In which section of the brain is the center for regulating the constancy of blood composition located?

it is in the hypothalamus. In vertebrates, the hypothalamus is the main nerve center responsible for the regulation of the internal environment of the body.
The functions of the hypothalamus: food intake and maintaining body weight, water intake and water-salt balance in the body, temperature regulation depending on the temperature of the environment, emotional experiences, muscle work and other factors, the function of reproduction.

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