Which of the following statements about the structure of the heart is (are) true? ANSWER The structure of the atrioventricular and semilunar valves helps to ensure that blood flows in only one direction through the heart and the innermost layer of epithelial cells, the endothelium, lines the inside of the entire cardiovascular system. The left ventricular wall is thicker than the right. The structure of the atrioventricular and semilunar valves helps to ensure that blood flows in only one direction through the heart. All of the listed responses are correct. The innermost layer of epithelial cells, the endothelium, lines the inside of the entire cardiovascular system.