Hyponatremia is an example of blood as a hypotonic solution when a person's blood has low sodium levels.
A hypotonic solution is any solution that has a lower osmotic pressure or less solute than another solution.
For example, when endurance athletes consume too much water and not enough electrolytes, they may become hyponatremic. Their blood does not contain enough sodium, so they water they drink cannot diffuse into their bloodstream via passive transport. Their blood is hypotonic and the liquid in their gut is hypertonic.