Blockchain technology has key role in electronic health record systems for storing, accessing and securing patient medical records. Patient medical information is being stored like personal bio, diagnosis, treatments etc. in electronic health records systems whenever he visits to any health sector. This information is very sensitive and private and it has been remaining a big challenge to access and secure patient medical records in a decentralized manner. Blockchain has become very important for its use in storing, accessing and managing patient medical records in a very secure and decentralized manner. In this paper, a systematic literature review has been done to review blockchain architecture for electronic health records system to store, access and secure patient medical records. The main objective of this paper is to highlight the use of blockchain in accessing and securing patient medical records. Moreover, some blockchain based electronic health records systems has been presented to secure the patient medical records. Lastly some challenges and gaps in using blockchain based medical health records systems has been presented.