The Following Questions Refer to the Data Shown Below Figure 421
-Blood Carbon Dioxide Levels Determine the PH of Other

The following questions refer to the data shown below.
Blood entering a capillary bed of a vertebrate was measured for the pressures exerted by various factors.
$$\begin{array} { l c c } & \begin{array} { c } \text { Arterial End of } \\\text { Capillary Bed }\end{array} & \begin{array} { c } \text { Venous End of } \\\text { Capillary Bed }\end{array} \\\hline \text { Hydrostatic pressure } & 10 \mathrm {~mm} \mathrm { Hg } & 14 \mathrm {~mm} \mathrm { Hg } \\\text { Osmotic pressure } & 26 \mathrm {~mm} \mathrm { Hg } & 26 \mathrm {~mm} \mathrm { Hg } \\\mathrm { PO } _ { 2 } & 100 \mathrm {~mm} \mathrm { Hg } & 42 \mathrm {~mm} \mathrm { Hg } \\\mathrm { PCO } _ { 2 } & 40 \mathrm {~mm} \mathrm { Hg } & 46 \mathrm {~mm} \mathrm { Hg }\end{array}$$
Figure 42.1
-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 directly measures and monitors carbon dioxide and causes breathing changes accordingly.
B) The medulla, which is in contact with cerebrospinal fluid, monitors pH and uses this measure to control breathing.
C) The brain alters the pH of the cerebrospinal fluid to force the animal to retain more or less carbon dioxide.
D) Stretch receptors in the lungs cause the medulla to speed up or slow breathing.
E) The medulla is able to control the concentration of bicarbonate ions in the blood.