In addition to a layer of mucus, the gastric mucosa barrier is mainly formed of a tight junction consisting of the surface membranes of epithelial cells. It is this that mainly impedes the flowback of H+ ions. Buffers conveyed by the blood neutralise the small numbers of ions that do flow back, even under physiological conditions. Damage to the barrier results in a considerable backflow of ions that enhance acid secretion and release of histamine and other vasoactive substances, so that further mucosal lesions occur. Such damage may be of exogenous or endogenous origin. Alcohol and acetylsalicylic acid are two examples. The latter is undissociated and hence liposoluble in the highly milieu of the stomach and can easily enter the cells and damage them. Severe hypovolaemia, stress and sepsis are instances of endogenous sources of damage. In the final analysis, the damage caused by sepsis is linked to cell anoxia.