The internet of things (IoT) refers to a framework of interrelated, web associated objects that can gather and move information over a remote network without human interference. With a quick development in the arrangement of IoT gadgets and expanding want to make medical care more financially savvy, customized, and proactive, IoT is ready to assume a solid function in all perspectives of the healthcare industry. In this context, IoT-based healthcare provides several benefits such as instant and reliable treatment, cost reduction, improved communication, etc. by using different new technologies. Wireless Body Area Networks (WBAN) technologies can enhance the quality of data gathering and data transferring in smart systems. Machine Learning(ML) are put to use at every level of smart healthcare systems. Fog computing reduces communication cost and provides low latency. Software-Defined Networking (SDN) and Network Function Virtualization (NFV) technologies provide less complex and more flexible network structures. Blockchain technology offers a better way of protection of users' sensitive information. This paper aims to provide comprehensive descriptions of ongoing research projects and the utilization of the above-mentioned technologies in smart healthcare systems. In this paper, the latest studies, proposed techniques, and the current solutions of smart healthcare systems are elaborated in the context of emerging technologies, applications and challenges of these systems to provide a better understanding of what IoT means in the healthcare industry now and what it will mean in the future.