We describe the foundational model of anatomy (FMA), reference ontology for the discipline of human anatomy. Using the semantic structure of the FMA as knowledge representation template, we propose a physiology reference ontology (PRO) as a corresponding ontology for "functional bioinformatics". We envision the PRO as a source vocabulary for building symbolic representations of human physiological states and actions that may ultimately be extensible to other species. We describe the evolving architecture of the PRO, in terms of simple examples based on the anatomical concepts encoded in the FMA.