M01.05.016 The Pericardium

The pericardium, often likened to the peel of an orange, may seem unexciting at first glance. However, its crucial physiological roles are fundamental to the heart’s function and overall cardiovascular health. This article explores the anatomy, functions, innervation, and clinical relevance of the pericardium, emphasizing its importance in maintaining heart health.

Anatomical Structure of the Pericardium

The pericardium is a fibro-serous, fluid-filled sac that surrounds the heart and the roots of the great vessels. It consists of two primary layers: the fibrous pericardium and the serous pericardium.

Fibrous Pericardium

• Structure: Made of tough connective tissue, this external layer is non-distensible and rigid.
• Attachment: Continuous with the central tendon of the diaphragm, it is also attached to the sternum and the adventitia of the great vessels.
• Function: Prevents rapid overfilling of the heart and offers a degree of protection to the heart from excessive motion.

Serous Pericardium

• Structure: A thinner, inner layer that is divided into two parts:
  1. Parietal layer: Lines the inner surface of the fibrous pericardium.
  2. Visceral layer: Forms the outer layer of the heart, also known as the epicardium.
• Mesothelium: Both layers consist of a single sheet of epithelial cells.
• Pericardial Cavity: A small amount of lubricating serous fluid is found between these layers, reducing friction as the heart beats.

Mnemonic for Layers of the Pericardium:

• Fart Police Smell Villains
  • F: Fibrous layer of the pericardium
  • P: Parietal layer of the serous pericardium
  • S: Serous fluid
  • V: Visceral layer of the serous pericardium

Functions of the Pericardium

The pericardium serves several key physiological roles in maintaining heart health:

1. Fixation of the Heart
   • Function: Secures the heart in the mediastinum by attaching to the diaphragm, sternum, and the tunica adventitia of the great vessels.
2. Prevention of Overfilling
   • Function: The fibrous pericardium’s rigidity prevents the heart from over-expanding rapidly, ensuring that the heart’s size is regulated.
3. Lubrication
   • Function: The pericardial cavity contains fluid that reduces friction during heart contractions, enabling smooth movement within the thoracic cavity.
4. Protection from Infection
   • Function: The fibrous pericardium acts as a physical barrier between the heart and surrounding organs, such as the lungs, that are susceptible to infection.

Clinical Relevance of the Pericardium

The pericardium’s structure and function are crucial in various clinical scenarios, particularly when complications arise.

Transverse Pericardial Sinus

• Description: A passage formed during embryological development.
• Location:
  • Posterior to the ascending aorta and pulmonary trunk.
  • Anterior to the superior vena cava.
  • Superior to the left atrium.
• Clinical Use: During coronary artery bypass grafting (CABG), the transverse pericardial sinus helps to isolate and ligate the heart’s arterial vessels (aorta and pulmonary trunk).

Cardiac Tamponade

• Description: A life-threatening condition where fluid accumulates in the pericardial cavity.
• Impact: The fibrous pericardium is inextensible, so as fluid accumulates, the heart becomes compressed, resulting in reduced cardiac output and impaired function.

Pericarditis

• Description: Inflammation of the pericardium, often caused by infection, myocardial infarction, or autoimmune diseases.
• Symptoms: Chest pain is the hallmark symptom. In severe cases, pericarditis can lead to cardiac tamponade due to the buildup of fluid in the pericardial cavity.

Innervation of the Pericardium

• Nerve Involved: The phrenic nerve (C3-C5) is responsible for the somatic innervation of the pericardium, as well as providing motor and sensory innervation to the diaphragm.
• Clinical Significance: The phrenic nerve can cause referred pain, with shoulder pain being a common symptom of pericarditis.

Points to Remember

• The pericardium is essential for fixing the heart in place, preventing overfilling, reducing friction, and offering protection from infection.
• It consists of two main layers: fibrous and serous, with the serous layer further divided into parietal and visceral layers.
• The transverse pericardial sinus is clinically significant during CABG surgery.
• Accumulation of fluid in the pericardial cavity can lead to life-threatening conditions like cardiac tamponade.
• The phrenic nerve is key in innervating the pericardium and can cause referred pain, especially in cases of pericarditis.

Summary Table of Key Features

By understanding the structure, function, and clinical importance of the pericardium, students can better appreciate its role in cardiovascular health and the potential consequences when it malfunctions.