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 Sexual selection in humans Sexual selection in humans concerns the concept of sexual selection, introduced by Charles Darwin as an element of his theory of natural selection, as it affects humans. The role of sexual selection in human evolution has not been firmly established although neoteny has been cited as being caused by human sexual selection. It has been suggested that the human brain is itself a product of sexual selection, i.e. it has developed as a sexual ornamentation to be used in courtship rather than for survival itself, and that it has developed in ways outlined by Ronald Fisher