to the pulmonary trunk. Although a small portion of this flow passes through
the lungs (where any residual oxygen is extracted by the tissue of the non-
respiring lung), most is shunted into the thoracic aorta via the ductus arterio-
sus and thereby lowers the oxygen content of that vessel. This occurs distal to
the origins of the carotid arteries and ensures that the rapidly developing brain
has the best oxygen supply. The pattern of blood supply in the fetus and the
changes that occur at birth are shown in the following figures.
86 Anatomy, Histology, and Cell Biology