The heart consists of four chambers: the two atria and the two ventricles. Blood returning to the heart enters the atria, and is then pumped into the ventricles. From the left ventricle, blood passes into the aorta and enters the systemic circulation. From the right, it enters the pulmonary circulation via the pulmonary arteries. The right ventricle pumps the oxygen-poor blood to the lungs through the pulmonary valve. The left atrium receives oxygen-rich blood from the lungs and pumps it to the left ventricle through the mitral valve. The left ventricle pumps the oxygen-rich blood through the aortic valve out to the rest of the body. ATRIA Right Atrium: The right atrium receives deoxygenated blood from the superior and inferior vena cavae, and from the coronary veins. It pumps this blood through the right atrioventricular orifice (guarded by the tricuspid valve) into the right ventricle. In the anatomical position, the right atrium forms the right border of the heart. Extending from the antero-medial portion of the chamber is the right auricle (right atrial appendage)- a muscular pouch that acts to increase the capacity of the atrium. The interior surface of the right atrium can be divided into two parts, each with a distinct embryological origin. These two parts are separated by a muscular ridge called the crista terminalis: Sinus venarum-located posterior to the crista terminalis. This part receives blood from the superior and inferior vena cavae. It has smooth walls and is derived from the embryonic sinus venosus. Atrium proper- located anterior to the crista terminalis, and includes the right auricle. It is derived from the primitive atrium, and has rough, muscular walls formed by pectinate muscles. The coronary sinus receives blood from the coronary veins. It opens into the right atrium between the inferior vena cava orifice and the right atrioventricular orifice. Interatrial Septum: The interatrial septum is a solid muscular wall that separates the right and left atria. The septal wall in the right atrium is marked by a small oval-shaped depression called the fossa ovalis. This is the remnant of the foramen ovale in the fetal heart, which allows right to left shunting of blood to bypass the lungs. It closes once the newborn takes its first breath. Left Atrium: The left atrium receives oxygenated blood from the four pulmonary veins, and pumps it through the left atrio-ventricular orifice (guarded by the mitral valve) into the left ventricle. In the anatomical position, the left atrium forms the posterior border (base) of the heart. The left auricle extends from the superior aspect of the chamber, overlapping the root of the pulmonary trunk. The interior surface of the left atrium can be divided into two parts, each with a distinct embryological origin: Inflow portion- receives blood from the pulmonary veins. Its internal surface is smooth and it is derived from the pulmonary veins themselves. Outflow portion – located anteriorly, and includes the left auricle. It is lined by pectinate muscles, and is derived from the embryonic atrium. Submit Your Manuscript at https://www.imedpub.com/submissions/anatomical-science-research.html or mail us to anatomicalsci@scholarlymed.com Regards Jacqueline ManagingEditor Journal of Anatomical Science and Research