Acclimatization, also physiological adaptation, is the process by which an organism becomes better adapted to 1 in an environment different from the one to which it was indigenous. If the environmental difference is extreme, changes take place in the   2    and physiology of the organism. Each organism, however, has certain limits of temperature and other conditions within which it can survive, and some alleged instances of acclimatizing have merely been   3   of unsuspected hardiness in the organism.Humans, for example, can acclimate to extreme conditions through changes in normal physiological 4 . Persons who move from a temperate to a hot, dry climate  5  , changes in heart rate and body temperature such that in time they perspire less, and their perspiration  6  less salt. Although at an altitude of 7, 600 m most persons need to. breathe high-pressure oxygen for survival, through  7  acclimatization they may become able to breathe unaided. This is due to an increase in the number of red blood cells, which contain oxygen-carrying hemoglobin. The increase is  8   by the hormone erythropoietin, secreted by the kidneys. In addition a change in chemical composition within the red blood cells promotes migration of hemoglobin into the body tissues, where oxygen is needed.The human organism also exhibits a  9   to the absence of natural light. As with most life forms, humans normally function in what is called a circadian rhythm corresponding to the length of the day. Humans who have lived underground under 10 conditions continue to show cyclical changes in physiology, demonstrating the existence of a natural biological clock. The period dictated by this internal clock, however is slightly longer than one day.[A] instances   [B] extreme   [C] processes     [D] response      [E] gradual[F] purchase   [G] undergo   [H] experimental   [I] conditions      [J] contains[K] exist     [L] structure   [M] within       [N] temperature      [O] mediated