Pulmonary alveolus thin fluid coating within all alveoli. The fluid coating is produced by the body in order to facilitate the transfer of gases between blood and alveolar air. The surfactant is produced by the type II cells which are the most numerous cells in the alveoli, yet do not cover as much surface area as the squamous alveolar cells (a squamous epithelium). Type II cells also repair the endothelium of the alveolus when it becomes damaged. Insufficient surfactant in the alveoli can contribute to atelectasis (collapse of part or all of the lung). Without pulmonary surfactant, atelectasis is a certainty; however,
Sexual selection in humans their beauty or fine plumages, and that the artistic faculties in humans belong to their spiritual nature and therefore cannot be connected to natural selection, which only affects the animal nature. Darwin was accused of looking to the evolution of early human ancestors through the moral codes of the 19th century Victorian society. Joan Roughgarden, citing elements of sexual behavior in animals and humans that cannot be explained by the sexual-selection model, suggested that the function of sex in human evolution was primarily social. Joseph Jordania suggested in 2011 that in explaining such human morphological and behavioral characteristics as singing,