Hpb

Superior vena cava

Sinus Septum venarum

.secundum

Septum primum

Figure 11.15 Coronal sections through the heart to show development of the smooth-walled portions of the right and left atrium. Both the wall of the right sinus horn (blue) and the pulmonary veins (red) are incorporated into the heart to form the smooth-walled parts of the atria.

veins of the developing lung buds. During further development, the pulmonary vein and its branches are incorporated into the left atrium, forming the large smooth-walled part of the adult atrium. Although initially one vein enters the left atrium, ultimately four pulmonary veins enter (Fig. 11.155) as the branches are incorporated into the expanding atrial wall.

In the fully developed heart, the original embryonic left atrium is represented by little more than the trabeculated atrial appendage, while the smooth-walled part originates from the pulmonary veins (Fig. 11.15). On the right side the original embryonic right atrium becomes the trabeculated right atrial appendage containing the pectinate muscles, and the smooth-walled sinus venarum originates from the right horn of the sinus venosus.

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