in the blood of one Overdose of diuretics, at Hormonal imbalances or in the case of a metabolic acidosis.
Metabolic acidosis leads to an accumulation of acidic substances in the blood (for example Lactate or Hydrogen ions) leading to a Acidification of the blood to lead. Usually there is one here Renal dysfunction fundamentally.

A decreased osmolarity in the blood can also be with neurological failure symptoms such as seizures, disorientation and coma.

Glucose and its influence on osmolarity

Elevated levels of glucose in the blood have serious consequences for the human body. So you can join a Damage to nerves, Vessels and the Kidneys lead, but also dangerous Electrolyte shifts cause.

If there are high levels of glucose in the blood, more glucose is excreted through the kidneys. According to the principle of osmosis, glucose follows water and increases often occur Sodium levels in blood.
This can lead to a Hyperosmolarity of the blood and are associated with neurological deficits.
Glucose levels can be measured either in the blood or in the urine.


The osmolarity or osmolality of the urine is mainly due to sodium and urea determined and regulated with the help of the renin-angiotensin-aldosterone system and the antidiuretic hormone in the kidney.

The osmolarity of the urine is important for the regulation of the salt and water balance.

Is the urine hyperosmolar, that means there are very many osmotically effective particles in a little urine, this indicates a lack of water in the body and it comes to Antidiuresis.
In return is the urine hypoosmolar, diuresis (excretion of water) is initiated.