Distribution and regulation of blood flow in the fetal and neonatal lamb

Abstract

The transfer of the function of oxygen uptake, from the placenta during fetal life to the lung after birth, is associated with dramatic alterations in the course and distribution of blood flow, as well as in myocardial demands and function. In the fetus, oxygenated blood returns to the body through the umbilical veins and about half of this blood bypasses the hepatic microcirculation through the ductus venous to enter the central circulation. Preferential streaming of blood in the thoracic inferior vena cava enhances the flow of well-oxygenated blood from the ductus venosus and left hepatic vein across the foramen ovale. This facilitates oxygen delivery to the fetal brain and myocardium. Current evidence indicates that the fetal myocardium is structurally and functionally immature as compared with that of the adult. The fetal heart appears to be functioning near the top of the ventricular function curve, and increases in filling pressure produce little increase in ventricular output. This could be related to low myocardial compliance or to reduced intrinsic myocardial contractility. Because of the limitation of myocardial performance, circulatory responses are largely related to adjustments in afterload and local vascular resistances. After birth, cardiac output as well as its response to increased filling pressure is enhanced. This ability to increase cardiac output appears to be related to the effects of thyroid hormone in late prenatal life. Postnatally, cardiac output requirements are high relative to body weight, and there is limited reserve for further increases in output with volume loading. During postnatal development, resting cardiac output decreases in relation to body weight, and there is greater reserve, so that the percentage increase in response to volume loading increases greatly.