Heart Anatomy: Chambers, Valves, and Blood Flow Pathway

The heart is a muscular pump located in the mediastinum, slightly left of midline. It has four chambers — two atria (receiving chambers) and two ventricles (pumping chambers) — separated by the interatrial and interventricular septa. The heart sits within the pericardium, a fibrous sac that contains a small amount of serous fluid for lubrication. The apex of the heart (the pointed inferior tip, formed by the left ventricle) is directed anteriorly, inferiorly, and to the left — it can be palpated in the fifth intercostal space at the midclavicular line (the "point of maximal impulse" or PMI). Understanding the external anatomy is important for clinical examination: the anterior surface is formed primarily by the right ventricle, which is why right ventricular injuries are most common in penetrating chest trauma.

The right atrium receives deoxygenated blood from the body via the superior vena cava (SVC, from above the diaphragm), inferior vena cava (IVC, from below), and the coronary sinus (from the heart's own venous drainage). Internally, it features the crista terminalis, pectinate muscles, and the fossa ovalis (a remnant of the fetal foramen ovale). The right ventricle receives blood from the right atrium through the tricuspid valve and pumps it into the pulmonary trunk through the pulmonary valve. Its interior has trabeculae carneae (muscular ridges), papillary muscles, and chordae tendineae. The left atrium receives oxygenated blood from the four pulmonary veins (two from each lung) and has a smooth-walled posterior portion. The left ventricle is the thickest chamber (2-3 times thicker than the right ventricle) because it must generate enough pressure to pump blood through the entire systemic circulation. It pumps blood through the aortic valve into the ascending aorta.

The heart has two types of valves that ensure unidirectional blood flow. The atrioventricular (AV) valves sit between the atria and ventricles: the tricuspid valve (right side, three cusps) and the mitral valve (left side, two cusps — also called the bicuspid valve). AV valves are anchored by chordae tendineae to papillary muscles, which prevent the valve leaflets from everting (prolapsing) into the atria during ventricular contraction. The semilunar valves guard the exits from the ventricles: the pulmonary valve (between right ventricle and pulmonary trunk, three semilunar cusps) and the aortic valve (between left ventricle and aorta, three semilunar cusps). The aortic valve cusps each have a corresponding aortic sinus (of Valsalva) — the right and left coronary arteries arise from the right and left aortic sinuses, respectively. The non-coronary (posterior) sinus does not give rise to a coronary artery.

Following a drop of blood through the heart: deoxygenated blood returns from the body via the SVC and IVC → enters the right atrium → passes through the tricuspid valve → enters the right ventricle → pumped through the pulmonary valve → enters the pulmonary trunk → divides into left and right pulmonary arteries → travels to the lungs for gas exchange → oxygenated blood returns via four pulmonary veins → enters the left atrium → passes through the mitral valve → enters the left ventricle → pumped through the aortic valve → enters the ascending aorta → distributed to the systemic circulation. A helpful mnemonic: the right side of the heart handles "pulmonary" circulation (to the lungs), the left side handles "systemic" circulation (to the body). Note that pulmonary arteries carry deoxygenated blood and pulmonary veins carry oxygenated blood — the opposite of systemic vessels.

The heart supplies itself through the coronary arteries, which are the first branches of the aorta. The left coronary artery (LCA) arises from the left aortic sinus and quickly divides into the left anterior descending artery (LAD, supplies the anterior left ventricle and anterior 2/3 of the interventricular septum) and the left circumflex artery (LCx, courses in the left coronary sulcus and supplies the lateral left ventricle). The right coronary artery (RCA) arises from the right aortic sinus, courses in the right coronary sulcus, and supplies the right ventricle, SA node (in ~60% of people), and AV node (in ~80% of people). In most people (~85%), the RCA gives off the posterior descending artery (PDA), which is called "right dominance." Coronary venous drainage occurs primarily through the coronary sinus into the right atrium. The LAD is the most commonly occluded coronary artery in myocardial infarction, which is why it's often called the "widow-maker."

Heart valves are auscultated (listened to with a stethoscope) at specific points on the chest wall that correspond to where blood flow turbulence from each valve is best heard — importantly, these points are NOT directly over the anatomical valve locations but downstream in the direction of blood flow. The aortic valve is best heard at the right second intercostal space (parasternal). The pulmonary valve at the left second intercostal space (parasternal). The tricuspid valve at the left fourth intercostal space (parasternal). The mitral valve at the left fifth intercostal space (midclavicular line — the apex). The mnemonic "All People Enjoy Time at the Mall" (from right 2nd to left 5th) helps remember: Aortic, Pulmonary, Erb's point, Tricuspid, Mitral. Understanding these points is essential for clinical cardiovascular examination and is heavily tested in both anatomy and clinical courses.