Development of Heart Tube

Parts of the Heart Tube

There are five parts of the heart tube;

• Sinus venosus
• Primitive atrium
• Primitive ventricle
• Bulbus cordis
• Truncus arteriosus- The oxygenated blood exits from here.

Heart Tube  

It has the heart tube has input and output channels.

Input Channels 

The branches of the sinus venosus are the Left horn and the Right horn. Sinus venosus carries the venous blood to the heart tube and drains into the sinus venosus. The Venous blood is received from a set of three veins on the left and right sides, namely, the Cardinal vein, Vitalien vein, and Umbilical vein.

Output Channels 

Truncus arteriosus is the terminal portion of the heart tube. It is the arterial trunk, which sends blood out in the lungs via their Pulmonary circulation and to all body parts via Systemic circulation. The Sinus Venosus has a Smooth inflow tract, and the Bulbus cordis has a Smooth outflow tract, which forms the smooth part of the ventricles.

The primitive atrium and ventricle forms a rough and trabeculated portion of 4 chambers of the heart. The future derivative of the Right horn of sinus venosus is Sinus venarum. It is the smooth posterior wall of the right atrium. The rough portion will be formed by the primitive atrium. It gets absorbed into the primitive atrium.

The future derivative of the Left horn of sinus venosus is the Coronary sinus. The left horn will become very small and become a coronary sinus. It carries venous blood from the heart muscle.

Septum

A septum is developed, and its name is dependent on its location. The septum in Primitive atrium is Interatrial septum. It divides the primitive atrium into the right and left atrium. The septum in the Primitive ventricle is known as the Interventricular septum. The interventricular septum divides the primitive ventricle into the right and left ventricles. Thus, the 4 chambers are formed.

Thus, we can infer that 4 chambers come from the primitive atrium and ventricle, which form the rough trabeculated portion. Bulbus cordis forms the smooth part of the ventricles. 

The AP septum, or the AortoPulmonary septum, is Derived from the neural crest cells. It divides the truncus arteriosus into two vessels, namely ascending aorta and pulmonary trunk. The Ascending aorta carries blood to the systemic circulation. The Pulmonary trunk carries blood to the pulmonary circulation. The aortopulmonary septum is a spiral septum as It spirals the vessel. It descends down to reach the ventricles. Thus dividing the bulbus cordis.

If the AP septum is not spiral (Straight), then it causes an anomaly known as Transposition of Great Vessel(TOGV) where the blood flow is reversed.

Folding of the Heart Tube

The heart tube is straight at the beginning and eventually bends and the upper arteries come anterior, and the lower veins remain posterior. The sinus between them is called transverse pericardial sinus. In the beginning, the Heart tube is straight with a structure under it known as Sinus venosus, and the structure Above is the Truncus arteriosus. The upper artery comes to the anterior, and the lower veins go posterior. In a converged state, the space formed between the sinus venosus and truncus arteriosus is called a transverse pericardial sinus.

During the transformation of Heart Tube, the upper artery comes to the anterior, and the lower veins go posterior.

The Right horn of the sinus venosus gets absorbed into the primitive atrium to form the sinus venarum, a smooth tract of the atrium. The primitive atrium forms a trabeculated or rough portion of the atrium. The Left horn of the sinus venosus will become very small and become a coronary sinus to collect venous blood from the heart muscle. The Sinus venarum forms the Posterior smooth wall of the right auricle. The Primitive atrium forms the Rough anterior trabeculated portion of the right auricle.

The left horn of the sinus venosus, or the Coronary sinus, will become very small and become a coronary sinus to collect venous blood from the heart muscle. Cardiac veins drain the blood to the coronary sinus, which then drains all the blood into the posterior wall of the right atrium.

The Posterior smooth part of the right atrium, or Sinus venarum, is also drained by the Superior vena cava and is Formed by the anterior cardinal vein. The Inferior vena cava is Formed by the posterior cardinal vein.

The left atrium has 4 pulmonary veins that open into the posterior wall. It is said that the 4 pulmonary veins contribute to the posterior wall of the left atrium (they are Absorbed partially into the left atrium). Four pulmonary veins open to the left atrium, and three veins open into the right atrium. This can only be seen posteriorly (to see anteriorly, open the right atrium).

In a primitive heart, we can observe a Rough anterior wall of the right atrium and a smooth posterior part of the wall, in which the following are open: the Superior vena cava, Inferior vena cava, and coronary sinus.

The superior and inferior vena cava are formed from cardinal veins. 

The Right horn of sinus venosus gets absorbed into the primitive atrium. Sulcus terminalis from the outside and crista terminalis from inside is the boundary between the smooth and rough part of the right atrium. The SA node is formed above the crista terminalis at the upper end of the crista terminalis in the subepicardial region.

Frequently Asked Questions

Q. What is true About Sinus Venosus

1. Forms rough wall of right atrium
2. Forms smooth wall of right atrium
3. Forms right coronary sinus
4. Forms left leaflet of coronary sinus

Ans. Forms smooth wall of right atrium.

Explanation

• Right horn of sinus venosus forms a smooth posterior wall of the right atrium, sinus venarum.
• Primitive atrium forms the rough wall of the right atrium.

Q: What structures are formed by the cardinal veins?

Answer: Superior and Inferior vena cava

Q: What structure is formed by the Sinus venarum?

Answer: The Posterior smooth wall of the right auricle. 

Q: What forms the Rough anterior trabeculated portion of the right auricle?

Answer: The Primitive atrium forms

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