The Cardiovascular System: Rivers of Life

The cardiovascular system — heart, arteries, capillaries, and veins — is the body's circulatory highway, delivering oxygen, nutrients, and hormones to every cell while removing carbon dioxide and metabolic waste. The adult heart beats approximately 60–100 times per minute at rest, pumping roughly 5 liters of blood per minute (cardiac output), a figure that can rise to 20–25 liters during maximal exercise in trained athletes. This guide is for educational purposes only.

## Heart Anatomy

The heart is a hollow, muscular organ roughly the size of a closed fist, situated in the middle mediastinum within the pericardial sac. The pericardium consists of an outer fibrous layer and a double-layered serous pericardium (parietal and visceral layers), with the pericardial cavity between them containing 15–50 mL of serous fluid that reduces friction during cardiac contraction.

The heart wall has three layers: the outer epicardium (visceral pericardium), the middle myocardium (the muscular bulk, composed of cardiac muscle fibers arranged in spiral sheets), and the inner endocardium (a smooth endothelial lining continuous with blood vessel endothelium).

The heart has four chambers. The right atrium receives deoxygenated blood from the body via the superior and inferior venae cavae and the coronary sinus. The right ventricle ejects blood into the pulmonary trunk. The left atrium receives oxygenated blood from the four pulmonary veins. The left ventricle — with walls two to three times thicker than the right — ejects blood into the aorta against the high resistance of the systemic circulation.

## Valves

Four valves ensure unidirectional blood flow. The tricuspid valve (right atrioventricular valve, three cusps) and the mitral valve (left atrioventricular valve, two cusps) prevent backflow from ventricles to atria during systole. The chordae tendineae — tendinous cords connecting the valve cusps to papillary muscles projecting from the ventricular walls — prevent valve prolapse. The pulmonary valve and aortic valve (both semilunar, three cusps each) prevent backflow from the great arteries into the ventricles during diastole. Valve disease — stenosis (narrowing) or regurgitation (incompetence) — produces characteristic heart murmurs.

## The Cardiac Cycle

The cardiac cycle consists of systole (contraction and ejection) and diastole (relaxation and filling). During diastole, the atria fill passively and then contract (atrial systole) to complete ventricular filling. Ventricular systole follows: the atrioventricular valves close (producing the first heart sound, S1 — "lub"), pressure rises rapidly (isovolumetric contraction), the semilunar valves open when ventricular pressure exceeds arterial pressure, blood is ejected (approximately 70 mL per beat — the stroke volume), the semilunar valves close (producing the second heart sound, S2 — "dub"), and diastole resumes. The ejection fraction — the percentage of ventricular end-diastolic volume ejected per beat — is normally 55–70%; values below 40% indicate heart failure with reduced ejection fraction.

## The Cardiac Conduction System

The cardiac conduction system initiates and coordinates the heartbeat. The sinoatrial (SA) node, located in the right atrial wall near the superior vena cava, is the primary pacemaker, generating impulses at 60–100 per minute. The impulse spreads across both atria, reaches the atrioventricular (AV) node at the interatrial septum — which introduces a 0.1-second delay allowing ventricular filling — then travels rapidly down the bundle of His, divides into the right and left bundle branches, and disperses via the Purkinje fiber network throughout the ventricular myocardium, triggering near-simultaneous contraction.

## Blood Vessels

Arteries carry blood away from the heart. The aorta (3 cm diameter) gives rise to all systemic arteries, branching progressively into arteries, arterioles (the primary site of vascular resistance regulation), and finally into capillary beds. Elastic arteries (aorta, pulmonary trunk) have abundant elastin, allowing them to distend during systole and recoil during diastole — smoothing pulsatile flow. Muscular arteries (most named arteries) have prominent smooth muscle in the tunica media, allowing vasoconstriction and vasodilation to regulate regional blood flow. Veins return blood to the heart; they have thinner walls, larger lumens, and venous valves in the limbs preventing retrograde flow. Capillaries (7–10 µm diameter, one cell thick) are the sites of gas, nutrient, and waste exchange.

## Coronary Arteries

The heart is supplied by the right and left coronary arteries, which arise from the aortic sinuses immediately above the aortic valve. The left main coronary artery rapidly divides into the left anterior descending (LAD) artery — supplying the anterior interventricular septum and anterior left ventricle — and the left circumflex artery, supplying the lateral and posterior left ventricle. The right coronary artery supplies the right ventricle and, in about 70% of people (right-dominant), also the inferior left ventricle and SA/AV nodes. Occlusion of the LAD — colloquially called the "widow-maker" — causes anterior ST-elevation myocardial infarction (STEMI).

## Blood Pressure Regulation

Blood pressure (BP = cardiac output × systemic vascular resistance) is regulated by multiple systems. The baroreceptor reflex, mediated by stretch receptors in the carotid sinus and aortic arch, provides rapid moment-to-moment correction. The renin-angiotensin-aldosterone system (RAAS) regulates volume over hours to days. Antidiuretic hormone (ADH/vasopressin) increases water reabsorption. Atrial natriuretic peptide (ANP), released when atrial walls are stretched, promotes sodium and water excretion, reducing BP. Hypertension (sustained BP above 130/80 mmHg by current ACC/AHA criteria) is the most prevalent cardiovascular risk factor globally, driving atherosclerosis, left ventricular hypertrophy, stroke, and renal disease.