Carbon Dioxide Alarm (Chemoreceptor Reflex for CO2/H+ Regulation)

carbon dioxide (CO2)

Gelo becomes a molecule of ___ , building up in the blood.

hypercapnia

This is ___(an abnormally high level of CO2 in the blood). Because CO2 is lipid-soluble, Gelo slips easily across the blood-brain barrier into the cerebrospinal fluid.

carbonic anhydrase

There the enzyme ___ (an enzyme that speeds the reaction of CO2 with water) helps convert Gelo into carbonic acid,

hydrogen ions

which splits into ___ (H+, the particles that make a fluid acidic) and bicarbonate.

central chemoreceptor

Now Gelo becomes a ___ (a neuron on the ventral surface of the medulla that senses H+ in cerebrospinal fluid). The blood-brain barrier is almost impermeable to H+ itself, which is why CO2 must carry the message across and generate H+ on the brain side.

respiratory center

The rising acidity excites Gelo, and she signals the ___ (medullary neuron groups that generate the rhythm of breathing).

peripheral chemoreceptor (in the carotid body)

Down in the neck, Gelo becomes a ___ (a sensory cell in the carotid or aortic bodies that monitors blood chemistry), sitting in the ___ (a small chemosensory organ near the carotid bifurcation). Rising arterial PCO2 and falling pH excite her too.

glossopharyngeal nerve

aortic body

vagus nerve
dorsal respiratory group

She fires up the ___ (cranial nerve IX), and her partner in the ___ (a chemosensory organ near the aortic arch) fires up the ___ (cranial nerve X). Both signals reach the___ (neurons in the medulla that drive inspiration through the phrenic nerve to the diaphragm)

negative feedback

The medulla concludes that ventilation is inadequate and increases both the rate and depth of breathing. More air moves through the alveoli and Gelo's CO2 companions are exhaled. Arterial PCO2 falls, and the carbonic acid reaction shifts back toward CO2 and water, so H+ falls and pH returns toward normal. As the stimulus fades, chemoreceptor firing eases and the respiratory drive relaxes. This is a rapid___ (a response that opposes and removes the original disturbance) loop.