The existence of human life in utero (inside the mother's uterus) is completely dependent on the mother's support. In fetal life, the mother's body is responsible for fetal circulation, oxygenation, waste management and hormone regulation via the placenta; the connection between the mother and fetus. The fetus and its internal organs are passive recipients during the process of growth and development in utero. Upon delivery and the separation from the placenta, the fetal lifeline, it's game on for the newly born human.
A human's entry into the world and the beginning of extrauterine (independent life outside of the mother's body) begins with a simple first breath of life; but the underlying process is more complex than one would imagine. The life process is initiated upon inspiration and a cascade of events and changes occur within the newborn's body. Flaws or obstacles can impede the process and require immediate external assistance establishing ventilation for a successful transition, and sometimes are necessary for survival.
Let us review the fetal system's functions and the expected changes during the transition to extrauterine life.
First, the fetal heart, lungs and liver are non-functional organs; are not required to perform their primary roles of circulation and oxygenation in utero.. Therefore, the fetus' circulatory system has a different pathway with three main bypasses shunting the majority of blood away from these organs. There is sufficient blood supply to nourish and grow these organs while reserving the flow of oxygenated blood to other parts of the fetal body. The lungs are actually fluid-filled and exhibit a passive influx of amniotic fluid. The umbilical cord delivers oxygenated, nutrient-rich blood from the placenta and returns deoxygenated blood to the placenta for exchange. Once the blood enters the fetus via the umbilicus and travels up the body via the inferior vena cava, bypassing the liver via the ductus venosus; the first of fetal structures to assist in regulating blood flow. The foramen ovale is the second bypass and is located between the right and left atrium of the heart; shunting blood away from the lungs. The third such structure is the ductus arteriosus; connecting the pulmonary artery and aorta, further shunting blood flow away from the lungs. In utero, the pulmonary pressures are higher than systemically. The blood exits the fetus via umbilical arteries (yes, arteries with deoxygenated blood) to the umbilical cord and back to the placenta.
Upon that first breath of life the circulatory system changes; delivering blood to the lungs and liver, respectively for oxygenation and waste management. The pressure gradients change and the resistance in the pulmonary beds fall while systemic pressures rise. The three fetal bypasses are no longer needed and close with continued increase in oxygen levels, decreased blood flow and increased vascular pressures. The heart takes on the dominant role of pumping blood to the body; and its dynamic changes. The fluid in the lungs needs to be replaced with oxygenated air, thereby deeming ventilation a crucial action.
This is a tremendous task for a being that is now required to be self-sufficient for survival. The amount of energy in this undertaking to be successful is assisted by metabolic and endocrine changes. Newborn metabolic rates rise. Energy in the form of glucose is no longer supplied by the mother and the newborn liver assumes responsibility to maintain blood glucose levels, compatible with life; until adequate oral intake is established. The hormonal system experiences a shift in the production and excretion of catecholamines and other hormones to sustain life by influencing blood pressures and energy reserves. Cortisol, epinephrine, norepinephrine, dopamine, renin-angiotensin and vasopressin all play a vital role in this process.
The observer of a newborn's birth should be in awe each time it is a flawless event. The months of preparing the fetus for that very moment of transition to extrauterine life can result in failure. The complex physiologic changes need to occur smoothly and precisely to ensure a successful and happy start to the first moments of independent life.
Works Cited:
- Morton, Sarah U, and Dara Brodsky. "Fetal Physiology and the Transition to Extrauterine Life." Clinics in perinatology vol. 43,3 (2016): 395-407. doi:10.1016/j.clp.2016.04.001