Chapter 42: Introduction to the Cardiovascular System

The heart is a hollow, muscular four-chambered organ, divided into upper atria (entryways for blood returning from veins) and lower ventricles (chambers that pump blood out). A septum divides the right side, which receives deoxygenated blood and pumps it through the pulmonary artery to the lungs for the pulmonary circulation, from the left side, which receives oxygenated blood from the lungs and directs it via the aorta into the systemic circulation. The cardiac cycle is a two-step process consisting of diastole, the resting phase when blood returns to and fills the heart chambers, followed by systole, the powerful contraction phase when blood is ejected. The functioning of one-way valves, including the tricuspid, mitral (bicuspid), aortic, and pulmonic valves, is crucial for maintaining unidirectional blood flow. The heart muscle, or myocardium, possesses inherent characteristics: automaticity, the ability of cells (particularly the primitive P cells) to spontaneously generate an action potential without external stimuli, and conductivity, the ability to rapidly transmit the impulse. The normal pacemaker is the sinoatrial (SA) node, located in the right atrium, which generates an impulse that travels through the atrial bundles and slows briefly at the atrioventricular (AV) node, before rapidly descending through the bundle of His, bundle branches, and Purkinje fibers to stimulate ventricular contraction. An electrocardiogram (ECG) is an electrical tracing that reflects this activity but does not measure mechanical action, displaying key waveforms such as the P wave (atrial depolarization), QRS complex (ventricular depolarization), and T wave (ventricular repolarization). Alterations in rate or rhythm are called arrhythmias or dysrhythmias and can significantly impact cardiac output, including conditions like sinus tachycardia, atrial fibrillation, heart block, or potentially fatal ventricular fibrillation. The strength of myocardial contraction relies on the functional unit called the sarcomere, where calcium inactivates troponin, enabling the reaction and sliding of actin and myosin filaments to form actomyosin bridges. This contractility is also described by Starling’s law, which states that the more the muscle fibers are stretched (by increased preload/volume return), the stronger the subsequent contraction will be, up to a maximal point. Crucially, the myocardium receives its oxygenated blood supply primarily during diastole via the coronary arteries. Systemic arterial blood pressure is regulated by several normal controls: the sympathetic nervous system increases heart rate and contraction strength; the renin–angiotensin–aldosterone system (RAAS) increases pressure by causing severe vasoconstriction via angiotensin II and increasing water and sodium retention via aldosterone and antidiuretic hormone (ADH); and natriuretic peptides (like ANP and BNP) oppose the RAAS, promoting natriuresis and diuresis to lower volume and pressure. Blood flow through the capillary system is governed by the capillary fluid shift, where fluid filters out at the arterial end due to hydrostatic pressure (HP) and is pulled back in at the venous end due to oncotic pressure (OP), exerted by plasma proteins; disruptions in this pressure balance can lead to edema, often seen in conditions like heart failure (HF).