Heart Anatomy: Chambers, Valves, and the Blood Flow Pathway Through the Heart Explained

Deoxygenated blood enters the right atrium via the superior and inferior vena cava, passes through the tricuspid valve into the right ventricle, is pumped through the pulmonary valve into the pulmonary arteries to the lungs, returns oxygenated via the pulmonary veins to the left atrium, passes through the mitral (bicuspid) valve into the left ventricle, and is pumped through the aortic valve into the aorta for systemic distribution. That is the complete circuit. Everything else is details about the anatomy and physiology of each step. The critical concept: the right side of the heart handles deoxygenated blood (to and from the lungs — pulmonary circulation). The left side handles oxygenated blood (to and from the body — systemic circulation). The two sides are separated by the interventricular septum and never mix blood in a healthy heart. A septal defect (a hole in the septum) allows mixing and is one of the most common congenital heart defects. The left ventricle has walls 3-4 times thicker than the right ventricle because it must generate enough pressure to push blood through the entire systemic circulation (mean arterial pressure ~93 mmHg), while the right ventricle only needs to push blood through the low-resistance pulmonary circuit (~15 mmHg). This wall thickness difference is why left ventricular hypertrophy (LVH) is a clinical finding in hypertension — the left ventricle enlarges when it has to work harder against chronically elevated systemic pressure. Snap a photo of any heart anatomy diagram or exam question and AnatomyIQ identifies each structure, traces the blood flow pathway, and explains the clinical significance. This content is for educational purposes only and does not constitute medical advice.

Right Atrium (RA): the receiving chamber for systemic venous return. The superior vena cava (SVC) enters from above, draining blood from the head, neck, upper limbs, and thorax. The inferior vena cava (IVC) enters from below, draining the abdomen, pelvis, and lower limbs. The coronary sinus opens into the RA posteriorly, draining the heart's own venous blood. The SA node (sinoatrial node — the heart's natural pacemaker) is located in the wall of the RA near the SVC opening. The RA has thin walls because it only needs to push blood a few centimeters into the RV through the tricuspid valve. Right Ventricle (RV): receives blood from the RA and pumps it to the lungs. The interior has thick muscular ridges called trabeculae carneae (which increase surface area for contraction) and papillary muscles that attach to the tricuspid valve leaflets via chordae tendineae (the tendinous cords). The moderator band (septomarginal trabeculation) carries part of the conduction system from the interventricular septum to the anterior papillary muscle — it is a clinical landmark visible on echocardiography. Left Atrium (LA): receives oxygenated blood from four pulmonary veins (two from each lung). The LA is posterior — it sits against the esophagus, which is why an enlarged LA can cause dysphagia (difficulty swallowing). The LA is also where most blood clots form in atrial fibrillation (specifically in the left atrial appendage), which is why A-fib is a major stroke risk factor — clots that form in the LA can travel to the brain via the aorta. Left Ventricle (LV): the strongest chamber, with walls 1-1.5 cm thick. It pumps blood through the aortic valve into the ascending aorta at peak systolic pressures of 120 mmHg. The LV is conical/bullet-shaped (compared to the crescent-shaped RV) and forms the apex of the heart — the point you feel when you palpate the PMI (point of maximal impulse) at the 5th intercostal space, midclavicular line. LV wall motion abnormalities on echocardiography are the hallmark of myocardial infarction — a segment that does not contract indicates dead or ischemic tissue in that territory.

The heart has four valves arranged in two functional pairs: the atrioventricular (AV) valves between atria and ventricles, and the semilunar valves between ventricles and great arteries. Tricuspid valve (right AV valve): three leaflets, between RA and RV. Opens during diastole (ventricular filling), closes at the beginning of systole to prevent backflow into the RA. Tricuspid regurgitation (leaking valve) is heard as a holosystolic murmur at the left lower sternal border, louder with inspiration (because inspiration increases venous return to the right side). Mitral valve (left AV valve, also called bicuspid valve): two leaflets, between LA and LV. The mitral valve is the most commonly diseased valve in rheumatic heart disease. Mitral stenosis (narrowed valve) produces a low-pitched diastolic rumble heard best at the apex with the patient in the left lateral decubitus position. Mitral valve prolapse (leaflets balloon backward into the LA during systole) is the most common valve abnormality, present in 2-3% of the population — usually benign, produces a mid-systolic click. Pulmonary valve (right semilunar): three semilunar cusps, between RV and pulmonary trunk. Opens during RV systole, closes at the end of systole. Pulmonary stenosis is usually congenital. Aortic valve (left semilunar): three semilunar cusps, between LV and ascending aorta. The coronary arteries originate from the aortic sinuses just above the aortic valve — which means coronary blood flow occurs during diastole (when the aortic valve is closed and blood pools in the sinuses). This is clinically important: tachycardia (fast heart rate) shortens diastole, which reduces coronary perfusion time and can trigger angina in patients with coronary artery disease. AnatomyIQ traces the valve locations on any heart diagram and explains the hemodynamic significance — including which murmurs are associated with each valve abnormality.

Coronary artery anatomy and myocardial infarction: the left anterior descending (LAD) artery supplies the anterior wall of the LV and the anterior 2/3 of the interventricular septum. LAD occlusion causes an anterior MI — the most dangerous type because it involves the largest territory of myocardium. The LAD is nicknamed the widow maker. The right coronary artery (RCA) supplies the right ventricle, the posterior descending artery (in 85% of people — right-dominant circulation), and the SA and AV nodes. RCA occlusion can cause heart block (the conduction system loses blood supply). The left circumflex (LCx) supplies the lateral wall of the LV. The cardiac conduction system: SA node → AV node → Bundle of His → left and right bundle branches → Purkinje fibers. The SA node fires at 60-100 bpm (normal sinus rhythm). If the SA node fails, the AV node takes over at 40-60 bpm (junctional rhythm). If both fail, the ventricles generate their own rhythm at 20-40 bpm (ventricular escape rhythm — barely compatible with life). Each level is a backup pacemaker. Fetal circulation differences: in the fetus, the lungs are not inflated, so blood bypasses them through two shunts. The foramen ovale (a hole in the interatrial septum) shunts blood from RA to LA. The ductus arteriosus connects the pulmonary trunk to the aorta, diverting blood away from the lungs. Both close at birth when the baby takes its first breath and pulmonary resistance drops. Failure to close the ductus arteriosus produces a patent ductus arteriosus (PDA) — a continuous "machinery" murmur that is one of the most tested heart sounds in pediatrics. AnatomyIQ generates practice questions for each of these clinical correlations — from basic identification (label the chambers) to advanced vignettes (given an EKG and coronary angiogram, identify the occluded vessel and the affected myocardial territory).