What do we mean when we talk about the osmolarity of the blood and why does it matter?

Osmolality indicates the concentration of all the particles dissolved in the body fluid (wherever in the body it is located). Higher osmolality indicates a higher concentration of particles (and as a result a lower concentration of water). Water can move between fluid compartments in the body e.g. between the intracellular and extracellular fluid. Provided the membrane separating the compartments is permeable to water, water moves from the compartment of low osmolality to the compartment of high osmolality. Another way of looking at this is to say that water is moving from the area or its concentration is higher (i.e. osmolality is lower) to the area where its concentration is lower-it is moving down its concentration gradient. When water moves between plasma and intracellular compartments, the movement direction depends on both compartments' osmolalities. For example, if a cell is in a relatively hyperosmolar solution, fluid will move out of the cell and it will shrink.