Systemic venous system

Abstract: 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. In this review, I have considered two important aspects of the circulation during fetal and neonatal life. First, I have described the patterns of blood flow in the fetal liver and heart, and the preferential streaming of blood flow, which favors distribution of oxygen to the brain and myocardium. Second, I have discussed the factors determining cardiac output in the fetus and the changes in cardiac output after birth. Although considerable work has been done largely in acute studies in exteriorized fetal lambs and in anesthetized newborn lambs, these maneuvers may profoundly affect circulatory responses. Therefore, this review is largely concerned with studies in chronically instrumented fetal and neonatal lambs. Patterns of Blood Flow in the Fetal Circulation Patterns of blood flow in the fetal heart had been examined previously by Pohlman (1907) who injected starch granules into superior and inferior vena caval tributaries, and by Barclay et al. (1944) who used cineradiography in exteriorized fetal lambs. These studies demonstrated that almost all superior vena caval blood passed through the tricuspid valve, with only small amounts entering the left atrium through the foramen ovale. Also, inferior vena caval blood largely passed through the foramen ovale into the left atrium. Blood ejected by the right ventricle was observed to pass to the lungs, but largely through the ductus arteriosus to the descending aorta, whereas blood ejected by the left ventricle entered the ascending aorta and vessels to the head and forequarters. Because the inferior vena caval blood that crossed the foramen ovale contained oxygenated umbilical venous blood passing through the ductus venosus, as well as distal inferior vena caval systemic venous blood, oxygen saturation of left atrial and left ventricular blood would be higher than that in the right ventricle. This information could thus explain the observation of Barcroft (1946) that, in fetal lambs, carotid arterial blood had a higher oxygen saturation than femoral arterial blood. These early studies defined directions of blood flow, but could not provide quantitative in