Water deprivation, plasma osmolality, blood volume, and thirst in young pigs.

When deprived of both drinking water and food, pigs failed to develop the hyperosmolality usually expected with dehydration. In further studies 12 pigs were deprived of drinking water and food, and the effects were compared with data from nondeprived pigs, pigs water deprived but with food available, and pigs with water but no food. When food was eaten during water deprivation, plasma osmolality rose to levels sufficient to stimulate drinking. During water and food deprivation, plasma osmolality failed to rise, even over 24 h, and usually fell. Blood volume changes were calculated from packed cell volume and plasma protein data, and it was found that blood volume fell significantly when both food and water were withheld, but not when only water was withheld. It appears that the conditions of deprivation determine the proportions of thirst stimulation that can be attributed to plasma hypertonicity and to hypovolemia.