Do you have access to a university or teaching hospital?
You can actually measure changes in bloodflow with doppler ultrasound. A very expensive piece of equipment run by an expensive tech. I used such a device when working on a DVT pump.
But if you are a student and have access to a teaching hospital, you have excellent odds of having them help you (for free) with your project. When I was in school I was absolutely surprised at the level of support hospitals and healthcare professionals gave me. I saw dozens of surgeries, went on flight for life runs and had access to many other areas as well.
I called on the cardio-lab to get access to the doppler ultrasound, but different hospitals will place it is different departments, ymmv.