The Atria

The atria are the receiving chambers of the heart. The right atrium receives venous blood from the superior vena cava, the inferior vena cava, and the coronary sinus. The left atrium receives oxygenated blood from the four pulmonary veins.

Although the atria continuously collect blood, most ventricular filling occurs passively while the ventricles are relaxed. Near the end of ventricular filling, atrial contraction provides additional force that helps complete the filling of the ventricles.

The atrial walls are relatively thin. The musculature of the auricular appendages is thicker than that of the venous portions of the atria. 

The appendages contain internal circular fibers and external longitudinal fibers that run parallel to their long axis, whereas the venous portions possess internal longitudinal fibers and external circular fibers. 

Superficial muscular bundles bridge the groove between the atria, especially on the anterior surface. 

The deeper muscle fibers of each atrium remain largely separate from one another and are almost completely independent of the ventricular musculature.

Interatrial Septum

The right and left atria are separated by the interatrial septum. This partition contains the fossa ovalis, a shallow depression that represents the remains of the fetal foramen ovale. 

Before birth, the foramen ovale allowed blood to pass directly from the right atrium to the left atrium, bypassing the fetal lungs. After birth, the opening closes, leaving the fossa ovalis.

The internal surface of the ventricle is lined by numerous muscular ridges known as the trabeculae carneae, which are formed by bundles of myocardium covered by endocardium. Among these muscular projections are the papillary muscles, which arise from the ventricular wall and give attachment to the chordae tendineae. The right ventricle contains three papillary muscles (the anterior, posterior, and septal muscles) corresponding to the three cusps of the tricuspid valve.

Each cusp of the tricuspid valve is connected to the papillary muscles by strong fibrous cords called the chordae tendineae. These cords consist primarily of collagen fibers, together with elastic fibers and endothelium. During ventricular contraction, the papillary muscles contract simultaneously, tightening the chordae tendineae and preventing the valve cusps from being pushed backward into the right atrium as pressure rises within the ventricle.

Superiorly, the cavity of the right ventricle gradually narrows as it approaches the pulmonary trunk. At the origin of this vessel lies the pulmonary semilunar valve, which prevents blood from flowing back into the ventricle after contraction.

Left Ventricle

The left ventricle forms the apex of the heart and possesses the thickest myocardium of all four chambers. It receives oxygenated blood from the left atrium through the mitral, or bicuspid, valve and pumps it into the aorta through the aortic semilunar valve.

As the principal pumping chamber of the systemic circulation, the left ventricle must generate much greater pressure than the right ventricle in order to distribute blood throughout the body.

The muscular wall of the left ventricle is therefore considerably thicker and stronger. Its myocardium is organized into three distinct layers. 

• The superficial layer consists mainly of oblique fibers that descend toward the apex of the heart. 

• Beneath this lies a powerful middle layer composed primarily of transverse fibers, which forms the bulk of the ventricular wall. 

• The deepest layer, located immediately beneath the endocardium, is formed by irregular muscular bundles that project into the ventricular cavity.

The superficial muscle fibers converge near the apex, where they form the vortex of the heart. From this whorl, fibers continue into the deeper layers of the myocardium, creating the complex spiral arrangement characteristic of the ventricular musculature.

Interventricular Septum

The right and left ventricles are separated by the interventricular septum. The uppermost portion remains membranous and lies between the bases of the posterior and right cusps of the aortic valve.

• This septum prevents mixing of oxygenated and deoxygenated blood while also contributing to the force of ventricular contraction.

This septum is much thicker than the interatrial septum because the ventricles generate significantly higher pressure during contraction as they pump blood out of the heart.

Atrioventricular Septum

Separating the atria from the ventricles is the atrioventricular septum. This septum contains four openings that allow blood to flow from the atria into the ventricles and from the ventricles into the pulmonary trunk and aorta. 

Each opening is guarded by a valve, which ensures that blood flows in only one direction. 

The valves between the atria and ventricles are called atrioventricular (AV) valves, while the valves that control blood flow out of the heart into the pulmonary trunk and aorta are called semilunar valves.

Because the atrioventricular septum contains multiple openings and valves, it is structurally weaker than other regions of the heart. To provide strength and support, this area is reinforced by dense connective tissue known as the cardiac skeleton. The cardiac skeleton forms four strong rings around the valve openings and serves as an attachment point for the heart valves.

Fibrous Skeleton of the Heart

Many cardiac muscle fibers arise from dense fibrous rings that surround the atrioventricular openings. These fibrous rings separate the musculature of the atria from that of the ventricles and provide attachment for the heart valves.

The right fibrous ring completely surrounds the right atrioventricular opening. The left fibrous ring is incomplete because the root of the aorta is fused with the left atrioventricular opening. Two dense connective tissue structures, known as the right and left fibrous trigones, connect the rings near the aortic root.

The fibrous skeleton supports the valves, strengthens the heart wall, and helps electrically isolate the atrial musculature from the ventricular musculature.

Functional Differences Between the Chambers

The four chambers work together to maintain circulation. The atria act primarily as receiving chambers and assist ventricular filling, whereas the ventricles generate the force necessary to propel blood through the lungs and body.

The right side of the heart pumps blood through the pulmonary circulation at relatively low pressure. The left side pumps blood through the systemic circulation and must therefore generate much greater pressure. As a result, the myocardium of the left ventricle is considerably thicker and stronger than that of any other chamber.