A 19-year-old primigravida at 38 weeks' gestation, in active labor for the past 8 hours, is admitted to the hospital accompanied by her mother. on admission, the client's cervix is 5 cm dilated, her blood pressure is 120/84 mm hg, and she is breathing rapidly, feeling dizzy and light-headed. the nurse determines that the client is most likely experiencing effects of which factor?