What makes the sensation of a dry mouth secondary, rather than the primary trigger for thirst?
Answer
The brain registers systemic osmolality changes well before salivary gland output significantly decreases.
The feeling of having a dry mouth is often mistakenly perceived as the initial cause of thirst. However, the text specifies that this is a secondary or localized symptom. The true primary trigger originates from deeper, central mechanisms where specialized osmoreceptors in the brain detect minute increases in blood concentration (osmolality). This systemic signal is registered well in advance of the salivary glands drastically reducing their moisture production. Consequently, by the time the mouth feels distinctly dry, the central regulatory system has already been signaling distress for a considerable duration.
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