A developing human inside the womb obtains oxygen without taking a breath of air by relying entirely on the maternal system for gas exchange. This involves remarkable adaptations, temporary structures, and unique circulatory pathways that deliver oxygen and nutrients, preparing the developing body for independent life after birth.
The Placenta: The Fetus’s Lifeline
The placenta, a temporary organ that develops in the uterus during pregnancy, serves as the primary interface between the mother and the fetus for all exchange processes. This organ implants into the uterine wall and connects to the fetus via the umbilical cord, which contains blood vessels for transport. Its structure includes chorionic villi, tiny, finger-like projections that extend into maternal blood spaces within the uterine lining.
Oxygenated blood from the mother flows into these intervillous spaces, bathing the chorionic villi. Oxygen and nutrients from the mother’s blood diffuse across a thin barrier into the fetal capillaries within the villi. Simultaneously, waste products like carbon dioxide from the fetal blood diffuse back into the maternal circulation for elimination. This specialized arrangement ensures efficient gas exchange without direct mixing of maternal and fetal blood.
The umbilical cord, containing two umbilical arteries and one umbilical vein, acts as the conduit for this exchange. The umbilical vein carries oxygen-rich blood and nutrients from the placenta to the fetus, while the umbilical arteries transport deoxygenated blood and waste products back to the placenta. The fetus depends completely on the placenta for its oxygen supply.
Fetal Circulation: A Unique Pathway
The fetal circulatory system is adapted to bypass the lungs and liver, which are not yet fully functional for gas exchange or metabolic processing. It employs three specialized shunts that redirect blood flow to ensure oxygenated blood reaches vital organs like the brain and heart. This pathway differs from the adult circulatory system.
One of these shunts is the ductus venosus, which allows most of the oxygenated blood from the umbilical vein to bypass the fetal liver and flow directly into the inferior vena cava. This highly oxygenated blood then travels to the right atrium of the heart. From the right atrium, a significant portion of this blood is shunted directly into the left atrium through an opening called the foramen ovale, bypassing the pulmonary circulation entirely.
Blood that does not pass through the foramen ovale proceeds from the right atrium into the right ventricle and then into the pulmonary artery. Since the fetal lungs are not inflated and offer high resistance to blood flow, most of this blood is diverted away from the lungs by the ductus arteriosus. This shunt connects the pulmonary artery directly to the aorta, ensuring that blood bypasses the lungs and flows to the rest of the body. These shunts ensure oxygen-rich blood is efficiently distributed to the developing body, particularly to the brain and heart, which have the highest oxygen demands.
Developing Lungs and Practice Breaths
While the lungs are not used for oxygen exchange in the womb, they undergo extensive development to prepare for their role after birth. Lung development progresses through several distinct stages, beginning early in pregnancy and continuing even after birth, including the embryonic, pseudoglandular, canalicular, saccular, and alveolar phases.
During the embryonic stage, around weeks 3-6, the lung bud forms from the foregut, and the major airways begin to develop. By the pseudoglandular stage (weeks 5-16), the bronchial tree forms, resembling a gland. The canalicular phase (weeks 17-26) involves the formation of respiratory bronchioles and the start of capillary development, as well as the initial production of surfactant, a substance that reduces surface tension in the lungs.
Later in pregnancy, during the saccular stage (weeks 26-36), primitive air sacs called saccules form, which will eventually develop into alveoli. Although the lungs are filled with amniotic fluid, fetuses engage in “fetal breathing movements.” These are rhythmic contractions of the diaphragm and chest muscles, causing amniotic fluid to move in and out of the developing lungs. These practice breaths are essential for promoting lung growth, maturing lung function, and strengthening the muscles necessary for sustained breathing after birth.
The First Breath: Transition to Air
Birth triggers a transformation in the newborn’s circulatory and respiratory systems. Once the umbilical cord is clamped, the placental oxygen supply is cut off, prompting the baby to initiate independent breathing. The baby’s first cry and subsequent breaths inflate the lungs, replacing amniotic fluid with air.
This inflation causes a significant decrease in pressure within the pulmonary blood vessels, allowing blood to flow freely to the lungs for the first time. This change in pressure, combined with the increase in oxygen concentration, signals the closure of the fetal shunts. The foramen ovale, the opening between the atria, functionally closes almost immediately as pressure in the left atrium rises above that in the right atrium.
The ductus arteriosus, connecting the pulmonary artery and aorta, also begins to close due to increased oxygen levels and changes in blood flow, typically within hours or days after birth. Similarly, the ductus venosus, which bypassed the liver, closes off as blood flow through the umbilical vein ceases. These closures redirect the entire cardiac output through the now-functional lungs, establishing the adult pattern of circulation and enabling the newborn to obtain oxygen directly from the air.