Ok, so you're using one of the out of the box pop up options. You could probably make a custom one that does what you want, but you'll have to brave working with XSLT.
XSLT is used to transform xml from one schema to another, often xml to html. It's composed of an xsl file which contains instructions for performing the transformation from the source xml to the output xml.
The way html popups work, in a nutshell, is ArcMap produces an xml document based on the layer settings (i.e only the fields checked on the 'fields' tab show up). That xml is converted to html using an xsl. That's what you see when you get the popup. ArcGIS has a few out of the box xsl files, and that's what those default options make use of.
I'm definitely no expert, but I've learned some basics. w3schools.com has some decent tutorials to lay down some foundation concepts. If you're not already familiar with xml or html, the whole prospect can be intimidating though.