Page 16, Later Development, Develop. Physiol., Dr. D. Penney

Immediate Postnatal Shunts:

Although the foramen ovale usually closes functionally within minutes of birth, anatomical closure may not occur for months or years, and complete closure never occurs in a significant percentage of people, (Figure 3.09). This leads to left to right shunting, but the degree of shunt is dependent upon the size of the "leak".

In a similar fashion complete closure of the ductus arteriosus, which begins shortly after birth, may take much longer. Again, complete closure fails to occur in some people, resulting in the left to right shunting of blood.

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