In the womb, a developing human does not breathe air. The fetal respiratory system is not yet adapted for gas exchange with the atmosphere. Instead, the oxygen a fetus needs for growth and development comes from an entirely different source.
The Placenta’s Role in Fetal Oxygen
The placenta functions as a temporary organ that facilitates the exchange of substances between the mother and the fetus. It serves as the fetus’s primary source of oxygen and nutrients. Oxygenated blood from the mother travels to the placenta, where oxygen diffuses across a barrier into the fetal blood.
Simultaneously, carbon dioxide and waste products from the fetus transfer back across this barrier to the mother’s bloodstream. The umbilical cord connects the fetus to the placenta, transporting oxygen and nutrient-rich blood to the fetus and carrying waste products back to the placenta for removal. This system ensures the fetus receives a continuous supply of oxygen without using its own lungs.
Fetal Lung Development and Breathing Movements
While not used for oxygen exchange, fetal lungs undergo significant development within the fluid-filled womb. The lungs are initially filled with amniotic fluid, which is essential for their normal expansion and growth. Lung development progresses through several stages, forming lung buds, branching airways, and eventually tiny air sacs (alveoli) with surrounding capillaries for gas exchange after birth.
A crucial component of lung maturation is the production of pulmonary surfactant, a substance that begins to form around 24 to 26 weeks of gestation. Surfactant coats the inner surface of the alveoli, reducing surface tension and preventing their collapse. Without sufficient surfactant, the lungs would struggle to inflate and remain open.
Fetuses also exhibit “fetal breathing movements,” rhythmic contractions of the diaphragm and other respiratory muscles. These movements, which can be observed as early as 10 to 11 weeks, involve the inhalation and exhalation of amniotic fluid. They do not provide oxygen to the fetus but are vital practice sessions. These practice movements help condition the respiratory muscles, promote lung growth, and stimulate surfactant production, preparing the lungs for independent breathing after birth.
The Transition to Breathing Air
Birth marks a dramatic physiological transition as the newborn prepares to breathe independently. When the umbilical cord is clamped, the fetus’s oxygen supply from the placenta is cut off. This cessation of placental blood flow, combined with sensory stimuli like temperature change, triggers the newborn’s first breath.
The initial breath is often a gasp, requiring significant effort to inflate the fluid-filled lungs. During a vaginal birth, some fluid is squeezed out of the lungs as the baby passes through the birth canal. The remaining fluid is then rapidly absorbed into the body’s tissues and lymphatic system. Surfactant plays a crucial role, allowing the alveoli to remain open and preventing their collapse with each exhalation, facilitating efficient gas exchange.
Simultaneously, the circulatory system undergoes significant changes. Increased oxygen levels and shifts in pressure cause specialized fetal shunts, such as the foramen ovale and ductus arteriosus, to close. Their closure redirects blood flow to the now-inflating lungs for oxygenation and establishes the adult pattern of circulation, where the lungs become the primary site for gas exchange.