A fetus obtains oxygen and nutrients without using its lungs while in the womb. Fetal respiration differs fundamentally from breathing after birth. Adaptations within the womb allow life to thrive in an aquatic environment, preparing the developing human for independent life outside the mother’s body.
The Placenta: The Fetus’s Lifeline
The placenta serves as the primary organ for gas exchange and nutrient transfer for the developing fetus. This temporary organ develops during pregnancy, attaching to the uterus wall and connecting to the fetus via the umbilical cord. Functioning as the fetus’s “lungs” and “digestive system,” the placenta manages the transfer of essential substances.
Oxygen and nutrients from the mother’s blood pass across the placental barrier into the fetal blood. Carbon dioxide and other waste products from the fetal blood are transferred back to the mother’s circulation for elimination. There is no direct mixing of maternal and fetal blood within the placenta. Instead, these substances diffuse across thin membranes separating the two distinct blood supplies. This separation protects the fetus from certain maternal infections and prevents the mother’s immune system from reacting to fetal antigens.
Umbilical Cord and Fetal Blood Flow
The umbilical cord acts as the conduit between the placenta and the developing fetus, facilitating the transport of oxygenated blood and nutrients while removing waste. This structure contains three blood vessels: two umbilical arteries and one umbilical vein. The umbilical vein carries oxygen-rich and nutrient-laden blood from the placenta directly to the fetus.
Conversely, the two umbilical arteries transport oxygen-poor blood and metabolic waste products from the fetus back to the placenta. This circulatory pathway ensures a continuous supply of necessary substances to the growing fetus and efficient waste removal. The coordinated function of the umbilical cord and placenta sustains life in the womb, establishing a direct link for all exchanges.
Fetal Circulation: Bypassing the Lungs
Within the fetus, the circulatory system features adaptations that enable blood to bypass the non-functional lungs, which are filled with fluid and not yet involved in gas exchange. Three main shunts redirect blood flow to ensure oxygenated blood reaches the developing body’s organs. The first, the ductus venosus, allows a significant portion of oxygenated blood from the umbilical vein to bypass the fetal liver and flow directly into the inferior vena cava. This ensures the most oxygenated blood reaches the heart and brain efficiently.
The foramen ovale is an opening between the right and left atria of the heart, serving as the second shunt. This opening permits blood to flow directly from the right atrium to the left atrium, bypassing the right ventricle and the pulmonary circulation. A third shunt, the ductus arteriosus, connects the pulmonary artery to the aorta. This connection diverts most blood pumped from the right ventricle away from the lungs and into the systemic circulation. These shunts are important because fetal lungs are collapsed and offer high resistance to blood flow.
Preparing for First Breath
At birth, the circulatory and respiratory systems undergo rapid changes to enable the newborn to breathe independently. As the umbilical cord is clamped, blood flow from the placenta ceases, leading to an immediate increase in systemic blood pressure. This pressure change, along with the first breath, triggers the closure of the fetal shunts.
The inflation of the lungs with air causes a significant drop in pulmonary vascular resistance, allowing blood to flow freely through the pulmonary arteries to the lungs. This shift in pressure and blood flow causes the foramen ovale to close within minutes of birth, becoming the fossa ovalis. Similarly, the ductus arteriosus constricts and eventually closes, becoming the ligamentum arteriosum, redirecting all blood from the right ventricle to the lungs. The ductus venosus also closes, becoming the ligamentum venosum, as blood flow through the liver becomes established.