Final answer:
At birth, the transition to independent respiration is prompted by increased carbon dioxide levels (chemical), the physical process of birth (mechanical), the change in temperature upon leaving the womb (thermal), and stimuli from the new environment (sensory).
Step-by-step explanation:
Upon birth, four factors combine to stimulate the respiratory center in the medulla, which are chemical, mechanical, thermal, and sensory influences. The direct answer in two lines to what initiates the newborn's first breath is the rise in carbon dioxide levels leading to acidosis, sensory signals from the environment, temperature change, and mechanical factors during birth. Here is an explanation in 200 words:
During life in utero, the fetus is oxygenated through the placenta with no need for breathing. At birth, contractions during labor, which are mechanical in nature, reduce oxygen flow and elevate carbon dioxide levels in the fetal blood, triggering the medulla oblongata to initiate breathing. The first breath also results from a change in the thermal environment as the newborn leaves the warm uterus for relatively cooler air, and the sensory stimuli from the external environment. These combined factors cause the deflation of the fluid-filled lungs and inflation with air, marking the first independent respiratory effort of the newborn.