The Role of Central Chemoreceptors in Carbon Dioxide Excretion

Your body rids itself of CO2 when you exhale.

If you've ever tried to hold your breath for too long, you'll probably remember that after a while, you start to feel an irresistible urge to breathe. In the absence of conscious effort, breathing is controlled by your brainstem or lower brain, the region of your brain just above your spinal cord. Centers in your brainstem adjust your breathing rate in response to changes in cerebrospinal fluid.

CO2 In the Bloodstream

When dissolved in water, carbon dioxide reacts with H2O to form carbonic acid. Consequently, as CO2 levels increase, the carbon dioxide will diffuse into cerebrospinal fluid, which is produced from your blood, and cause a slight decrease in pH. The decrease is very slight because the bicarbonate and hemoglobin in your blood act as buffers to take up the hydrogen ions released by the carbonic acid. Nonetheless, this decrease is significant enough for the involuntary mechanisms in control of your breathing to sense it and respond.

Brainstem

Breathing control centers in the medulla oblongata (the two lowest regions of your brainstem) use the pH of cerebrospinal fluid as an indicator of blood CO2 concentration. Chemoreceptors, sensory neurons that can detect the pH, turn this chemical change into nerve impulses to the breathing control center, which responds by modulating your breathing. If pH rises, the medulla oblongata decreases breathing rate and depth, and if pH falls, it increases breathing rate and depth.

Pons

While the medulla oblongata sets the rhythm of your breathing, another breathing control center in the pons determines the tempo. It moderates the breathing rate set by the medulla oblongata to smooth out transitions. By breathing, you eliminate CO2 from your bloodstream. CO2 concentration is the most important factor in determining your breathing rate. Although sensors in your aorta and the carotid arteries of your neck do monitor oxygen concentration, they typically only send signals to the brainstem when oxygen concentration drops very low -- as when you are at high altitude, for example.

Considerations

Breathing control is an example of a negative feedback mechanism because the response to an input acts to diminish the cause of the input. If CO2 levels rise, pH falls and your medulla oblongata increases your breathing, decreasing CO2 concentrations and thus increasing pH. The same applies in reverse. You could consciously override these mechanisms by forcing yourself to breathe very rapidly or holding your breath, but once you stop or pass out, your brainstem will bring the rhythm of your breathing back to normal.

References

• "Progress in Neurobiology"; CO2, Brainstem Chemoreceptors and Breathing; E. Nattie; August 4,1999
• "Biology"; Neil A. Campbell, et al.; 2008