A patient with atherosclerotic heart disease experiences chest pain during exertion because:

In the case of a patient with atherosclerotic heart disease, experiencing chest pain during exertion is primarily due to the narrowing of the lumen of the coronary arteries. Atherosclerosis leads to the buildup of plaque within the arterial walls, which constricts the space through which blood flows. During periods of increased physical activity or exertion, the heart requires more oxygenated blood to meet the heightened demand. However, the narrowed coronary arteries cannot sufficiently increase blood flow to meet this demand, resulting in ischemia and subsequent chest pain, commonly referred to as angina.

This situation outlines the relationship between the structural changes caused by atherosclerosis and the functional limitations in the blood supply to the myocardium. Hence, the patient's symptoms during exertion can be directly attributed to the inability of the narrowed coronary arteries to adequately supply increased blood flow.