Human Heart:

The heart is a muscular organ responsible for pumping blood through the blood vessels by repeated contractions. The term cardiac means "related to the heart" The bulk of the walls of the heart chambers is made of cardiac muscles. In the human body, the heart is situated between the lungs, in the middle of the chest cavity (thorax) underneath the breastbone.

Pericardium:

The heart is enclosed in a sac known as the pericardium. There is a fluid, known as pericardial fluid, between the pericardium and the heart walls. It reduces friction between the pericardium and the heart, during heart contractions. Like birds and other mammals, the human heart consists of four chambers. The upper thin-wallet chambers are called the left and right atria (singular 'atrium'), and the lower thick-walled chambers are called the left and right ventricles. The left ventricle is the largest and strongest chamber in the heart.