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The sixth branchial arch arteries are the last of the branchial arch arteries to appear. They appear around day 28 of gestation. By this time, the truncus arteriosus of the heart has divided up into pulmonary trunk and ventral aorta.
The sixth branchial arch arteries become continuous with the pulmonary trunk proximally and the lung buds distally. However, they are also in continuity with the dorsal aortae via a pair of vessels. These act to shunt blood away from the lungs during intrauterine life as part of the fetal circulation.
During further development, the connections to the lung buds become the pulmonary arteries. The connection to the dorsal aorta on the right degenerates. The connection to the dorsal aorta on the left is the ductus ateriosus that normally seals after birth.