Summary
Highlights
The circulatory system, also known as the cardiovascular system, consists of the heart, arteries, veins, and capillaries. While the brain handles thought and emotion, the heart functions primarily as a pump, located in the mediastinum.
The heart participates in two circuits: the pulmonary circuit, where blood is sent to the lungs for oxygenation and then returns to the left side of the heart, and the systemic circuit, where oxygenated blood is distributed throughout the body and then returns to the right side of the heart.
The heart has four chambers: the right and left atria (receiving chambers) and the right and left ventricles (pumping chambers). The heart is protected by the pericardium, a dense connective tissue, which has fibrous and serous layers including the epicardium, myocardium, and endocardium.
The heart contains four chambers, two atria and two ventricles, separated by the interatrial and interventricular septa. Oxygen-poor blood enters the right atrium via the superior vena cava, inferior vena cava, and coronary sinus. Oxygenated blood enters the left atrium via the pulmonary veins. The ventricles are the main pumps, with the right ventricle sending blood to the lungs and the left ventricle propelling blood into the aorta.
Heart valves ensure unidirectional blood flow. There are two atrioventricular (AV) valves (tricuspid on the right, mitral/bicuspid on the left) and two semilunar valves (aortic and pulmonary valves). These valves close due to pressure changes, preventing backflow.
Cardiac muscle is unique to the heart, striated like skeletal muscle, but with short, fat, branched fibers containing one or two nuclei. Intercalated discs with desmosomes and gap junctions connect cardiac cells, allowing for coordinated contraction and rapid spread of depolarization. Cardiac muscle also has a longer refractory period to ensure proper heart function.
The heart has an intrinsic cardiac conduction system with specialized pacemaker cells that can self-excite. These cells continuously depolarize until a threshold is reached, initiating contractions and ensuring coordination. Problems with this system can lead to arrhythmias. The heart also has its own coronary arteries and veins for blood supply.