Primitive Heart Tube Diagram – Parts and What They Become

All 11 parts on the diagram

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The tube 5

Truncus arteriosus The head end of the tube, where blood leaves.
Bulbus cordis The outflow part, between the ventricle and the truncus arteriosus.
Primitive ventricle Becomes the rough-walled main part of the two ventricles.
Primitive atrium Becomes the rough-walled parts of the two atria.
Sinus venosus The tail end, where veins bring blood into the tube.

What it becomes 6

Ascending aorta Forms, with the pulmonary trunk, from the truncus arteriosus.
Pulmonary trunk Forms, with the aorta, from the truncus arteriosus.
Conus arteriosus The smooth outflow part of the right ventricle, from the bulbus cordis.
Aortic vestibule The smooth outflow part of the left ventricle, from the bulbus cordis.
Sinus venarum The smooth part of the right atrium, from the sinus venosus.
Coronary sinus Forms from the left horn of the sinus venosus.

How this list was compiled: Every name and every fate checked on 2026-10-01 and kept only where two independent sources give it. (1) Gray's Anatomy (1918), Development of the Vascular System, https://www.bartleby.com/107/135.html : the five parts of the tube (sinus venosus, primitive atrium, primitive ventricle, bulbus cordis, truncus arteriosus), the division of the truncus into the aorta and pulmonary artery, the left horn of the sinus venosus persisting as the coronary sinus; and The Heart, https://www.bartleby.com/107/138.html , for the sinus venarum. (2) Kenhub, Embryology of the heart, https://www.kenhub.com/en/library/anatomy/embryology-of-the-heart : the bulbus cordis becoming the conus arteriosus and the aortic vestibule, the sinus venosus becoming the sinus venarum, the left horn the coronary sinus. (3) Wikipedia, Tubular heart, Truncus arteriosus, Bulbus cordis, Primitive ventricle, Primitive atrium and Sinus venosus, https://en.wikipedia.org/wiki/Tubular_heart ; its Heart development paragraph on the five regions rests on a textbook this site does not use and was not followed. Licence: own work

The drawing: Own drawing from plain geometry, 2026-10-01; nothing traced. Licence: own work

How it works

The primitive heart tube is the heart of the early embryo, before it folds. The first drawing names its five parts from the head end down. The second shows where each ends up in the finished heart.

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The tube

Blood enters at the tail end, the sinus venosus, and moves toward the head end. It passes through the primitive atrium, the primitive ventricle and the bulbus cordis, and leaves by the truncus arteriosus. The tube then bends and folds into a loop.

What each part becomes

A septum divides the truncus arteriosus: aorta and pulmonary trunk. The bulbus cordis becomes the smooth outflow parts of the ventricles: the conus arteriosus on the right and the aortic vestibule on the left. The sinus venosus becomes the smooth part of the right atrium and the coronary sinus.

The two drawings share their shading: each part of the finished heart is shaded like the part of the tube it came from.
Lists differ. Some books count the truncus arteriosus as the upper part of the bulbus cordis.

Questions and answers

What are the five parts of the primitive heart tube?

From the head end to the tail end: truncus arteriosus, bulbus cordis, primitive ventricle, primitive atrium and sinus venosus. Blood flows the other way, from the sinus venosus to the truncus arteriosus.

What does the bulbus cordis become?

The smooth-walled outflow parts of the two ventricles: the conus arteriosus of the right ventricle, below the pulmonary valve, and the aortic vestibule of the left ventricle, below the aortic valve.

What does the sinus venosus become?

Its right part is taken into the right atrium as the smooth-walled sinus venarum. Its left horn becomes the coronary sinus.