In the human body, the [?] is normally situated slightly to the left of the middle of the thorax, underneath the [?] . It is enclosed by a sac known as the [?] and is surrounded by the [?]. In adults, it weighs about 300-350 g. It consists of [?] chambers, the two upper atria (singular: atrium) and the two lower [?].
A thick muscular wall, the septum, divides the right atrium and ventricle from the left atrium and ventricle, preventing blood from passing between them. [?] between the atria and ventricles (atrioventricular valves) maintain coordinated unidirectional flow of blood from the atria to the ventricles.