The atria are the heart’s upper chambers, receiving blood before it is pumped into the lower ventricles. The right atrium collects deoxygenated blood from the body, while the left atrium receives oxygenated blood from the lungs. This article explores the anatomical components that cover the atria, from the outermost protective sac to the internal linings and features.
The Pericardium
The heart, including its atrial chambers, resides within a double-walled sac called the pericardium. This protective layer secures the heart’s position within the chest cavity and offers mechanical protection. The pericardium consists of two primary layers: an outer fibrous pericardium and an inner serous pericardium.
The fibrous pericardium is a tough, inelastic layer of dense connective tissue. This outer sac prevents the heart from over-expanding with blood and protects it from external forces. It anchors the heart to the diaphragm and great vessels, limiting excessive movement.
Beneath the fibrous layer lies the serous pericardium, a thinner, double-layered membrane. It comprises a parietal layer, which lines the inside of the fibrous pericardium, and a visceral layer, also known as the epicardium, which directly covers the heart’s surface. Between these two serous layers is the pericardial cavity, a space containing a small amount of pericardial fluid. This fluid acts as a lubricant, reducing friction as the heart beats and allowing for smooth movement within the sac.
The Heart Wall Layers
Directly beneath the visceral layer of the serous pericardium are the three distinct layers that form the wall of the atria. These layers are the epicardium, myocardium, and endocardium, each contributing to the atrial chambers’ structure and function.
The epicardium is the outermost layer of the atrial wall, also referred to as the visceral layer of the serous pericardium. This layer contains adipose tissue, blood vessels, and nerves that supply the heart muscle.
The myocardium forms the middle and most substantial layer of the atrial wall, composed of specialized cardiac muscle cells. While considerably thinner in the atria compared to the ventricles, this muscle tissue is responsible for the contractile force that propels blood into the ventricles.
The endocardium is the innermost layer, a smooth lining that covers the internal surfaces of the atria and extends over the heart valves. This single layer of endothelial cells creates a non-thrombogenic surface, preventing blood clotting and facilitating unobstructed blood flow.
Key Internal Atrial Features
Beyond the layers of the heart wall, several internal anatomical features help define the atrial chambers and contribute to their function. These structures shape the internal space and guide blood flow within the atria.
The interatrial septum is a muscular wall separating the right and left atria, preventing the mixing of oxygenated and deoxygenated blood. A shallow depression on its right atrial side, the fossa ovalis, is a remnant of the fetal foramen ovale, which allowed blood to bypass the lungs.
Pectinate muscles are prominent in the right atrium and its auricle. These parallel ridges of muscle tissue give the atrial walls a rough appearance. They increase the atria’s contractile force without significantly increasing mass, helping push blood into the ventricles.
A smooth, C-shaped muscular ridge, the crista terminalis, is located on the posterolateral wall of the right atrium. This ridge separates the rough, pectinate muscle-lined anterior portion from its smooth-walled posterior section. The heart’s natural pacemaker, the sinoatrial node, is located near its superior aspect.