Chapter 13: Cardiovascular System

Cardiovascular System outlines the cardiovascular system, a crucial transport network that moves blood and lymph to and from the body’s tissues, composed fundamentally of the heart, blood vessels, and lymphatic vessels. Blood circulates through the low-pressure pulmonary circuit, which conveys blood to the lungs for oxygenation, and the high-pressure systemic circuit, which distributes blood to all other tissues. The heart, functioning as a four-chambered muscular pump, is essential to this process. Its wall is organized into three layers: the epicardium (outer layer, containing coronary vessels), the thick middle myocardium (cardiac muscle), and the inner endocardium, where the subendocardial layer houses components of the conducting system. The rhythmic, coordinated contraction is initiated intrinsically by the specialized conducting system, starting with the sinoatrial (SA) node (the dominant pacemaker), propagating to the atrioventricular (AV) node, and spreading rapidly via the AV bundle and Purkinje fibers. This activity is regulated by the autonomic nervous system, with sympathetic input increasing heart rate (tachycardia) and force of contraction, while parasympathetic input decreases the rate (bradycardia). Blood vessels are characterized by three distinct layers, or tunics: the inner tunica intima (lined by endothelium), the muscular tunica media, and the outer tunica adventitia. The vascular endothelium is highly active, maintaining a selective permeability barrier, preventing thrombosis, regulating immune responses, and modulating blood flow through the release of potent vasodilators, such as nitric oxide (NO), and vasoconstrictors, like the endothelins. Arteries are structurally classified as elastic (large, resistance vessels that maintain flow) or muscular (medium, distribution vessels) and progressively narrow into small arteries and arterioles, which control access to the capillary beds. Capillaries, the primary sites of molecular exchange, are categorized into continuous, fenestrated, and discontinuous (sinusoidal) types based on their permeability and presence of junctions. Veins collect blood, generally having thinner walls and larger lumens than corresponding arteries, and often feature valves, especially in the limbs, to prevent backflow. Lastly, the lymphatic vessels, which are more permeable than blood capillaries, return interstitial fluid and proteins (lymph) back to the bloodstream, starting with blind-ended lymphatic capillaries that possess button-like endothelial cell junctions.