1. A healthy, 37-year-old, recently divorced woman loses her job at th e auto factory. She picks up her three young children from the factory day care center and gets into an automobile accident on the way home. Her 5-year-old son, who was not wearing a seat belt, sustains a severe head injury. The woman was not hurt in the accident, but is hyperventilating as she sits in the waiting room at the hospital. She complains of feeling faint and has blurred vision. Which of the following is decreased in this woman?