Final answer:
The mesothelium, a macroscopic layer of the heart wall, secretes the lubricating serous fluid that fills the pericardial cavity and reduces friction as the heart contracts.
Step-by-step explanation:
The lubricating serous fluid that fills the pericardial cavity and reduces friction as the heart contracts is secreted by the mesothelium, a type of simple squamous epithelium, that is part of the heart wall. The mesothelium is a macroscopic layer that attaches to the pericardium and is made up of loose, irregular connective tissue. The serous fluid is produced by the mesothelium and fills the pericardial cavity, reducing friction and allowing the heart to contract smoothly.