The left and right atria contract roughly simultaneously, followed by the left and right ventricles. Left ventricle 1st, left atrium 2nd, right ventricle 3rd, right atrium 4th. Mammals have a two-chambered heart. All arteries carry oxygenated blood. All veins carry deoxygenated blood.
All arteries carry blood to the heart.
All veins carry blood away from the heart. Imagine that you inject a rabbit with a small amount inactivated smallpox virus. You extract the antibodies that the rabbit made and inject it into a human subject who is later exposed to the virus, but remains healthy. The human subject is said to have acquired a. active immunity. During congestive heart failure, a. the heart-muscle fibers are stretched beyond their optimal length.