Fetal respiratory features
During fetal development, the infant's own external respiration system does not function. Gas exchange depends entirely on the maternal organism, which effectively replaces the external gas environment for the fetus. The fetus receives necessary oxygen ($O_2$) and eliminates carbon dioxide ($CO_2$) exclusively via placental circulation. During this period, the external link of respiratory self-regulation in the fetus is inactive and unused due to lack of need.
Birth and changes in blood gas composition
The pivotal moment in physiological change occurs at birth and subsequent clamping of the umbilical cord. At this second, placental circulation ceases instantaneously. The infant loses connection with maternal gas exchange. As a result, the newborn's $O_2$ level begins to drop and a sharp accumulation of $CO_2$ occurs. It is precisely the high concentration of carbon dioxide—rather than nitrogen, oxygen, or carbon monoxide—that is the absolute primary factor stimulating the onset of independent respiration.
Neurophysiological chain of the first breath
The initiation of the respiratory act represents a precise sequence of neural structure excitation. The entire process can be divided into several stages:
- The $CO_2$ accumulating in the blood primarily affects peripheral chemoreceptors located in tissues and vessels.
- In parallel, central chemoreceptors detect the increased $CO_2$ level and the concomitant decrease in pH (acidosis).
- These signals cause powerful excitation of inspiratory neurons. Anatomically, they are localized in the dorsal respiratory group of the medulla oblongata, near the structure known as the obex.
- From the activated inspiratory neurons, the nerve impulse is transmitted via axons downward to spinal motor neurons.
- Spinal motor neurons provide direct innervation to the main respiratory muscles: the diaphragm and external intercostal muscles.
- The contraction of these muscles ensures the expansion of the thoracic cavity and the physical execution of the first breath.
Briefly: answers to topical questions
What role does surfactant play in the execution of the first breath?
Surfactant ensures adequate expansion of the newborn's lungs. Its synthesis by alveolar cells begins well in advance—as early as the 2nd trimester of intrauterine development. A deficiency of surfactant leads to low alveolar surface tension and the development of atelectasis.
Memory chain: CO2 (stimulus) → Chemoreceptors (sensors) → Medulla oblongata (inspiratory neurons) → Spinal cord (motor neurons) → Diaphragm (effector muscles).
Class and exam questions
What is the leading factor of the newborn's first breath?
The absolute primary factor is the accumulation of carbon dioxide (CO2) in the infant's body after the cessation of placental circulation.
Where are the inspiratory neurons that initiate respiration located?
They are localized in the dorsal respiratory group of the medulla oblongata, located near the obex.
Which muscles execute the first breath?
The main work is performed by the diaphragm and external intercostal muscles, which receive impulses from spinal motor neurons.
How do receptors react to the cessation of placental blood flow?
Peripheral and central chemoreceptors detect excess CO2 and a drop in pH (acidosis), after which they send a signal to the respiratory center.
What else to study on this topic
- The role of peripheral and central chemoreceptors in respiratory regulation
- Anatomy and functions of the dorsal respiratory group of the medulla oblongata
- The effect of acidosis (decreased pH) on the excitability of neural centers
- The mechanism of impulse transmission from the medulla oblongata to spinal motor neurons
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