68. Blood carbon dioxide levels determine the pH of other body fluids as well as blood, including the pH of cerebrospinal fluid.How does this enable the organism to control breathing?
(A) The brain alters the pH of the cerebrospinal fluid to force the animal to retain more or less carbon dioxide.
(B) The medulla is able to control the concentration of bicarbonate ions in the blood.
(C) The brain directly measures and monitors carbon dioxide and causes breathing changes accordingly.
(D) Stretch receptors in the lungs cause the medulla to speed up or slow breathing.
(E) The medulla, which is in contact with cerebrospinal fluid, monitors pH and uses this measure to control breathing.第 8 頁，共 8 頁