Question

Why saline water is mixed with medicine before injected into the blood of a patient?

Hint:

Normal saline ($0.9\% NaCl$) is the "gold standard" for IV fluids because it creates an osmotic balance that prevents damage to blood cells.

Answer 1

Concept: The process is based on the principle of Osmosis. Osmosis is the movement of solvent molecules through a semi-permeable membrane from a region of lower solute concentration to a region of higher solute concentration. For medical injections: Isotonic Solutions: Have the same osmotic pressure as blood ($0.9 %$ w/v $NaCl$).
Hypotonic Solutions: Lower concentration than blood; causes cells to swell and burst (hemolysis). 
Hypertonic Solutions: Higher concentration than blood; causes cells to shrink (plasmolysis). 

Step 1: Matching Osmotic Pressure.Human blood cells have an osmotic pressure equivalent to a $0.9\%$ (mass/volume) sodium chloride solution, commonly known as normal saline. When medicine is mixed with this saline, the resulting solution remains isotonic with the fluid inside the red blood cells. 

Step 2: Preventing Cell Damage. If pure water (hypotonic) were injected, water would enter the red blood cells by osmosis, causing them to swell and potentially burst. Conversely, if a highly concentrated solution (hypertonic) were injected, water would leave the cells, causing them to shrivel. Mixing with saline ensures the cells maintain their normal shape and function.