A developing human must maintain a continuous supply of oxygen to grow, yet the entire process occurs submerged within the amniotic fluid of the womb. Since the lungs are filled with fluid and not air, they cannot perform the gas exchange necessary to sustain life. The solution involves a specialized temporary organ and a completely re-routed circulatory system. This unique fetal physiology ensures that oxygen transfer happens efficiently until the moment of birth, when the body must rapidly convert to independent pulmonary respiration.
The Placenta: The Fetus’s External Lung
The organ that handles all gas exchange for the fetus is the placenta, which functions as a temporary external lung. Oxygen transfer occurs across a microscopic barrier where maternal blood and fetal blood flow in close proximity. This exchange happens primarily through a passive process called diffusion, driven by the difference in oxygen concentration between the mother’s and the fetus’s bloodstreams.
Maternal blood, rich in oxygen, flows into the spaces surrounding the chorionic villi, moving across the thin membrane into the fetal circulation. The process is highly efficient due to fetal hemoglobin (HbF), which has a higher chemical affinity for oxygen compared to the adult hemoglobin. This enhanced attraction allows the fetal blood to effectively “pull” oxygen molecules away from the maternal supply, while carbon dioxide waste diffuses in the opposite direction for the mother’s lungs to exhale.
Unique Features of Fetal Circulation
Because the lungs are bypassed, the fetal circulatory system must use specialized shortcuts to direct oxygenated blood. Blood returning from the placenta via the umbilical vein is the most highly oxygenated blood. This blood must be swiftly distributed to the most demanding organs, particularly the brain and heart, while avoiding the non-functional lungs and liver.
One of the first bypasses is the ductus venosus, which allows most oxygen-rich blood from the umbilical vein to skip the liver and flow directly into the inferior vena cava toward the heart. Once the blood reaches the right atrium, the foramen ovale allows the majority of the blood to pass immediately into the left atrium. This shunt bypasses the right ventricle and pulmonary circulation, sending the most oxygenated blood straight to the left side of the heart, from where it is pumped to the head and upper body.
A smaller amount of blood enters the pulmonary artery, but high resistance in the fluid-filled lungs limits flow. This blood is diverted through the third major bypass, the ductus arteriosus, which connects the pulmonary artery directly to the aorta. This mechanism ensures that only a small portion of blood goes to the developing lungs, while the rest is routed into the systemic circulation.
Fetal Breathing Practice and Lung Fluid
Despite not using its lungs for oxygen supply, the fetus engages in rhythmic movements called Fetal Breathing Movements (FBMs). These movements are not for gas exchange but are essential practice for the respiratory muscles and nervous system control. FBMs involve the diaphragm and chest wall muscles contracting, causing the fetus to inhale and exhale amniotic fluid rather than air.
These practice movements help maintain the necessary expansion of the lungs, which are filled with a specialized fluid secreted by the lung tissue itself. This fluid and the stretching action of the movements are necessary for the proper structural development of the airways and the alveoli that will eventually perform gas exchange. The fluid also contains surfactant, a substance that reduces surface tension and is necessary for the alveoli to inflate properly after birth.
The Physiological Transition at Birth
The moment of birth triggers a shift from placental oxygenation to independent pulmonary respiration. When the umbilical cord is clamped, the low-resistance placental circuit is cut off, causing a rise in systemic blood pressure. Simultaneously, the first breaths and the influx of air into the lungs cause pulmonary vascular resistance to drop sharply.
This decrease in resistance allows blood to flow freely into the lungs for gas exchange. The massive increase in blood flow returning to the left atrium raises the pressure on the left side of the heart. This pressure shift forces the flap of the foramen ovale to slam shut, ending the mixing of blood.
The increased oxygen concentration in the blood prompts the muscles surrounding the ductus arteriosus to constrict, functionally closing this shunt within minutes or hours of birth. The ductus venosus also constricts, ensuring that all blood passes through the liver. These synchronized closures permanently reroute the blood flow, establishing the adult pattern of circulation.