Cardiac structure and chambers 

The human heart is a muscular organ that functions as the central pump of the circulatory system, maintaining the continuous flow of blood throughout the body to supply oxygen and nutrients while removing waste products. Structurally, the heart is divided into four chambers—two atria and two ventricles—that, in normal function, work together in a coordinated manner to ensure efficient circulation. 
 
External Structure 
 
The heart is enclosed within a protective double-walled sac known as the pericardium, which contains pericardial fluid to reduce friction during contractions. The outer layer of the heart wall is the epicardium, the middle layer is the myocardium (composed of cardiac muscle responsible for pumping action), and the inner layer is the endocardium, which lines the chambers and valves. 
 
Externally, the heart shows a few visible features such as the auricles—small flap-like projections on the atria that increase their capacity—and several grooves or sulci that mark the boundaries between chambers and house the major coronary blood vessels. The coronary arteries supply oxygenated blood to the heart muscle itself, while the cardiac veins drain deoxygenated blood into the coronary sinus. 
 
Internal Structure and Chambers 
 
Internally, the heart is divided into four chambers: the right atrium, right ventricle, left atrium, and left ventricle. A muscular wall called the septum separates the right and left sides, preventing the mixing of oxygenated and deoxygenated blood. 
 
Right Atrium: 
The right atrium receives deoxygenated blood from the body through three major veins—the superior vena cava, inferior vena cava, and coronary sinus.  
It acts as a receiving chamber, holding blood temporarily before it passes through the tricuspid valve into the right ventricle. The Tricupsid valve ensures one-way flow (from the right atrium to the right ventricle), preventing backflow during ventricular contraction (systole). 
 
Right Ventricle: 
The right ventricle pumps deoxygenated blood to the lungs for oxygenation through the pulmonary artery. The flow is regulated by the pulmonary valve, which opens during contraction and closes to prevent backflow when the ventricle relaxes. The muscular wall of the right ventricle is thinner than the left because it only needs to pump blood to the nearby lungs under low pressure. 
 
Left Atrium: 
The left atrium receives oxygenated blood from the lungs via four pulmonary veins. It then pushes the blood through the mitral valve into the left ventricle. Like the right atrium, its wall is thin since it only needs to move blood a short distance. 
 
Left Ventricle: 
The left ventricle has the thickest muscular wall because it must generate enough force to pump blood through the aorta to the entire body, during systole. The aortic valve regulates blood flow from the left ventricle into the aorta and prevents backflow during relaxation (diastole). 


 
The heart’s four-chambered structure allows for the complete separation of oxygenated and deoxygenated blood, a key adaptation for efficient double circulation in humans. This design ensures that oxygen-rich blood is delivered to tissues at high pressure, while deoxygenated blood is sent to the lungs for gas exchange at lower pressure. Through its rhythmic contractions and coordinated valve system, the heart maintains a precise, unidirectional flow that sustains life.