Final answer:
True, the endothelium is a single layer of thin, flat cells that lines the heart and all blood vessels. It forms the tunica intima, is continuous with the endocardium of the heart, and plays vital roles in physiological processes such as blood flow regulation and blood pressure maintenance.
Step-by-step explanation:
A single layer of thin, flat cells that lines the heart and all blood vessels is indeed called endothelium. The endothelium is a specialized simple squamous epithelium which forms the innermost layer of the heart and blood vessels, a fact that supports the true statement to the student's question. This layer is known as the tunica intima or the endothelial tunic and is continuous with the endocardium of the heart. The endothelium plays an essential role in various physiological processes, including the regulation of capillary exchange, blood flow, and blood pressure.
In addition to lining the heart valves and chambers, the endothelium is attached to a basal lamina or basement membrane. This attachment provides stability and sustains the delicate tissue. The endothelial cells are crucial in maintaining homeostasis and, when damaged, can lead to clot formation. Furthermore, the endothelium produces chemicals called endothelins that can influence vascular resistance and pressure.