WebMD Medical News: LINK: Michael Miller, MD, director of preventive cardiology at the University of Maryland Medical Center: "The endothelium is the first line in the development of atherosclerosis or hardening of the arteries, so, given the results of our study, it is conceivable that laughing may be important to maintain a healthy endothelium, and reduce the risk of cardiovascular disease. At the very least, laughter offsets the impact of mental stress, which is harmful to the endothelium." Researchers found that watching a funny movie had a healthy effect on blood vessel function, allowing them to expand and contract more effectively in response to changes in blood flow. BUT...watching a mentally stressful movie, like a war drama, may have the opposite effect, causing the lining of the blood vessels, known as the endothelium, to narrow and restrict blood flow