Why is menstrual blood considered impure and a menstruating woman prohibited from fasting and praying, even though it is merely blood vessels and cells like any other cell in the body?

A menstruating woman being prohibited from fasting and praying does not necessarily mean it's solely due to the impurity of menstrual blood. Rather, it might be for other reasons, such as the woman's weakness. All blood, once it leaves its source, is impure. This ruling is based on শরীয়ত (Islamic law) evidence, not laboratory analyses. A Muslim must submit to Allah's حکم (judgment), for He has wisdom in everything He has commanded and forbidden, whether that wisdom is apparent or not.