Blood Osmolality

Blood osmolality is a test that is done to check for the levels of dissolved osmoles in the blood. These are in their ionic form and include substances like sodium, chloride, glucose, and urea among others. This is a base test that could form the basis of further testing of kidney function and other tests too are based on the findings. Blood Osmolality is the property that is used to judge the health of cells and the process of osmoregulation within cells. To understand this, it is important to understand the relationship of cells and the surrounding environment that they exist in.

The body is made of cells and these are living structures with a central nucleus that contains DNA. There is a gap between the center and the outer membrane of the cell. This space is called the cystosol. Here structures like the mitochondria and other organelles exist. There is always a movement of material from within the cells to the outside environment and this is the movement of nutrients, fuel, and excretory material. This movement occurs due to the action of carrier proteins and also with the action of osmosis. The balance of osmosis depends on the concentration of water in the blood, interstitial fluid, and the intracellular fluid. Since the concentration of osmoles in the cells is higher than the surrounding fluid of the environment, items can move in and out freely. Osmolality is of clinical significance because the osmolality of extracellular fluids and intracellular fluids are usually the same. This is what allows water to freely move between cells. When there is a decreased osmolality, this means that there is too much water and the kidneys then have to expel water. Conversely, where the osmolality increases, antidiuretic hormone is secreted to save water. If the surrounding blood was too hypotonic, the water would readily diffuse into the cells. This will cause the cell to swell up and burst.

The ideal levels of osmolality lie at 275-299 mili-osmoles per kilogram. Increased levels are usually an indication of impeding stroke. Deficient kidney function can also be judged by osmolality tests. The kidneys regulate the amount of fluid volume in the body by the action of system called the renin-angiotensin-aldesterone system. This controls fluid volume by saving or excreting more or less sodium from the body in the urine. Breakdown’s of this system cause hypertension and water retention as can also be seen in decreased osmolality.