Chapter 25: Structure and Function of the Cardiovascular and Lymphatic Systems

The heart operates as two serially connected pumps, with the right ventricle directing deoxygenated blood through pulmonary circulation and the left ventricle propelling oxygenated blood through systemic circulation. Cardiac architecture comprises four chambers, four valves, and a fibrous skeleton, enclosed within the pericardium and lined by endocardium, while the myocardium generates the contractile force necessary for effective pumping. The sinoatrial node initiates electrical activity, which propagates through the atrioventricular node, bundle of His, and Purkinje fibers to coordinate atrial and ventricular contractions; these electrical events correspond to the P wave, QRS complex, and T wave visible on electrocardiography. Cardiac performance depends on multiple regulatory factors including preload, afterload, contractility, and heart rate, which collectively determine cardiac output according to the Frank-Starling mechanism. The coronary circulation perfuses the myocardium, while myoglobin facilitates oxygen storage within cardiac muscle. Systemic circulation consists of arteries that branch into arterioles and precapillary sphincters controlling microcirculation, followed by capillaries for gas and nutrient exchange, then venules and veins that return blood centrally. Blood vessel walls contain three distinct layers: the tunica intima with endothelial lining, tunica media with smooth muscle, and tunica externa providing structural support. The endothelium actively regulates vasomotor tone, hemostasis, coagulation, inflammation, and angiogenesis. Blood flow is governed by pressure gradients, resistance as described by Poiseuille's law, vessel compliance, and the distinction between laminar and turbulent flow patterns. Blood pressure regulation integrates baroreceptor and chemoreceptor reflexes, autonomic nervous system control, and hormonal systems including the renin-angiotensin-aldosterone mechanism, vasopressin, catecholamines, and natriuretic peptides. The lymphatic system functions as a unidirectional network collecting interstitial fluid, proteins, and immune cells from tissues and returning them to central circulation via the thoracic and right lymphatic ducts. Lymph nodes filter antigens and coordinate immune responses while supporting overall fluid homeostasis and lipid transport. Together, these systems establish the foundational physiology underlying cardiovascular and immunological health.