5.1 The fetal circulation
In the fetus, the circulatory system develops from a single circuit into two circuits acting in parallel. The purpose of this pattern of distribution of blood is to deliver oxygenated blood to those parts of the body which have the highest require
Blood returning from the placenta in the umbilical vein to the fetus is shunted through the liver via the ductus venosus and enters the heart via the caudal vena cava. The majority of it (approximately 80%) is directed through the foramen ovale by the shape of the structure of the junction of the vena cava and atrium. Oxygenated blood therefore enters the left atrium (LA) and is ejected into the aorta and the coronary and carotid arteries supplying the heart and brain. A small portion of the caudal vena caval blood flow is directed by the lower edge of the atrial septum secundum into the right atrium (RA) where it mixes with deoxygenated blood returning from the head and forelimbs and passes into the right ventricle (RV) and pulmonary artery (PA). However, the pulmonary circuit is the high resistance circuit in the fetus and this results in the majority of the deoxygenated blood being diverted through the ductus arteriosus into the aorta and thence to the systemic circulation (but not to the brain). Blood returns to the placenta via the umbilical arteries (Figure 5.1).