Chapter 4: Cardiovascular System

The heart operates as a four-chambered muscular pump whose electrical activity originates in the sinoatrial node and propagates through specialized conduction pathways to coordinate atrial and ventricular contractions in a synchronized manner. The ventricular action potential involves sequential ion flux across the myocyte membrane, particularly calcium entry during the plateau phase, which triggers the release of additional calcium from intracellular stores to activate the contractile apparatus. The electrocardiogram provides a noninvasive method to assess cardiac electrical activity by recording the depolarization and repolarization events corresponding to atrial and ventricular activity. Cardiac output, the volume of blood ejected per unit time, depends on both heart rate and stroke volume, with stroke volume being regulated by contractility, preload, and afterload. Structural or functional impairments lead to systolic or diastolic heart failure, prompting compensatory mechanisms such as fluid retention through the renin-angiotensin-aldosterone system, which paradoxically worsens cardiac performance. The peripheral vasculature is organized hierarchically from large elastic arteries through muscular arterioles that regulate regional blood flow to microscopic capillaries where nutrient and waste exchange occurs. Blood pressure homeostasis is maintained through baroreceptor reflexes and sympathetic nervous system modulation of heart rate and vascular tone. Capillary fluid exchange is determined by Starling forces, balancing hydrostatic and oncotic pressures; disruption of this equilibrium results in interstitial fluid accumulation and edema. Coronary atherosclerosis develops through accumulation of oxidized lipoproteins and immune cells within arterial walls, forming plaques that can rupture and occlude vessels, precipitating myocardial infarction. The cardiovascular system also serves as the transport medium for hematopoietic cells that originate in bone marrow, including erythrocytes, leukocytes, and platelets, with disorders such as sickle cell disease exemplifying how mutations in hemoglobin structure compromise red blood cell function. The lymphoid system, integrated throughout the cardiovascular system, includes primary organs where immune cells develop and secondary organs where immune surveillance and response to pathogens occur.