Development of the Respiratory System

Development of the Respiratory System

Lung development can be divided into distinct stages: the pseudoglandular period, the canalicular period, and the terminal saccular period.

Learning Objectives

Evaluate the ability of a fetus to survive birth based on the development of the respiratory system

Key Takeaways

Key Points

  • During the pseudoglandular period (also known as the glandular period), all major lung elements except those required for gas exchange (e.g. alveoli ) develop.
  • During the canalicular period, the lumen of the bronchi enlarge, lung tissue becomes highly vascularized, and respiratory bronchioles and alveolar ducts develop from the terminal bronchioles.
  • The terminal saccular period establishes the important blood-air barrier and specialized cells of the respiratory epithelium emerge.
  • The lungs expand after the first breath.

Key Terms

  • terminal saccular period: The important blood-air barrier is established during this period. Specialized cells of the respiratory epithelium appear at this time, including type I alveolar cells across which gas exchange occurs and type II alveolar cells which secrete pulmonary surfactant.
  • first breath: At birth, the baby’s lungs are filled with fluid secreted by the lungs and are not inflated. When the newborn is expelled from the birth canal, its central nervous system reacts to the sudden change in temperature and environment. This triggers it to take the first breath within about 10 seconds after delivery.
  • canalicular period: The lumens of the bronchi enlarge and lung tissue becomes highly vascularized during the canalicular period.

The respiratory system lies dormant in the human fetus during pregnancy. The development of the lungs is divided into three stages. The pseudoglandular period (also known as the glandular period) spans weeks six to 16, during which time the developing lung resembles an endocrine gland. By the end of this period, all major lung elements, except those required for gas exchange (e.g. alveoli), have appeared. Respiration is not possible during this phase and fetuses born during this period are unable to survive.

The canalicular period spans weeks 16 to 26, during which the lumens of the bronchi enlarge, lung tissue becomes highly vascularized, and respiratory bronchioles and alveolar ducts develop from the terminal bronchioles. Respiration is possible towards the end of this period, but few fetuses born during this time will survive.

The terminal saccular period spans from week 26 to birth. During this time, the important blood-air barrier is established. Specialized cells of the respiratory epithelium appear, including type II alveolar cells that secrete pulmonary surfactant. This surfactant is important in reducing the surface tension at the air-alveolar surface, allowing expansion of the terminal saccules. The lungs at this stage are rock-like and will sink if placed in water but expand after the first breath, a trait which used to determine if babies were born alive. Figure 1 shows the right and left lung buds from which the bronchi and lungs will develop.

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Respiratory buds: Lung buds from a human embryo of about four weeks, showing commencing lobulations.

At birth, the respiratory system becomes fully functional upon exposure to air, although some development and growth continues throughout childhood. Preterm birth can lead to infants with under-developed alveolar type II cells. This causes a lack of surfactant, leading to increased surface tension within the alveoli, subsequent alveoli collapse, and absence of gas exchange, a condition known as respiratory distress syndrome.

Lastly, the alveolar period spans from birth to eight years of age, during which the terminal saccules, alveolar ducts, and alveoli increase in number. True alveoli appear as indentations in the saccular wall, and septae form to produce divisions in the wall.

Aging and the Respiratory System

In mammals, breathing is produced by a flattening of the diaphragm and lung expansion. Lung elasticity declines with aging.

Learning Objectives

Describe the alterations to the respiratory system that occur with aging

Key Takeaways

Key Points

  • Expiration involves relaxation of the diaphragm that causes the air be expelled, largely dependent on lung elasticity.
  • Emphysema is a long-term, progressive lung disease causing shortness of breath due to loss of lung elasticity with age.
  • Deterioration of lung capacity and function is a natural part of aging in healthy people.

Key Terms

  • emphysema: An abnormal accumulation of air in tissues, especially the lungs.
  • forced expiration: The speed at which air is expelled from the lungs during the middle portion of a forced expiration.
  • wheezing: A continuous, coarse, whistling sound produced in the respiratory airways during breathing. This occurs when part of the respiratory tree is narrowed or obstructed or airflow velocity heightened.

Examples

  • Chronic obstructive pulmonary disease (COPD) is the occurrence of chronic bronchitis or emphysema, a pair of commonly coexisting lung disease in which airways become narrowed.
  • This leads to a limitation of the flow of air to and from the lungs, causing shortness of breath (dyspnea) in the aging population.
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Lateral chest X-ray of a patient with emphysema: Emphysema is a common lung disease in the elderly. Note the barrel-shaped chest and flat diaphragm.

In mammals, breathing (inhaling) occurs during the contraction and flattening of the diaphragm, a domed muscle separating the thorax and abdomen. If the abdomen is relaxed, its volume is increased; the fall in pressure in the thorax is met by the entry of air. When the diaphragm relaxes, air is expelled by the lung’s elasticity. This quiet, relaxed breathing state requires little energy. When requirements increase, the abdominal muscles resist expansion. Increased abdominal pressure then tilts the diaphragm and ribcage upwards, increasing volume and air entry.

Expiration follows relaxation of diaphragm and abdominal muscles, but can be increased by the downward action of abdominal muscles on the rib cage. This forced expiration increases pressure across walls of airways and may lead to narrowing or even wheezing. Auxiliary intercostal muscles stiffen and shape the rib cage. Speech depends on the balance between the two forms of breathing. In humans, conscious change often modifies autonomous reaction to need, a pattern that can vary due to things like fear or anxiety, loss of lung elasticity due to aging, pulmonary diseases such as emphysema, or abdominal expansion from obesity.

Some types of emphysema occur as a normal part of aging, commonly in those older than age 85. At about 20 years of age, humans stop developing new alveolar tissue. In the years following this cessation, lung tissue begins to deteriorate, albeit at a relatively slow rate. As alveoli die, the number of lung capillaries decreases and the elastin of the lungs begins to break down, causing loss of pulmonary elasticity. This is a natural part of aging in healthy people.

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Diagram of an Alveoli: An alveoli with both cross-section and external views, bronchiole, pulmonary vein, pulmonary artery, alveolar sacs, and alveoli.

Age also contributes to loss of strength and mass in the chest muscles—these weaken, bones and cartilage start to deteriorate, and posture changes. Together, such age-related changes in respiratory system structures can cause or contribute to the development of emphysema. Though not all elderly people will develop clinically evident emphysema, all are at risk of decreasing respiratory function, which limits maximum lung performance and causes discomfort at higher levels of exertion.