Chemoreceptors detect the levels of carbon dioxide in the blood. To do this, they monitor the concentration of hydrogen ions in the blood, which decreases the pH of the blood. This is a direct consequence of an increase in carbon dioxide concentration, because carbon dioxide becomes carbonic acid in an aqueous environment.
The response is that the inspiratory centre (in the medulla), sends nervous impulses to the external intercostal muscles and the diaphragm, via the intercostal nerve and the phrenic nerve, respectively, to increase breathing rate and the volume of the lungs during inhalation.
Chemoreceptors which affect breathing rate are broken down into two categories.
In response to this high concentration, a nervous impulse is sent to the cardiovascular centre in the medulla, which will then feedback to the sympathetic ganglia, increasing nervous impulses here, and prompting the sinoatrial node to stimulate more contractions of the myogenic cardiac muscle increasing heart rate by causing the secretion of nor-adrenaline directly on to the sinoatrial node.
Noses in vertebrates and antennae in many invertebrates act as distance chemoreceptors. Molecules are diffused through the air and bind to specific receptors on olfactory sensory neurons, activating an opening ion channel via G-proteins.
When inputs from the environment are significant to the survival of the organism the input must be detected. As all life processes are ultimately based on chemistry it is natural that detection and passing on of the external input will involve chemical events. The chemistry of the environment is, of course, relevant to survival, and detection of chemical input from the outside may well articulate directly with cell chemicals.
For example: The emissions of a predator's food source, such as odors or pheromones, may be in the air or on a surface where the food source has been. Cells in the head, usually the air passages or mouth, have chemical receptors on their surface that change when in contact with the emissions. The change does not stop there. It passes in either chemical or electrochemical form to the central processor, the brain or spinal cord. The resulting output from the CNS (central nervous system) makes body actions that will engage the food and enhance survival.