What is the difference between plasma and blood osmolarity

Blood osmolarity is a broader term that encompasses the osmolarity of both the plasma and the cellular components of blood (red blood cells, white blood cells, and platelets). However, in most physiological contexts, when people refer to blood osmolarity, they are essentially referring to plasma osmolarity because the cells contribute relatively little to the overall osmotic pressure compared to the plasma. The cellular components are largely contained within the plasma, and their internal osmolarity is very similar to that of the plasma to prevent osmotic lysis or crenation.

So, while technically blood osmolarity includes cellular contributions, it is functionally equivalent to plasma osmolarity in most clinical and physiological discussions.