Title: Pericardium
Content:  The pericardium, also called pericardial sac, is a double-walled sac containing the heart and the roots of the great vessels. It has two layers, an outer layer made of strong connective tissue (fibrous pericardium), and an inner layer made of serous membrane (serous pericardium). It encloses the pericardial cavity, which contains pericardial fluid, and defines the middle mediastinum. It separates the heart from interference of other structures, protects it against infection and blunt trauma, and lubricates the heart's movements. The English name originates from the Ancient Greek prefix "peri-" (περί; "around") and the suffix "-cardion" (κάρδιον; "heart").
Title: Pericardium
Content:  The pericardium is a tough fibroelastic sac which covers the heart from all sides except at the cardiac root (where the great vessels join the heart) and the bottom (where only the serous pericardium exists to cover the upper surface of the central tendon of diaphragm). The fibrous pericardium is semi-rigid, while the serous pericardium is quite pliable. The same mesothelium that constitutes the serous pericardium also covers the heart as the epicardium, resulting in a continuous serous membrane invaginated onto itself as two opposing surfaces (over the fibrous pericardium and over the heart). This creates a pouch-like potential space around the heart enclosed between the two opposing serosal surfaces, known as the pericardial space or pericardial cavity, which is filled with a small amount of serous fluid to lubricate the heart's movements and cushions it from any external jerk or shock.
Title: Heart
Content:  The pericardium is the sac that surrounds the heart. The tough outer surface of the pericardium is called the fibrous membrane. This is lined by a double inner membrane called the serous membrane that produces pericardial fluid to lubricate the surface of the heart. The part of the serous membrane attached to the fibrous membrane is called the parietal pericardium, while the part of the serous membrane attached to the heart is known as the visceral pericardium. The pericardium is present in order to lubricate its movement against other structures within the chest, to keep the heart's position stabilised within the chest, and to protect the heart from infection.
Title: Pericardium
Content: The parietal serous pericardium, which lines the interior side of the superficial portion of the pericardial sac, is fused to and inseparable from the fibrous pericardium ; The visceral serous pericardium, also known as the epicardium, covers the myocardium of the heart and can be considered its serosa. It is largely made of a mesothelium overlying some elastin-rich loose connective tissue.  During ventricular contraction, the wave of depolarization moves from the endocardial to the epicardial surface. The serous pericardium, in turn, is divided into two parts: Both of these layers function in lubricating the heart to prevent friction during heart 
Title: Pericardium
Content:  The pericardium sets the heart in mediastinum and limits its motion, protects it from infection, lubricates it and prevents excessive dilation in cases of acute volume overload.
Title: Pericardial fluid
Content:  The pericardial fluid reduces friction within the pericardium by lubricating the epicardial surface allowing the membranes to glide over each other with each heart beat.
Title: Heart
Content:  The sac which surrounds the heart, called the pericardium, can become inflamed in a condition known as pericarditis. This condition typically causes chest pain that may spread to the back, and is often caused by a viral infection (glandular fever, cytomegalovirus, or coxsackievirus). Fluid can build up within the pericardial sac, referred to as a pericardial effusion. Pericardial effusions often occur secondary to pericarditis, kidney failure, or tumours, and frequently do not cause any symptoms. However, large effusions or effusions which accumulate rapidly can compress the heart in a condition known as cardiac tamponade, causing breathlessness and potentially fatal low blood pressure. Fluid can be removed from the pericardial space for diagnosis or to relieve tamponade using a syringe in a procedure called pericardiocentesis.
Title: Peritoneal cavity
Content:  The peritoneal cavity is a potential space between the parietal peritoneum (the peritoneum that surrounds the abdominal wall) and visceral peritoneum (the peritoneum that surrounds the internal organs). The parietal and visceral peritonea are layers of the peritoneum named depending on their function/location. It is one of the spaces derived from the coelomic cavity of the embryo, the others being the pleural cavities around the lungs and the pericardial cavity around the heart. It is the largest serosal sac, and the largest fluid-filled cavity, in the body and secretes approximately 50 ml of fluid per day. This fluid acts as a lubricant and has anti-inflammatory properties. The pericardial cavity is divided into two compartments – one above, and one below the transverse colon.
Title: Pericardial fluid
Content:  Pericardial fluid is the serous fluid secreted by the serous layer of the pericardium into the pericardial cavity. The pericardium consists of two layers, an outer fibrous layer and the inner serous layer. This serous layer has two membranes which enclose the pericardial cavity into which is secreted the pericardial fluid. The fluid is similar to the cerebrospinal fluid of the brain which also serves to cushion and allow some movement of the organ.
Title: Pericardial sinus
Content:  The pericardial sinuses are impressions in the pericardial sac formed between the points where great vessels enter it.

Question: what is the function of the pericardial sac?
Answer: Lubricates the heart.