Chapter 21: Cardiovascular System: The Heart

The cardiovascular system is kept functional by the heart, a muscular organ roughly the size of a clenched fist, which tirelessly pumps blood to maintain physiological stability. This pumping action drives blood through two continuous circuits: the pulmonary circuit, which carries carbon dioxide-rich blood to the lungs for gas exchange; and the systemic circuit, which transports oxygen-rich blood to the body’s cells. The heart resides within the pericardial cavity located in the mediastinum, enveloped by the tough outer fibrous pericardium and the inner serous pericardium, which includes the epicardium (visceral pericardium). The muscular wall, the myocardium, is composed of specialized cardiac muscle cells, or cardiocytes, which are highly reliant on aerobic respiration. These cardiocytes are structurally and electrically linked by specialized connections called intercalated discs, featuring gap junctions that enable the tissue to act like a single, massive cell, known as a functional syncytium. Internally, the heart contains four chambers separated by the interatrial and interventricular septa. Deoxygenated blood enters the right atrium via the superior vena cava and inferior vena cava and passes through the three-flapped tricuspid valve (right atrioventricular valve) into the right ventricle. The right ventricle then propels this blood through the pulmonary valve into the pulmonary trunk. Oxygenated blood returns to the left atrium from the pulmonary veins and flows through the two-flapped mitral valve (left AV or bicuspid valve) into the left ventricle. Notably, the left ventricle has the thickest myocardium to generate the high pressure needed to pump blood across the entire systemic circuit. The AV valves are secured by chordae tendineae anchored to papillary muscles, which tense to prevent the valves from swinging backward (regurgitation) during ventricular contraction. The heart muscle itself is supplied by the coronary circulation, originating from the ascending aorta, which includes the right coronary artery and the left coronary artery (with its circumflex and anterior interventricular branches). The heart’s intrinsic rhythm is governed by automaticity, initiated by the pacemaker cells found in the sinoatrial (SA) node. The electrical stimulus proceeds through the internodal pathways to the atrioventricular (AV) node, the AV bundle, the bundle branches (like the moderator band), and finally the Purkinje fibers, ensuring that the atria contract (systole) just before the ventricles. This basic rate is modulated by the autonomic nervous system via the cardiac centers in the medulla oblongata: sympathetic neurons release norepinephrine to increase the heart rate and contraction force, while parasympathetic neurons release acetylcholine to decrease them. Pathological conditions range from valve leakage leading to a heart murmur (like in mitral valve prolapse) to severe issues like Coronary Artery Disease (CAD), which can restrict blood flow (ischemia), cause chest pain (angina pectoris), and ultimately lead to a myocardial infarction (MI) or heart attack. Disorders in the electrical pathway, or cardiac arrhythmias (such as tachycardia or the life-threatening ventricular fibrillation), often necessitate the use of medical devices like artificial pacemakers.