Heart development
The heart raises from a little group of special cells located in the embryological region which will become the chest. During the first four weeks of pregnancy this cells grow until they form a tube called the primitive heart tube. It is constituted of four segment: the seno-atrium which forms the atria, the primitive ventricle which forms the left ventricle, the heart bulb which forms the right ventricle and the truncus arteriosus which forms the aorta and the pulmonary artery. In the primitive heart tube the blood arrives in the senoatrium, travel through the tube and go out through the truncus arteriosus.

-Fig. 2.1
Embryological development of the heart. A. Primitive heart tube stage. The senoatrium (SA) will give right and left atrium; the primitive ventricle (V) is the precursor of left ventricle; the bulbous cordis (BC) will give the right ventricle; the truncus arteriosus (TA) will branch into aorta (A) and pulmonary artery (P). B. C. D. Successive stages of the development up to an heart with 4 cavities-.

During the next step the primitive heart tube coils up in a handle giving two atria and two ventricles. The right atrium connects to right ventricle through the tricuspid valve, left atrium connects to the left ventricle through the mitral valve. Contemporaneously are formed the atrial and the ventricular septum. The truncus arteriosus branches into aorta and pulmonary artery which connect respectively to the left and right ventricle . At ten weeks the heart is completely formed and functional and during the rest of the pregnancy it will grow to supply the demand of developing fetus.

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