Central Thailand (central plain) is a region of Thailand, covering the broad alluvial plain of the Chao Phraya River. It is separated from northeast Thailand (Isan) by the Phetchabun mountain range. The Tenasserim Hills separate it from Myanmar to the west. In the north it is bounded by the Phi Pan Nam Range, one of the hilly systems of northern Thailand. The area was the heartland of the Ayutthaya Kingdom, and is still the dominant area of Thailand. Central Thailand contains the Thai capital of Bangkok and it is the most populated region in the country.