Principally encountered in 'ductus arteriosus" discussed here a whiie ago. Before a baby is born,
only a small amount of blood from right ventricle goes through the lung, but the right ventricle
muscle must develop fully so it can meet the demands when baby's lungs inflates so it can breathe.
The ductus arteriosus joins pulmonary artery and the aorta and in utero most of output of right
ventricle goes through the ductus arteriosus into the aorta. Normally it soon closes off. My
brother had his only son die soon after birth because of abnormality of the ductus. He was born
about ten years before corrective surgery became possible.