Combining mathematics and physiology? I am currently studying Mathematics as a undergraduate, but I'm also very interested in Physiology. I was also recently accepted to an Applied Mathematics master's program.

Is there a field of research in physiology/pharmacology that admits, or even encourages mathematicians with a decent knowledge of physiology? 

I've found that there are a few undergraduate math modules named "Mathematical physiology/biology"; this is sort of the reason for this question. There are also books upon books on the field of mathematical physiology, which makes sense as in many cases it seems like rates of change are important (glycolysis, etc).

This seems like a radical change in subject, but perhaps someone who has been through the rigor can shine light on this.
Typically, you will be better off studying mathematics with additional knowledge in X rather than a studying X with additional mathematics. Why? Because you can always enter X as a mathematician, they will welcome you with that. At least for academic jobs. 

Talking about mathematical biology: it is mostly mathematicians who do biology rather than the other way around. Granted, you will also find physicists and computer scientists there. 

A friend of mine claimed that the members of mathematical biology groups get their biology from Wikipedia and then apply their mathematics to it, most of which is settled in analysis of PDE and numerical analysis. He certainly exaggerated a bit, but there is truth in that statement.

Hence my recommendation: if these topic interest you, then pick a math graduate school that feature collaboration with math biology/physiology.