1. A blood vessel with a thick layer of smooth muscle and elastic fibers is A. a vein B. an artery C. a capillary D. the vena cava2. The circulatory or cardiovascular system comprises A. the lungs, heart, and brain B. the lungs, blood vessels, and heart C. the heart, blood, and blood pressure D. the heart, arteries, and veins 3. Which cells protect the body by engulfing invaders or producing antibodies? A. red blood cells B. plasma cells C. platelets D. white blood cells 4. Capillaries are part of which body system? A. skeletal B. nervous C. digestive D. circulatory 5. A blood vessel one cell thick for the absorption of nutrients and removal of wastes is A. a vein B. an artery C. a capillary D. the pulmonary artery 6. Hemoglobin carries oxygen to the body's cells. Which system contains it? A. respiratory B. circulatory C. nervous D. endocrine 7. The cells in cardiac muscle would probably have a high number of A. mRNA molecules B. Golgi apparatus C. lysosomes D. mitochondria