Type I pneumocyte
Type I pneumocyte
Two types of pneumocytes contribute to the maintenance of the alveoli of the lungs: Type I pneumocytes and Type II pneumocytes. These cells function to aid in gas exchange, secretion of pulmonary surfactant, and self-regeneration...

(simple squamous alveolar cells) are responsible for gas exchange
Gas exchange
Gas exchange is a process in biology where gases contained in an organism and atmosphere transfer or exchange. In human gas-exchange, gases contained in the blood of human bodies exchange with gases contained in the atmosphere. Human gas-exchange occurs in the lungs...

 in the alveoli and cover a majority of the alveolar surface area (>95%). While type I pneumocytes account for most of the surface area, they are approximately half as numerous as type II cells
Type II pneumocyte
Type II pneumocytes also called great alveolar cells or septal cells are granular and roughly cuboidal in shape. Type II pneumocytes are typically found at the alveolar-septal junction...

, due to differences in size. Type II pneumocytes are large round cells, while type I pneumocytes are smaller cells stretched across a large surface area.

Type I pneumocytes are unable to replicate
Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...

 and are susceptible to toxic
A toxin is a poisonous substance produced within living cells or organisms; man-made substances created by artificial processes are thus excluded...

insults. In the event of damage, Type II cells can proliferate and/or differentiate into type I cells to compensate.
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