Associated with the switch in the site of gas exchange during
hatching is a shift in the pattern of blood flow. Prior to internal
pipping, the embryo has interatrial foramina and two patent
ductus arteriosi that allow for a right-to-left shunt of blood flow
away from the non-ventilating lungs and towards the CAM and
systemic tissues (White, 1974; Tazawa and Takenaka 1985;
Rahn et al., 1985). With the initiation of internal pipping, right
atrial and ventricle output flowing to the lungs increases and
output flowing to the systemic tissues and CAM decreases. At
this point, approximately 40% of the right atrial outflow passes
through the pulmonary arteries to the lungs while 60% bypasses
the lungs through the ductus arteriosi and interatrial foramina
(Rahn et al., 1985). As hatching progresses, blood flow through
the ductus arteriosi decreases such that during external pipping