1. use PageUrls.ParseUrl(uri.LocalPath) to get a UrlData object which contais PageId
2. Page content are stores in instances of IPagePlaceholderContent which holds a reference to a PageId and PlaceHolderId. By using the PageId from step 1, you just need to query for the specific placeholders you need, and then extract content via the IPagePlaceholderContent.Content property.