English: Anatomy of DA and the adjacent arteries. (a) A patent DA is found in the fetus before respiration starts. Oxygen-rich blood (in red) from the placenta passes though the aorta. Oxygen-poor blood from the fetus (in blue) passes the DA entering the descending aorta (in purple). (b) After the start of respiration the DA regresses into a ligamentum arteriosum. Aortic (red) and pulmonary (blue) circulation are disconnected. (c) In aortic coarctation, a left sided heart malformation, the DA is kept open by administration of PGE allowing blood to enter the systemic circulation. Dashed lines indicate the plane of tissue sections shown in d–h. Transverse sections of the muscular DA (d;1 in c), the elastic aorta (e;2 in a), and the ligamentum arteriosum (f;3 in b). Intimal thickening in the DA is indicated in yellow. Areas with CN are indicated in white and the wavy elastic lamellae with blue. Endothelial cells (red outlines) line the inside of the vessels. Longitudinal sections through the DA (4 in a, providing Fig. 1g, and 5 in c providing Fig. 1h). (g) DA tissue and aorta tissue are merging and can be separately distinguished by their characteristic histology. (h) In coarctation, inner media and intimal thickening of the DA extend into the aorta sometimes almost encircling the lumen of the aortic arch at that site (as depicted). The aortic tissue will always be present in the roof of the coarctation.