The Formation of the Atria and Venous Return

Starting from the 4th week of embryonic development, a partition begins to form in the atrioventricular canal. This process gradually divides the primitive atrium into two distinct chambers: the right atrium and the left atrium.

The interatrial septum thus created plays a crucial role by establishing a bypass pathway for the lungs, which are still non-functional at this stage. All vital exchanges, including oxygenation and waste elimination, occur exclusively at the placenta, ensuring the embryo’s survival and development.

Development of the Atria and Venous Return

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Chapter 1 – Fertilization – Embryology
Chapter 2 – Development of the Cardiovascular System
Chapter 4 – Development of the Ventricles and Major Arterial Vessels

The Early Stages of Cardiac Development

By the 22nd day, the cardiac tube undergoes significant changes, forming bulges (or swellings) and narrowing zones. These primitive structures mark the beginnings of the developing heart.

Folding of the Cardiac Tube

Due to uneven growth, the cardiac tube begins to fold in an orderly manner around the 23rd day.

This critical process shifts the primitive atrium and venous connections behind the primitive ventricle and arterial bulb, establishing the initial configuration of the future heart.

Formation of the Right and Left Atria

Starting from the 4th week of embryonic development, a septum begins to form within the atrioventricular canal.

A Wall at the Center of the Primitive Atrium

This crucial structure, essential for dividing the primitive atrium, separates the cavity into two distinct chambers: the right atrium and the left atrium.

This process, known as atrial septation or the formation of the atrial septum, occurs in two successive stages.

1-The First Septum: Septum Primum

A thin crescent-shaped membrane, called the septum primum, grows downward from the roof of the primitive atrium toward the endocardial cushions at the center of the heart.

During its descent, a temporary opening called the ostium primum remains between the edge of the septum primum and the endocardial cushions.

Once the ostium primum closes, a new opening, the ostium secundum, forms in the upper part of the septum primum. This opening allows continuous bloodBlood is composed of red blood cells, white blood cells, platelets, and plasma. Red blood cells are responsible for transporting oxygen and carbon dioxide. White blood cells make up our immune defense system. Platelets contribute to blood flow between the two atriaThe atria are the two upper chambers of the heart. They act as reservoirs for blood that will fill the ventricles. during fetal life.

2-The Second Septum: Septum Secundum
A second, thicker membrane, called the septum secundum, develops in parallel.

It partially covers the ostium secundum, leaving a semi-oval opening at its base called the foramen ovale.

This structure ensures controlled communication between the two atriaThe atria are the two upper chambers of the heart. They act as reservoirs for blood that will fill the ventricles. throughout fetal life.

A Temporary Passage Between the Two Atria

During fetal life, the combination of the ostium secundum and the foramen ovale allows bloodBlood is composed of red blood cells, white blood cells, platelets, and plasma. Red blood cells are responsible for transporting oxygen and carbon dioxide. White blood cells make up our immune defense system. Platelets contribute to blood to flow directly from the right atrium to the left atrium, bypassing the lungs, which are not yet functional.

Closure at Birth

At birth, with the onset of breathing, the pressure in the left atrium surpasses that in the right atrium.

The pressure difference pushes the septum secundum against the septum primum, permanently sealing the passage between the two atriaThe atria are the two upper chambers of the heart. They act as reservoirs for blood that will fill the ventricles..

This mechanism marks the functional closure of the foramen ovale, enabling normal bloodBlood is composed of red blood cells, white blood cells, platelets, and plasma. Red blood cells are responsible for transporting oxygen and carbon dioxide. White blood cells make up our immune defense system. Platelets contribute to blood circulation between the heart’s chambers.

A Bypass Pathway for the Lungs

During fetal life, the foramen ovale (or interatrial communication, IAC) plays a crucial role in bypassing the non-functional lungs. At this stage, only minimal bloodBlood is composed of red blood cells, white blood cells, platelets, and plasma. Red blood cells are responsible for transporting oxygen and carbon dioxide. White blood cells make up our immune defense system. Platelets contribute to blood flow is required to support lung development.

Respiratory function is entirely managed by the placenta, where vital exchanges such as oxygenation of the bloodBlood is composed of red blood cells, white blood cells, platelets, and plasma. Red blood cells are responsible for transporting oxygen and carbon dioxide. White blood cells make up our immune defense system. Platelets contribute to blood and removal of carbon dioxide occur.

Formation of the Left Atrium

As the partitioning of the primitive atrium begins, much of the left atrium’s growth is attributed to the incorporation of the primitive pulmonary vein. This fusion provides part of the left atrium with a smooth surface, in contrast to the section derived from the primitive atrium, which remains rough and folded.

This rough structure corresponds to the left auricle or left atrial appendage, a small finger-shaped pouch.

A Valve Between the Left Atrium and the Left Ventricle

The remaining opening between the left atrium and the left ventricle will develop into the mitral valve.

This valve plays a crucial role in regulating the unidirectional flow of bloodBlood is composed of red blood cells, white blood cells, platelets, and plasma. Red blood cells are responsible for transporting oxygen and carbon dioxide. White blood cells make up our immune defense system. Platelets contribute to blood between these two chambers, preventing any backflow into the atrium when the ventricle contracts.

The Return of Blood to the Heart

At the primitive stage, the venous network is symmetrical, with identical venous structures on both sides of the body. This network plays a crucial role in the formation of the cardiac chambers and their connections.

The Venous Sinuses

The embryonic heart receives bloodBlood is composed of red blood cells, white blood cells, platelets, and plasma. Red blood cells are responsible for transporting oxygen and carbon dioxide. White blood cells make up our immune defense system. Platelets contribute to blood via the venous sinuses, initially present on both the left and right sides, which drain into their respective primitive atrium.

Over time, the left venous sinus migrates to merge with the right venous sinus, forming a single common opening that notably accommodates the inferior vena cava.

Formation of the Right Atrium

The integration of the right venous sinus into the primitive atrium is key to the development of the right atrium.

This integration creates a smooth interior section derived from the venous sinus.
However, a rough and folded portion remains, forming the right auricle or right atrial appendage.

Formation of the Tricuspid Valve

The communication between the right atrium and the right ventricle, which develops in parallel with the venous sinus, will become the site of the tricuspid valve.

This valve will play an essential role in regulating bloodBlood is composed of red blood cells, white blood cells, platelets, and plasma. Red blood cells are responsible for transporting oxygen and carbon dioxide. White blood cells make up our immune defense system. Platelets contribute to blood flow from the right atrium to the right ventricle, while preventing backflow during ventricular contractions.