The heart is an active organ of the human body. Its pumping action, which begins in the very early embryo, continues without stopping until death.

During each cycle of its activity, the heart changes in shape and size and tends also to rotate. (The number of cycles per minute is called heart rate.)

To reduce the amount of friction resulting from this activity, the heart is included within a serous (secretory) sac, called the pericardium, or pericardial sac.

Label 1Serous Space
Between the two serous pericardia is a very thin space containing a thin film of pericardial fluid. This lubricating fluid makes the action of the heart much less strenuous.

Label 2 Visceral Pericardium
The visceral pericardium intimately covers the surface of the heart.
Earlier, we referred to this as the epicardium.

Label 3 Parietal Pericardium
The parietal pericardium is the outer serous membrane.

Label 4 Fibrous Pericardium
The parietal pericardium is covered with a very dense fibrous envelope.

This envelope forms the outer portion of the pericardial sac.