Sehlmeyer et al. SP-C/Cholesterol Relationships in Alveoli diffusion through lipid membranes is not well understood. However, it has been proposed that the interactions between cholesterol and the acyl carbon chains in phospholipids (stabilized by Van der Waals' forces) (Wennberg et al., 2012) create tightly packed arrangements that limits small molecule diffusion (Zocher et al., 2013). By decreasing trans-gauche rotation of the phospholipid acyl chains, membrane rigidity increases (Cassera et al., 2002; Molugu and Brown, 2019) leading to less free volume and free pockets which may accommodate oxygen, therefore reducing its flux (its partition and diffusion) through membranes (Zuniga-Hertz and Patel, 2019). Mammals developed sophisticated organs to optimize the uptake of oxygen during the life essential breathing cycle. From conducting airways to the second biggest surface exposed to the environment, the alveolar human surface (Ochs et al., 2004). In addition, the first membrane that oxygen encounters in the mammalian lungs is a complex mixture of lipids (mainly phosphatidylcholines (PC), such as dipalmitoylphosphatidylcholine (DPPC), up to a 90%) and proteins (10%) with a 5-10% of cholesterol (14-20% mol) (Zuo et al., 2008; Bernhard, 2016), called lung surfactant (surface active agent). Interestingly two surfaces in the human body present with abnormally higher cholesterol molar ratio, and both of them are in contact with the environment tightly regulating the uptake of oxygen. On the one hand, as previously explained, lung surfactant presents around a 14-20% mol cholesterol in its composition. On the other hand the eye lens surface contains up to 35% mol of cholesterol (Raguz et al., 2008; Mainali et al., 2013), compared to a normal cell plasma membrane with a 0.5 phospholipid to cholesterol molar ratio (van Meer et al., 2008; Widomska et al., 2017). Although the main function of lung Abbreviations: α-SMA, alpha smooth muscle actin; aaAMs, alternatively activated alveolar macrophages; ABCA1, ATP binding cassette transporter A1; ABCG1, ATP binding cassette transporter G1; ADA, adenosine-deaminase; ADP, adenosine diphosphate; AEC, alveolar epithelial cell; AE1C, alveolar epithelial type 1 cell; AE2C, alveolar epithelial type 2 cell; ALI