Bathing in hot springs containing high concentrations of carbon dioxide (CO_2) remarkably influences cardiovascular function more than bathing in fresh water. The CO_2-ennriched water in hot springs generally contains many salts among which interactions remain unknown. We separately evaluated the actions of individual factors in CO_2-water and confirmed that CO_2 and NaCl have combined effects on blood pressure fluctuations in anesthetized rats. The animals were equipped with sensor probes to monitor body temperature, skin blood flow and arterial blood pressure, and immersed in bathwater (35°C) containing CO_2 with NaCl, KCl or sucrose. The effects of these factors on cardiovascular functions were evaluated using power-spectral analysis of the fluctuation in blood pressure and heart rate. Compared with immersion in tap water, heart rate and skin vascular resistance were reduced during immersion in CO_2-water as reported irrespective of the ingredients. In terms of the very low frequency range (0.02 ~ 0.195 Hz), the power of blood pressure fluctuation during immersion was significantly reduced when the CO_2-water contained more than 1.5 % NaCl but was not influenced by other ingredients with a similar osmotic pressure and the same specific gravity. The results indicated that coexistent CO_2 and sodium ions in bathwater reduce blood pressure fluctuations, and suggested that this combination effect of CO_2 and salt contributes to the sedative effect on human cardiovascular functions while bathing in CO_2-hot springs.
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