Everything I've been reading about blockchain from my understanding says that even on a private blockchain, every participant can view all transactions. I've seen it mentioned that a use case for block chain could be the sharing of medical data. So for example if I had a blockchain that holds the medical history of every person from birth to death in a country. Is there no way of setting up permissions so that only data relating to a person and those who have been given permission to that person's data can view it? If the data is stored on every node in a blockchain, how is a person's computer supposed to have the capacity to store the medical data of every person in a country?