9. Body water and its distribution in the human body. Osmolality and osmolarity - methods for determination – reference range, informative significance.

body water and its distribution in the human body. Osmolality and osmolarity-methods of investigation, reference ranges, result interpretation.

Function of water in the body:

1. solvent – many of the biochemical reactions proceed only when the reactive ingredients are dissolved
2. Plays important role in the transport of different substances
3. Regulation of the body temperature

Body water and its distribution in the human body:

Water makes up 45-75% of the total weight of an adult human body.

• newborns and breast-fed babies about 75%
• young men 64%
• adult men 53%
• young women 53%
• adult women 46%

Adipose tissue contains approx. 20% water, while the other tissues contain aprrox.73% water. We distinguish between intracellular and extracellular areas. Extracellular space contains about 15 litters of water while the Intracellular space contains 25L

Osmolality and osmolarity methods of investigation and Reference Range:

1. Osmolarity: number of osmotically active particles, dissolved in 1 litre of body fluid. Range: 280-295 mosm/l in serum.
2. Osmolality: number of osmotically active particles, in 1 kg of body. Range 281-297 mosm/kg in serum. measured using osmometer.
3. Serum osmolal gap = measured serum osmolality - calculated serum osmolality Range < 10 mosml/kg

Method: Osmometry is the technique employed to measure osmolality of solutions of a biological sample, by means of a device called osmometer, which uses the colligative properties to take the measurements. Colligative properties of a solution depend on the number of solute particles dispersed at a certain quantity of solvent, regardless of the chemical nature of the solute

Result interpretation:

Osmolality of blood increases with dehydration and decreases with overhydration. In normal people, increased osmolality in the blood will stimulate secretion of antidiuretic hormone (ADH). This will result in increased water reabsorption, more concentrated urine, and less concentrated blood plasma. A low serum osmolality will suppress the release of ADH, resulting in decreased water reabsorption and more concentrated plasma.

Increased osmolarity frequently occurs following illness due to chronic neurotoxic diseases such as Lyme disease. Elevation may be associated with stroke mortality. Hyperosmolality due to: Hyperglycaemia, Hypernatremia (high Na) with/without changes in water balance Hypoosmolality due to: Hyponatremia with hypovolemia (low blood vol), with/without changes in water balance.