What covers the heart and blood vessels?

The heart and blood vessels are covered by a protective layer known as the pericardium, which is a double-walled sac. The outer layer, called the fibrous pericardium, provides structural support, while the inner layer, known as the serous pericardium, produces a lubricating fluid that reduces friction during heartbeats. Additionally, blood vessels are lined with a layer of endothelial cells, which help regulate blood flow and maintain vascular health.