Respiratory System

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Respiratory System

Lower Respiratory System

The primordium of the lower respiratory system develops in about the fourth week. The laryngotracheal diverticulum arises from endoderm on the ventral wall of the foregut. Tracheoesophageal folds develop on either side and join to form a tracheoesophageal septum that separates it from the rest of the foregut. This divides the foregut into the laryngotracheal tube (ventral) and the esophagus (dorsal). The caudal end of the laryngotracheal diverticulum enlarges to form the lung bud, which is surrounded by splanchnic mesenchyme.

Larynx

The opening of the laryngotracheal tube becomes the inlet of the larynx. The laryngeal cartilages are derived from the fourth and sixth pharyngeal arches.

Trachea

The epithelium and glands of the trachea develop from the endoderm of the laryngotracheal tube. The cartilage, connective tissue, and smooth muscle are derived from the surrounding splanchnic mesenchyme.

Bronchi

At the end of the fourth week the lung bud divides into two bronchial buds, which enlarge to form the primary bronchi. The right bronchus is larger and more vertically oriented than the left one, and this relationship persists throughout life. In the fifth week, each bronchial bud divides into secondary bronchi. In the eighth week the secondary bronchi divide to form the segmental bronchi (tertiary bronchi), ten in the right lung and eight in the left. Each segmental bronchus becomes a bronchopulmonary segment (segment in a lung). The smooth muscle, connective tissue, and cartilaginous plates in the bronchi are derived from splanchnic mesenchyme.

Table 14 - Stages of Lung Development

Time period	Stage	Notes
Weeks 5 – 17	Pseudoglandular	Developing lungs resemble an exocrine gland. Respiration is not possible. Fetuses born during this period cannot survive.
Weeks 16 – 25	Canalicular	Terminal bronchioles divide and primitive alveolar sacs (terminal sacs) develop. Some respiration may be possible towards the end of this stage. Fetuses born towards the end of this period (weeks 22-25) can survive if given intensive care but often die anyway.
Week 24 – birth	Terminal sac	Many more alveoli develop, and the epithelium lining the terminal sacs become thin enough to allow respiration. Type I and Type II pneumocytes develop. Type II pneumocytes begin producing pulmonary surfactant, which counteracts surface tension and facilitates expansion of the terminal sacs at birth. Fetuses born after 24 weeks may survive, and those born after 32 weeks have a good chance of survival.
Birth – year 8	Alveolar	Respiratory bronchioles, terminals, alveolar ducts continue to increase in number

Figure 13 - Development of Lungs

Clinical Correlations

Tracheoesophageal Fistula

An abnormal communication between the trachea and esophagus due to incomplete separation of the trachea and esophagus during the fourth week of development. It is commonly associated with esophageal atresia. Newborn infants with these malformations cough and choke during eating due to the aspiration of food and saliva into the lungs.

Respiratory Distress Syndrome

Respiratory Distress Syndrome is common in premature infants and is due to a deficiency of surfactant. It is commonly associated with hyaline membrane disease in which the alveolar surfaces of the lungs are coated with a glassy hyaline membrane. Treatment with thyroxin and cortisol can increase production of surfactant.

Pulmonary Hypoplasia

Pulmonary hypoplasia results when lungs are compressed by abnormally positioned abdominal viscera and cannot develop normally or expand at birth. It is commonly caused by congenital posterolateral diaphragmatic hernia.