Heart wall

Heart wall

Main article: Cardiac muscle

Layers of the heart wall, including visceral and parietal pericardium.

The heart wall is made up of three layers: the inner endocardium, middlemyocardium and outer epicardium. These are surrounded by a double-membraned sac called the pericardium.

The innermost layer of the heart is called the endocardium. It is made up of a lining of simple squamous epithelium, and covers heart chambers and valves. It is continuous with the endothelium of the veins and arteries of the heart, and is joined to the myocardium with a thin layer of connective tissue.^[7] The endocardium, by secreting endothelins, may also play a role in regulating the contraction of the myocardium.^[7]

The swirling pattern of myocardium helps the heart pump effectively

The middle layer of the heart wall is the myocardium, which is the cardiac muscle– a layer of involuntary striated muscle tissue surrounded by a framework of collagen. The myocardium is also supplied with blood vessels, and nerve fibers by way of the epicardium that help to regulate the heart rate.^[7] Cardiac muscle tissue hasautorhythmicity, the unique ability to initiate a cardiac action potential at a fixed rate – spreading the impulse rapidly from cell to cell to trigger the contraction of the entire heart. This autorhythmicity is still modulated by the endocrine and nervous systems.^[7]

There are two types of cardiac muscle cell: cardiomyocytes which have the ability to contract easily, and modified cardiomyocytes the pacemaker cells of the conducting system. The cardiomyocytes make up the bulk (99%) of cells in the atria and ventricles. These contractile cells are connected by intercalated discs which allow a rapid response to impulses of action potential from the pacemaker cells. The intercalated discs allow the cells to act as a syncytium and enable the contractions that pump blood through the heart and into the major arteries.^[7]

The pacemaker cells make up 1% of cells and form the conduction system of the heart. They are generally much smaller than the contractile cells and have few myofibrils which gives them limited contractibility. Their function is similar in many respects to neurons.^[7]

The cardiac muscle pattern is elegant and complex, as the muscle cells swirl and spiral around the chambers of the heart.^[7]They form a figure 8 pattern around the atria and around the bases of the great vessels.^[7] Deeper ventricular muscles also form a figure 8 around the two ventricles and proceed toward the apex. More superficial layers of ventricular muscle wrap around both ventricles.^[7] This complex swirling pattern allows the heart to pump blood more effectively than a simple linear pattern would.^[7]

As with skeletal muscles the heart can increase in size and efficiency with exercise.^[7] Thus endurance athletes such asmarathon runners may have a heart that has increased in size by up to 40%.^[17]

The pericardium surrounds the heart. It consists of two membranes: an inner serous membrane called the epicardium, and an outer fibrous membrane.^[18] These enclose the pericardial cavity which contains the pericardial fluid that lubricates the surface of the heart.

Chambers

Heart being dissected showing right and left ventricles, from above

The heart has four chambers, two upper atria, the receiving chambers, and two lower ventricles, the discharging chambers. The atria are connected to the ventricles by the atrioventricular valves and separated from the ventricles by thecoronary sulcus. There is an ear-shaped structure in the upper right atrium called the right atrial appendage, or auricle, and another in the upper left atrium, the left atrial appendage. The right atrium and the right ventricle together are sometimes referred to as the right heart and this sometimes includes the pulmonary artery. Similarly, the left atrium and the left ventricle together are sometimes referred to as the left heart. The ventricles are separated by the anterior longitudinal sulcus and the posterior interventricular sulcus.

The cardiac skeleton is made of dense connective tissue and this gives structure to the heart. It forms the atrioventricular septum which separates the atria from the ventricles, and the fibrous rings which serve as bases for the four heart valves.^[19] The cardiac skeleton also provides an important boundary in the heart’s electrical conduction system since collagen cannot conduct electricity. The interatrial septum separates the atria and the interventricular septum separates the ventricles.^[7] The interventricular septum is much thicker than the interatrial septum, since the ventricles need to generate greater pressure when they contract.