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| the first menstrual period; the establishment of menstruation |
| the ability of an organism or part of an organism to react to stimuli; degree of susceptibility to stimulation |
diving reflex n.
A reflexive response to diving in many aquatic mammals and birds, characterized by physiological changes that decrease oxygen consumption, such as slowed heart rate and decreased blood flow to the abdominal organs and muscles, until breathing resumes. Though less pronounced, the reflex also occurs in humans, upon submersion in water.