Type I alveolar cells, also known as type I pneumocytes, are thin, flat cells that make up the majority of the surface area of the alveoli in the lungs. Their primary function is to facilitate gas exchange between the air in the alveoli and the blood in the surrounding capillaries. These cells cover about 95% of the alveolar surface and are crucial for efficient oxygen and carbon dioxide transfer. Type II alveolar cells, or type II pneumocytes, are cuboidal in shape and are responsible for producing and secreting surfactant, a substance that reduces surface tension in the alveoli. This surfactant prevents the alveoli from collapsing during exhalation and helps maintain their stability. Type II cells also play a role in repairing the alveolar epithelium after injury and can differentiate into type I cells when needed.