A 68 year old woman presents with an obviously incarcerated umbilical hernia. She has gross abdominal distention, is clinically dehydrated, and repos persistent fecaloid vomiting for the past 3 days. Although tired, weak, and thirsty, she is awake and ale and her sensorium is not paicularly affected. Laboratory analysis reveals a serum sodium concentration of 118 mEq/L. Which of the following is the most likely physiologic explanation for the serum sodium?