On your main PubMed page, in the left hand column under “PubMed Services”, click on MeSH Database. On the next screen, enter a term in the search box and hit go. Try
“hypertension”. At the next screen, you will see several hypertension terms, if you want #1 hypertension, click in the box. If you click “go” near the top, it will search all hypertension, just like a plain Pubmed search. But, if you click on the Hypertension link, that will take to a screen where you can choose subheadings and also see the scope note, tree structure, etc. Mark subheadings of, say, all therapy terms. Then click on “Send to: search box”, then go to “History” to combine sets.