A portal system is a special type of circulation that incorporates two capillary beds. Normally, blood passes from the heart through arteries to a single capillary bed and then back to the heart through veins. In a portal system, blood first travels to one capillary bed and then to a second capillary bed before entering the venous system. The two capillary beds are connected by a portal vein (Figure SF-1).
Two important portal systems are found in the body: The hepatic portal system and the hypophyseal portal system. These portal systems are described in the chapters on the endocrine system (hypophyseal portal) and digestive system (hepatic portal). The hepatic portal system is shown in Figure SF-2.
Blood circulation in the fetus is very different than in the newborn or adult (Figure SF-3). Before birth, the fetus in the womb does not breath air, but instead obtains oxygen from the mother’s blood via the placenta. Indeed, the lungs of the fetus are collapsed and will not inflate until birth. As a result, the pulmonary circuit in the fetus is nearly completely shut down. Two detours redirect the blood in the fetus away from the pulmonary circuit. First, a hole called the foramen ovale penetrates the wall separating the left and right atria. As a consequence, blood returning from the systemic circuit and collecting in the right atrium can flow freely into the left atrium. From there, the blood enters the left ventricle and enters again into the systemic circuit. Second, a small artery called the ductus arteriosus connects the pulmonary trunk to the aorta. As a result, blood pumped from the right ventricle enters the aorta (and the systemic circuit) rather than the pulmonary circuit.
Immediately upon birth a newborn must inflate his lungs and obtain oxygen on his own. Thus, the pulmonary circuit must quickly become functional. After taking his first breath, pressure changes will cause the foramen ovale to close. Over the next few days the ductus arteriosus begins to shrivel. A thin connective tissue “scar” called the ligamentum arteriosum is all that remains of this artery in the adult. Likewise, a thin membranous remnant of the foramen ovale called the fossa ovalis is usually present in adults.
Without the lungs, how does the baby receive oxygen in the womb? The baby draws oxygen from the mother's blood. At the placenta, capillary exchange of oxygen moves the gas from the mother’s blood to the baby’s blood. One large umbilical vein conveys the oxygenated blood to the baby's inferior vena cava, where it again enters systemic circulation. Two small arteries carry deoxygented blood from the common iliac arteries through the umbilical cord back to the placenta.