6.1 Central chemoreceptors
Changes in PCO2, and therefore in pH, are detected largely by chemoreceptors within the respiratory centres of the brain (Figure 16). During increased metabolic activity, such as exercise, the PCO2 in the arterial blood increases.
Question 12 Increased exercise
What happens to the P50 (the PO2 at which 50% of Hb molecules are saturated with O2) of the oxygen–haemoglobin dissociation curve during increased exercise? (see Section 4.2)
it stays the same
The correct answer is a.
Increasing exercise will shift the oxygen–haemoglobin dissociation curve to the right, so the P50 will increase.
As CO2-rich blood reaches the brain, CO2 diffuses across the blood–brain barrier into the interstitial fluid and cerebrospinal fluid that surrounds the medulla.
Activity 11 Reaction components
Enter the components represented by x and y that complete the formula below.
There are superscript and subscript buttons in the formatting bar. Make sure to use these to enter the correct chemical formula, including the associated positive and negative charges:
Using the completed formula above, what will happen to levels of H+ in the brain as CO2-rich blood reaches the medulla?
levels of H+ will increase
levels of H+ will decrease
levels of H+ will stay the same
The correct answer is a.
Adding more CO2 will increase the production of H+ and HCO3−. Increased H+ will make the tissue more acidic, meaning that the pH will decrease.
Neurons within the medullary and pontine respiratory centres will fire action potentials in response to the change in pH, via activation of receptors that are sensitive to protons (Guyenet and Bayliss, 2015). These neurons synapse onto the phrenic and intercostal nerves which innervate the diaphragm and intercostal muscles (see Section 1.2) and stimulate increased breathing (Figure 16).
As the pH returns to homeostatic levels, the chemoreceptors stop being activated and the breathing rate returns to normal. Therefore, the respiratory centres act as the ‘pacemakers’ of respiration during both resting and stimulated conditions, via communication with the muscles that control the expansion and contraction of the lungs (McKay et al., 2003). Fine-tuning of the breathing pattern is controlled by inputs from the pontine respiratory group (Figure 16). Information from stretch receptors in the lungs is also used by the respiratory centres to determine when the lungs have expanded to full capacity.
Some neurodegenerative diseases, such as motor neurone disease, are characterised by respiratory problems that are caused by the gradual loss of innervation to the diaphragm and intercostal muscles, despite the fact that the respiratory centres are intact. In other cases, when the respiratory centres of the medulla are damaged, individuals may require artificial ventilation of the lungs to regulate their breathing rate.