If you haven't already, do some wider reading and write about that - write about why you were interested in it, what you learnt from it, what particularly interested you about what you read, and what you now want to learn more about (i.e. why you want to go and study Biomed at Uni basically). Basically read smth, reflect on it + discuss it in depth.
Any experiences you've had relating to Biomed are great to write about too, e.g. work experience, Uni taster days, lectures, even stuff you've learnt in class. What's even better is if you tie this to something you've read.
It should be ok to write about your kidney problems etc. but I'd be careful writing stuff like 'Ever since I was a child I've wanted to study this blah blah blah' because Unis assume that you are obviously devoted to your subject and know it's your one true calling in life etc etc. For example, I didn't write about my childhood love of chemistry because they don't tend to like that sort of thing and because it would've been super cringe ahaha. Your example is probs a bit better than mine would've been, but even so I'd still write about experiences, wider reading, etc. instead.