Carbon dioxide levels in the blood and cerebrospinal fluid affect blood ph. this enables the organism to sense a disturbance in gas levels as a. the brain directly measures and monitors oxygen levels and causes breathing changes accordingly. b. the brain alters the ph of the cerebrospinal fluid to force the animal to retain more or less carbon dioxide. c. the medulla oblongata, which is in contact with cerebrospinal fluid, monitors ph and uses this measure to control breathing. d. stretch receptors in the lungs cause the medulla oblongata to speed up or slow breathing.

C. the medulla oblongata, which is in contact with cerebrospinal fluid, monitors pH and uses this measure to control breathing.

Explanation:

The central chemoreceptors are located near the medulla inside the blood-brain barrier. When CO2 in cerebrospinal fluid increases, pH of the fluid lower (because CO2 becomes hydrated to carbonic acid). Change in pH stimulates the central chemoreceptors to activate respiratory centers which will increase the respiratory rate and depth of breathing.

blood enters the heart through two large veins, the inferior and superior vena cava, emptying oxygen-poor blood from the body into the right atrium of the heart. as the ventricle contracts, blood leaves the heart through the pulmonic valve, into the pulmonary artery and to the lungs where it is oxygenated.

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