The human respiratory system is designed for efficient gas exchange, with the alveoli serving as the primary sites where oxygen enters the bloodstream and carbon dioxide is expelled. Within these tiny air sacs, specialized cells known as alveolar type II cells play an important role in maintaining lung function. These cuboidal-shaped cells are found throughout the alveoli and are responsible for several functions that contribute to lung health.
Primary Secretion: Pulmonary Surfactant
The main secretion of alveolar type II cells is pulmonary surfactant, a mixture composed primarily of lipids (about 90%) and proteins (5-10%). The most abundant lipid is dipalmitoylphosphatidylcholine (DPPC). The protein component includes four main surfactant proteins: SP-A, SP-B, SP-C, and SP-D.
Surfactant lipids and proteins are synthesized and processed within the cells. They are then assembled and stored in specialized intracellular organelles called lamellar bodies. When stimulated, lamellar bodies release pulmonary surfactant into the fluid lining the alveoli through exocytosis.
Essential Role of Pulmonary Surfactant
Pulmonary surfactant plays an important role in the mechanics of breathing by regulating surface tension at the air-liquid interface within the alveoli. Alveoli are wet, spherical structures, and the inherent surface tension of the watery lining would cause them to collapse, similar to how a soap bubble shrinks. Surfactant counteracts this force. Its unique molecular structure, with both hydrophilic (water-attracting) and hydrophobic (water-repelling) regions, allows it to adsorb to the air-water interface.
The main lipid component, DPPC, positions its hydrophilic head groups in the water and its hydrophobic tails towards the air, reducing surface tension. This reduction is particularly pronounced during exhalation when alveoli become smaller and surfactant molecules become more concentrated. By lowering surface tension to very low levels at the end of expiration, surfactant prevents the tiny air sacs from collapsing. This action reduces the effort required to reinflate the alveoli during the next inhalation, ensuring continuous and efficient gas exchange.
Additional Secretions and Functions
Beyond their primary role in producing surfactant, alveolar type II cells perform other functions that contribute to lung health. They secrete anti-inflammatory cytokines, which help modulate the lung’s immune response. They also produce growth factors, important for tissue repair and regeneration by stimulating the proliferation of other cells.
Alveolar type II cells also serve as progenitor cells for alveolar repair and regeneration after injury. When lung tissue is damaged, these cells can proliferate and differentiate into alveolar type I cells, which are thin, flat cells that form the majority of the alveolar surface area for gas exchange.
Clinical Impact of Dysfunction
Dysfunction of alveolar type II cells or their secretions can lead to serious respiratory conditions. A well-known example is Respiratory Distress Syndrome (RDS) in preterm infants. This condition arises due to insufficient production of pulmonary surfactant, as type II cells typically begin producing significant amounts around 35 weeks of gestation. Without enough surfactant, the alveoli collapse with each breath, leading to impaired gas exchange and severe breathing difficulties.
In adults, dysfunction of these cells can contribute to Acute Respiratory Distress Syndrome (ARDS). ARDS is a severe inflammatory lung disease characterized by widespread damage to the alveolar-capillary membrane. Injury to type II cells in ARDS can reduce surfactant production, leading to alveolar instability and worsening atelectasis, which is the collapse of part or all of the lung. This impairment in fluid transport and surfactant function contributes to the severe hypoxemia and respiratory failure observed in ARDS patients.