Wearable sensor devices organized in a Wireless Body Area Network (WBAN) have been widely used in healthcare monitoring. However, energy efficiency and reliability are still major problems in this area. An efficient media access control (MAC) protocol is a promising solution. Traditional protocols are too generic and usually used in busy frequency band which are not suitable for a particular medical scenario. On the basis of Human Body Communication (HBC), a MAC protocol for WBAN is proposed in this work which can avoid interference and make non-RF technique available. It also helps reduce power consumption and prolong the network lifetime by reducing transmitting frequency and increasing sleep time of a WBAN node. The protocol utilizes a new ALOHA medium access which defines user priorities for different types of data. Compared with the Slotted ALOHA (S-Aloha) suggested in the IEEE 802.15.6 standard, our method shows good performance in terms of power consumption when the information rate is low. In addition, a circuit system is designed for a medical scenario according to our new protocol.