The heart has four chambers: two thin-walled atria (receive blood) and two thick-walled ventricles (pump blood). The interatrial and interventricular septa separate left and right sides. Four valves (tricuspid, pulmonary, mitral, aortic) prevent backflow. This structure enables one-way circulation to the lungs and body.
From your study of cardiac muscle, you know that the heart wall is made of cardiomyocytes that contract rhythmically and are electrically coupled. Now consider the structural problem that cardiac anatomy solves: the body needs two parallel pumps operating in perfect coordination — one that sends oxygen-depleted blood to the lungs at relatively low pressure, and one that drives oxygenated blood to the entire body at much higher pressure. The four-chamber design achieves this with a single organ.
The two atria are thin-walled receiving chambers. The right atrium collects deoxygenated blood returning from the body via the superior and inferior vena cava. The left atrium receives freshly oxygenated blood from the four pulmonary veins. Atria have thin walls because their job is low-pressure collection and priming of the ventricles — they generate only modest force. In contrast, the ventricles do the heavy pumping work, and their wall thickness reflects the pressure they must generate. The right ventricle drives blood through the pulmonary circuit at roughly 25 mmHg — a low-resistance system. The left ventricle must push blood against systemic vascular resistance (typically 120 mmHg), so its wall is three times thicker.
The septa are the walls separating right from left. The interatrial septum divides the two atria; the interventricular septum divides the two ventricles. These walls keep oxygenated and deoxygenated blood completely separate — a defect in either (a septal "hole") causes mixing and reduces circulatory efficiency. The four valves solve a different problem: ensuring blood moves only forward. The atrioventricular valves — the tricuspid (right side, three leaflets) and the mitral or bicuspid valve (left side, two leaflets) — open when atrial pressure exceeds ventricular pressure, and snap shut when the ventricles contract and pressure reverses. The semilunar valves — the pulmonary and aortic — guard the exits to the pulmonary artery and aorta respectively, opening during ventricular ejection and closing to prevent backflow when the ventricles relax. Valvular disease (stenosis = narrowing, regurgitation = backflow) disrupts these pressure gradients with predictable hemodynamic consequences you will study next.