Explain chemical control of respiratory rhythm

Large number of chemoreceptors are located in the carotid bodies (lie bilaterally in the bifurcations of the common carotid arteries).
Their afferent nerve fibres pass through glossopharyngeal cranial nerve and thus to dorsal respiratory area of the medulla oblongata, sizeable number of chemoreceptors are also located in the aortic bodies (which lie along the arch of aorta).
Their afferent nerve fibre pass through the vagus cranial nerve and thus to dorsal respiratory group of neurons.