BRIEF REVIEW Distribution and Regulation of Blood Flow in the Fetal and Neonatal Lamb

AFTER the postnatal mammalian circulation has been established, blood flows serially from the systemic veins through the right side of the heart to the lungs, then to the left atrium and ventricle, which ejects the blood into the aorta to be distributed to the body, after which it returns to the systemic venous system. Apart from a small amount of coronary venous blood which enters the left ventricular cavity through Thebesian veins, there is essentially no admixture of oxygenated pulmonary venous blood and venous blood from the systemic circulation. The tissues therefore are supplied by blood that is fully oxygenated. During fetal life, the placenta serves as the site for gas exchange, and blood that is almost fully oxygenated returns to the fetal body through the umbilical veins. The common umbilical vein enters the porta hepatis, where it is joined by the portal vein. From the portal sinus, branches are provided to the left and right lobes of the liver, and the ductus venosus connects the sinus to the inferior vena cava; the ductus venosus thus permits umbilical venous blood to bypass the hepatic microcirculation. The connections between umbilical and portal veins and ductus venosus and inferior vena cava provide sites for admixture of well-oxygenated umbilical venous and poorly oxygenated systemic venous blood in the fetus. In addition to the ductus venosus shunt, the fetal circulation is characterized by the presence of two other shunts, the foramen ovale and the ductus arteriosus, which serve to direct blood returning to the heart away from the pulmonary circulation.