It is important to recognize that primary cardiac disease is rare in the foal. It is also important, when evaluating for cardiac disease, to remember that systemic disease can affect dardiovascular parameters. In considering the cardiovascular system of neonates, recognize that significant circulatory changes occur at parturition. In the fetus, oxygenated blood is supplied by the placenta.
Blood flows from the placenta through the umbilical vein to the fetus.
It passes through the liver via the ductus venosus, and enters the heart through the caudal vena cava. It then flows into the aorta and the coronary and carotid arteries. The brain and heart therefore receive highly oxygenated blood. A small amount of blood from the caudal vena cava passes through the right atrium and ventricle and into the pulmonary artery. Because the pulmonary circulation is a high-resistance pathway in the fetus, the majority of this blood passes through the ductus arteriosus into the aorta, again bypassing the pulmonary circulation.