Primary hypothesis: osmolality changes influence the sensitivity of the respiratory center to carbon dioxide, hyponatraemia causing hyperventilation, and hypernatraemia depressing ventilation.

Secondary hypothesis: There are gender differences in the sensitivity to osmolality changes.

10 women and 10 men will on different occasions drink water or receive hypertonic saline intravenously, in order to lower or increase plasma osmolality. The women will participate during both faces of the menstruation cycle. On each occasion the subject´s sensitivity to carbon dioxide will be tested, and blood samples will be drawn for analysis of blood gases,electrolyte and osmolality.Subjects who interrupt participation before completion of all planned occasions, will be substituted, so that 10 subjects of either sex will have participated as planned. All results from all participants will be analyzed.

Healthy volunteers will on different occasions be subject to reduced plasma osmolality caused by drinking water, and increased osmolality caused by intravenous infusion of hypertonic saline.Before and after each osmolality change, sensitivity to carbon dioxide will be tested by partial rebreathing through a so called Bain-system. Throughout the whole experiment heart rate, blood pressure and oxygen saturation will be recorded.Blood samples will be collected before each rebreathing test and every 20 minutes during the two hours of water or salt load. Urine will be collected and analysed.

Inclusion Criteria:

• Females with regular menstruations, males

Exclusion Criteria:

• Consumption of nicotine, BMI > 26,
• pregnancy,any hormone treatment,
• treatment with diuretics,
• diabetes or kidney disease,
• BMI > 26,