When deprived of both drinking water and food, pigs failed to develop
the hyperosmolality usually expected with dehydration. In further stud
ies 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 e
aten during water deprivation, plasma osmolality rose to levels suffic
ient to stimulate drinking. During water and food deprivation, plasma
osmolality failed to rise, even over 24 h, and usually fell. Blood vol
ume 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. I
t appears that the conditions of deprivation determine the proportions
of thirst stimulation that can be attributed to plasma hypertonicity
and to hypovolemia.