Fetal Respiratory Development

Fetal respiratory development is the process by which the lung is built and prepared for air breathing before birth. Throughout gestation the lung is filled not with air but with a secreted liquid, and this liquid, together with rhythmic fetal breathing movements, regulates how large the lung grows and how its airways and air sacs are shaped. Gas exchange itself takes place across the placenta until birth.

Definition

Fetal respiratory development is the prenatal growth and structural and biochemical maturation of the lung, driven in part by the volume of liquid distending the developing airways and by fetal breathing movements, culminating in a lung capable of gas exchange after birth.

Scope

The entry covers the developmental stages of lung growth, the role of fetal lung liquid in maintaining lung distension, the contribution of fetal breathing movements, and the maturation of surfactant-producing cells that prepare the lung for air breathing. It treats fetal lung development as normal physiology and is not a guide to managing prematurity or fetal disease.

Core questions

• What determines how large the fetal lung grows?
• What is fetal lung liquid and why does it matter for lung growth?
• How do fetal breathing movements influence lung development?
• How does the fetal lung become biochemically ready for air breathing?

Key concepts

• Stages of lung development
• Fetal lung liquid
• Lung distension and basal lung volume
• Fetal breathing movements
• Surfactant maturation
• Lung hypoplasia from inadequate distension

Mechanisms

The fetal lung secretes a liquid into its lumen that keeps the developing airways and air sacs distended; the volume and pressure of this liquid are a major determinant of lung growth, and experimental drainage of the liquid reduces lung size while obstruction that increases its volume enlarges the lung. Fetal breathing movements, episodic contractions of the diaphragm, contribute to maintaining lung expansion and to normal growth. As gestation advances, type II alveolar cells differentiate and produce pulmonary surfactant, the surface-active material that will lower surface tension once the lung is aerated; insufficient surfactant at birth underlies the surface-tension problems Avery and Mead identified in the immature lung.

Clinical relevance

Knowledge of how the fetal lung grows and matures provides the physiological background for understanding conditions in which lung growth or surfactant maturation is incomplete. This entry describes normal developmental physiology and is educational context, not a basis for diagnosing or managing any fetal or neonatal condition.

History

Much of what is known about fetal lung development came from experiments in fetal sheep and lambs showing that the lung is liquid-filled and that altering the volume of this liquid changes lung growth. Avery and Mead's 1959 observation that immature lungs have abnormal surface properties linked surfactant deficiency to the difficulty such lungs have in becoming aerated, framing a central theme of fetal lung maturation.

Key figures

• Richard Harding
• Stuart B. Hooper
• Mary Ellen Avery
• Jere Mead

Seminal works

• harding-1996
• hooper-1995
• avery-mead-1959

Frequently asked questions

Does the fetus breathe in the womb?

The fetus makes rhythmic breathing movements that help the lung grow, but it does not breathe air; the lung is filled with liquid and gas exchange occurs across the placenta until birth.

Why is fetal lung liquid important?

The liquid keeps the developing lung distended, and the degree of distension is a major determinant of how the lung grows; too little distension is associated with a smaller, underdeveloped lung.