Final answer:
At birth, the infant's lungs do not collapse entirely after the first exhalation due to pulmonary surfactant, which reduces alveoli surface tension. Labor triggers the first breath, prompting circulatory adjustments, enabling the lungs to perform gas exchange.
Step-by-step explanation:
The question pertains to the reason why a newborn infant's lungs do not collapse completely after the first exhalation. Following birth, the lungs are inflated, and the remaining amniotic fluid is either expelled or absorbed by the baby's body shortly thereafter. This is made possible largely due to the presence of pulmonary surfactant, which plays a crucial role in reducing the surface tension within the alveoli making it easier for these tiny air sacs to remain open upon exhalation.
Respiratory Adjustments
During gestation, the lungs contain amniotic fluid, limiting their activity, but this changes dramatically at birth. The fetus is prompted to take its first breath due to elevated carbon dioxide levels from reduced umbilical blood flow during labor contractions. This process causes pressures to change within the lungs and body, assisting in the drainage or absorption of amniotic fluid, thus allowing the lungs to take over the task of gas exchange, previously performed by the placenta.
Circulatory Adjustments
Once the infant has taken the first breath, a reconfiguration of the circulatory system occurs. The alveolar capillaries fill with blood, and the lungs begin their function of exchanging carbon dioxide for oxygen, now essential for the infant's survival.