As part of the IA requirements you may well be asked to plan and carry out an experiment on a topic of your choosing pertaining to a central theme. My students are given free reign to look at the human body and construct an experiment investigating the manipulation of one variable on another.
I came across this excellent package online that is bustling with possible experiments you could explore. Just remember to reference/footnote your source. The experiments they discuss are mostly very accessible in the classroom within most schools and I encourage you to go look.
Word of warning though, keep away from the majority of activities on the brain. e.g. effect of music on reaction time etc. Though they are presented here, they are best suited to a psychology investigation and not a Biology one, and to correctly carry out an experiment is more complex than perhaps indicated there.
One of the simplest physiological tests to perform involves the use of a spirometer and a measure of lung volume.
Some examples of how to incorporate these into an experiment can be viewed at http://www2.vernier.com/sample_labs/HP-A-19-COMP-lung_volume_capacities.pdf
and http://www2.vernier.com/sample_labs/HP-A-21-COMP-analysis_lung_function.pdf which are worth reading.
In addition you should watch the following videos that give an overview of the differing lung volumes / definitions together with practical and authentic applications of the tests you may perform.
Once again, do not forget to reference your sources, nothing wrong with taking an established method and applying it to your specific needs.