Answered by Shaykh Faraz Rabbani
Why is urine najis? When it is inside, it is pure, but outside our bodies it is deemed najis. Same question regarding blood.
The things excreted from the body only take their legal rulings when they actually come out. This is because legal rulings relate to human actions, and it would be impossible to consider the human being (who is full of blood!) impure.
Once these things exit the body, they take their rulings…