How is $PATH created?
On OS X you'll have following :  

The base PATH used is located in /etc/paths. It is used to set $PATH. 

By default, the file contains : 

/usr/bin
/bin
/usr/sbin
/sbin
/usr/local/bin


It will also load the paths located in /etc/path.d/. 

When opening the terminal I will load : 


~/.bash_profile 
~/.bash_login, (if .bash_profile doesn't exist)
~/.profile (if .bash_login doesn't exist). 


These files can load some other files, for example .bashrc. 

If you want to expand your path you can do it in one of the files by adding your  export command you gave in your question.