Cardiovascular System: The Heart

This chapter will discuss the microscopic and gross anatomy of the heart. The wall of the heart has three tissue layers. The innermost layer is the endocardium, which is made of simple squamous epithelium and is continuous with the endothelium of the large blood vessels that enter and leave the heart. The middle layer, which make up the bulk of the thickness, is the myocardium. The myocardium is made up of cardiac muscle tissue which is organized by connective tissue. The outermost layer, the epicardium is part of the serous pericardium, which forms the pericardial cavity (labeled in Figure 4).

Cardiac Muscle Tissue

The cardiac muscle tissue in our heart is responsible for contraction, which generates the pressure needed to pump blood throughout our body. Cardiac muscle cells (cardiomyocytes) are branched, and connected to each other through unique connections called intercalated discs, which contain gap junctions and desmosomes (Figures 1-3). Typically, cardiac muscle cells have one larger, centrally-located nucleus (uninucleated). Cardiac muscle tissue is striated, which refers to the perpendicular “striped” appearance. Contraction of cardiac muscle tissue is involuntary, meaning that you cannot consciously control contraction of your heart (Table 1). Figures 1-3 below show three separate representations of cardiac muscle tissue.

Table 1: Cardiac muscle

Figure 1: Cardiac muscle (version A) with and without illustration overlay

Figure 2: Cardiac muscle (version B) with and without illustration overlay

Figure 3: Cardiac muscle (version C) with and without illustration overlay

Systemic and pulmonary Circuits

Blood vessels circulate blood throughout your body helping to transport gases, nutrients, hormones, and waste products to the appropriate organs and tissues. The blood vessels are part of two distinct circulations: the pulmonary and systemic circulation. The pulmonary circuit is a low pressure system that moves blood between the heart and the lungs. Deoxygenated blood is pumped from the right ventricle to the lungs, where it releases carbon dioxide and absorbs oxygen. The oxygenated blood then returns to the heart. Vessels carrying oxygenated blood are colored red on  anatomical models, vessels carrying deoxygenated blood are colored blue on the anatomical models. A third circulation, the coronary circulation, supplies the tissue of the heart.

In the systemic circuit,  blood moves between the heart and the rest of the tissues of the body. Oxygenated blood is pumped from the left ventricle of the heart  to the body’s tissues where it delivers oxygen and absorbs carbon dioxide. Deoxygenated blood returns back to the heart. Arteries carry blood away from the heart, while veins carry blood to the heart.  The systemic circuit is a very high pressure system that loses pressure as blood moves from the arteries to the veins. Because the systemic circuit is a higher pressure system than the pulmonary circuit, the myocardium of the left ventricle must be thicker than the myocardium of the right ventricle to achieve this higher pressure.

Figure 4: Pathway of flow of blood through the heart

The heart is divided into four chambers – two atria (singular, atrium) and two ventricles (Figure 4 and 5). Figure 4 illustrates the internal heart structures and great vessels entering and exiting the heart, with the white arrows indicating the direction of blood flow through these structures. The superiorly-positioned atria receive blood from either the system (right atria) or the lungs (left atria). Blood moves inferiorly through the atrioventricular valves into the ventricles. Blood is pumped from the right ventricle to lungs through the pulmonary circulation and from the left ventricle to body through the systemic circulation. To reach these circulations, blood passes through valves which are named for the vessel they are in. The pulmonary semilunar valve separates the right ventricle and the pulmonary trunk and the aortic valve separates the left ventricle and the aorta. All four heart valves ensure that blood flows in the correct direction, preventing backflow of blood.

The right side of the heart is typically considered the deoxygenated side. Deoxygenated blood returns to right atrium of the heart through the superior vena cava, inferior vena cava and coronary sinus (coronary circulation = blood supply to the heart muscle). Blood moves from the right atrium through the right atrioventricular valve or tricuspid into the right ventricle. Upon contraction, blood moves from the right ventricle through the pulmonary semilunar valve into the pulmonary trunk. The pulmonary trunk divides into the right and left pulmonary arteries and carries this deoxygenated blood to the lungs for gas exchange.

The pulmonary veins return oxygenated blood to the left atrium of the heart. Blood moves from the left atrium through the left atrioventricular valve or bicuspid/mitral valve into the left ventricle. Upon contraction, blood moves from the left ventricle through the aortic semilunar valve into the aorta. Many vessels branch off the aorta and carry blood throughout the systemic circuit.

Figure 5: Gross anatomy of the heart with and without labels, anterior view

Intrinsic conduction network of the heart

The heart contains a network of non-contractile autorhythmic cells that are responsible for setting the “rhythm” or frequency of heart beats. These cells are called pacemaker cells and they are part of a network of cells that stimulate contraction of the cardiac muscle cells. This network is called the intrinsic conduction network, and is shown in yellow on Figure 6. When these cells depolarize, the wave of depolarization spreads to nearby contractile cardiac muscle cells and stimulates them to depolarize and contract.

Conduction begins at the sinoatrial node (SA node), which is located on the posterior wall of the right atrium. The wave of depolarization spreads across both atria and stimulates the atria to contract. The wave of depolarization reaches the atrioventricular node (AV node), which is located on the inferior wall of the right atria between the right atria and the right ventricle.  After a brief delay, the wave of depolarization spreads from the AV node to the bundle of His (AV bundle), where it branches into left and right bundle branches as it spreads down the septum that divides the right and left ventricles. The wave of depolarization spreads throughout the muscle of the ventricles through Purkinje fibers that branch off of the right and left bundle branches. This depolarization from the Purkinje fibers spreads across both ventricles and stimulates the ventricles to contract beginning at the apex of the heart. Initiating contraction at the apex pushes blood superiorly towards the great vessels.

Figure 6: Gross anatomy of the heart with and without labels, right lateral view 

Coronary Circulation

The tissue of the heart is supplied by the coronary circulation. Blood enters the coronary circulation through small branches off the aorta. The anterior view of the external heart structures, including great vessels and coronary vasculature is shown in Figure 7.

Figure 7: External structures of the heart with great vessels and coronary vasculature

Chapter Illustrations By:

Georgios Kallifatidis, Ph.D.

Soma Mukhopadhyay, Ph.D.

Juan Manuel Ramiro Diaz, Ph.D.