instruction: Given this passage about the human heart, tell me about the major structural features of the human heart as well as the primary purpose of the organ.
context: The heart is a muscular organ in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon dioxide to the lungs. In humans, the heart is approximately the size of a closed fist and is located between the lungs, in the middle compartment of the chest, called mediastinum .
  
  In humans, other mammals, and birds, the heart is divided into four chambers: upper left and right atria and lower left and right ventricles. Commonly the right atrium and ventricle are referred together as the right heart and their left counterparts as the left heart. Fish, in contrast, have two chambers, an atrium and a ventricle, while most reptiles have three chambers. In a healthy heart blood flows one way through the heart due to heart valves, which prevent backflow. The heart is enclosed in a protective sac, the pericardium, which also contains a small amount of fluid. The wall of the heart is made up of three layers: epicardium, myocardium, and endocardium.
response: The major structural features of the human heart are its four chambers for pumping blood. The heart is commonly viewed as having a left side and a right side, with each side comprising two chambers -- an atrium and a ventricle. The four total chambers make up the organ's primary structural features. Together, they are responsible for maintaining blood flow throughout the body via the circulatory system. The heart is also responsible for moving waste products through the bloodstream to the organs that process them.