title: Water drinking causes a biphasic change in blood composition in humans.
abstract: To investigate precisely the fluid shifts associated with water drinking in humans, we measured continuously blood density and plasma electrolyte concentrations using the mechanical oscillator technique and ion-selective electrodes, respectively, in healthy young volunteers before (10 min) and after (48 min) water drinking for a period of 2 min. Beat-by-beat blood pressure was also monitored throughout the experiment. Drinking 1 l tap water caused a transient increase in blood density immediately after the drinking episode (from 1051.1+/-0.5 g/l before drinking to 1051.8+/-0.5 g/l 4 min after the start of drinking, P<0.05), followed by a gradual reduction (1050.1+/-0.5 g/l at 31 min). This drinking-induced change paralleled those of haematocrit, plasma density and plasma volume. Plasma [Na+] and [Cl-] and osmolality decreased after drinking without transient increases and reached minima at about 30 min. A transient increase in mean arterial blood pressure was observed prior to the increase in blood density. These findings suggest that water drinking causes a biphasic change in plasma volume: initial haemoconcentration, probably due to sympathetic acceleration, followed by haemodilution due to the post-absorptive effect, and further suggest that the fluid shift associated with the initial haemoconcentration is isosmotic.
authors.affiliation: Department of Physiology, School of Medicine, University of Occupational and Environmental Health, Kitakyushu, Japan.
authors.forename: Y
authors.initials: Y
authors.lastname: Endo
authors.forename: R
authors.initials: R
authors.lastname: Torii
authors.forename: F
authors.initials: F
authors.lastname: Yamazaki
authors.forename: S
authors.initials: S
authors.lastname: Sagawa
authors.forename: K
authors.initials: K
authors.lastname: Yamauchi
authors.forename: Y
authors.initials: Y
authors.lastname: Tsutsui
authors.forename: T
authors.initials: T
authors.lastname: Morikawa
authors.forename: K
authors.initials: K
authors.lastname: Shiraki
mesh_terms: D000328:Adult; D001794:Blood Pressure / Q000502:physiology; D001810:Blood Volume / Q000502:physiology*; D002712:Chlorides / Q000097:blood; D004326:Drinking / Q000502:physiology*; D018495:Fluid Shifts / Q000502:physiology; D006339:Heart Rate / Q000502:physiology; D006400:Hematocrit; D006438:Hemodilution; D006801:Humans; D008297:Male; D009994:Osmolar Concentration; D012964:Sodium / Q000097:blood
chemical_list: D002712:Chlorides; D012964:Sodium