And so as the blood moves from the high pressure into the low pressure, this forayman O'Valley opens up and the blood travels into the left atrium and from the left atrium, it passes into the left ventricle and then it moves into our aorter. Now, of course, a small portion of that blood will leak from the right atrium and into the right ventricle and then will pass into the pulmonary trunk. Now, in the pulmonary trunk, remember, we don't want to waste any of that oxygen and nutrients on the lungs because the lungs inside that fetal individual are not yet developed. And so that means they cannot exchange any of that oxygen for carbon dioxide. And so we need another duct. And this duct is called the ductus arteriosis.