Page 4, Early Development, Devel. Physiol., Dr. D. Penney

Somewhat Later Heart Development:

Prior to birth pressures and workloads of the right ventricle and left ventricle are about equal. Postnatally, growth of the left ventricle increases faster than that of the right ventricle due to a rapid rise in systemic vascular resistance, while pulmonary resistance declines. An increased workload on either ventricle results in compensatory hypertrophy, either normal as during growth to adulthood, or abnormal as in response to unusual pressure or volume loading of the heart.

At the 9 somite stage the heart consists of a single ventricle, while the atria are paired sacs (Figure 2.01).

At the 11 somite stage the "S" shaped heart is made up of 4 divisions from caudal to cranial end, the atria, the ventricle, the bulbus, and the truncus (Figure 2.02). The entrance to the atrium is guarded by the sinus valve.

At the 16 somite stage the common atrium is divided into the right and left halves.

By the 22 somite stage, the heart is very compact. The right and left horns of the sinus venosus are mere bumps on the atria.

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