How do universities know about extenuating circumstances??

I got 3 A's at As level and BBD at A2 as I was waiting on an operation and wanted to at least try to take the A2 exams, my teachers know about it as I took a LOT of time off last year (around 8-10% off school!).

I had the operation a couple of weeks after exams and now I have nearly fully recovered and can't believe how i functioned before having it! I really want to retake the exams and get at least 3 A's and apply to good universities.

How will the universities know about my circumstances and complications last year? Do i have to write it in the personal statement or contact them directly?