We've learned David Lynch's cause of death ... his death certificate was released Friday.

In the L.A. County Dept. of Public Health document, David's immediate cause of death is listed as cardiac arrest ... with chronic obstructive pulmonary disease as the underlying condition. The document also lists dehydration as a significant contributor.

Lynch himself revealed last year he was diagnosed with emphysema after years of smoking cigarettes, and likely wouldn't be able to direct anymore, or even leave his house. In November, the director responsible for "Blue Velvet," "Mulholland Drive" and "Twin Peaks" admitted he couldn't even walk without oxygen ... saying smoking had been an integral part of his life since he started at just 8 years old.

As we reported ... a family member told TMZ Lynch died on Wednesday, January 15, at his daughter's house -- where he had been staying during the devastating wildfires in Los Angeles.

According to the certificate, he was cremated and his remains were buried on January 22 at the Hollywood Forever Cemetery.

Lynch was 78 years old.

RIP