A healthy, 37-year-old woman loses her job at the auto factory. She picks up her three young children from school and met with a road traffic accident. Her 5-year-old son, sustains a severe head injury. The woman was not hu 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?