Does bleeding nullify ablution? The Hanafites are very strict about it but at the same time I have heard of some Ahādīth which describe the story of a companion of the Prophet (sws), who was hit by an arrow while he was offering his prayers and he didn't discontinue his prayers. Please elaborate.

The basic objective of our religion is purification. Cleansing one's body is purifying it from all physical impurities.

There are certain impurities after which ablution becomes necessary and there are others after which does not. In the latter case, ablution can be counted as a good deed done to achieve the basic objective of our religion as much as possible. Doing ablution after bleeding falls in this second category.