参考答案：

Two groups of chemoreceptors, medullary (1`) and peripheral (1`), send afferent information to the medulla and influence the depth and frequency of respiration.(2`)Medullary chemoreceptors are sensitive to pH and increase ventilation when pH falls.(3`)Peripheral chemoreceptors are sensitive to pH↓, PO2↓ , and PCO2↑, with PCO2 being most effective to intensify pulmonary ventilation.(3`)