Constructive criticism is an opportunity to learn. She is either giving you technical guidance related to your area of expertise or she is guiding you on how to be a better employee. Either way, learn from it. It is incredibly useful and will help you develop your career. Thank him (or her) and put the suggestions into practice as soon as possible. Constructive criticism is never a personal attack. Your question hints that that is how you would expect someone to respond. That would be a big mistake.

Constructive criticism fuels growth. It tells you where you can improve and what you need work at. Never take the rightful criticism as a personal attack. Accept and assess the feedback. Recognise the opportunity to grow and put in the work. That's the only way to maintain professionalism - by taking it for what it is, a critique.