The respiration must always be regulated so that breathing is not excessive and not insufficient. If breathing were insufficient then the body wouldn’t get all the oxygen it needs, and it wouldn’t get rid of all the CO2 it produces. If breathing were excessive the body would lose too much CO2, which will cause respiratory alkalosis as we will see in the next topic.
The respiration is controlled by three mechanisms:
- Centrally by the brainstem and cerebral cortex
- By chemoreceptors – described in topic 45
- By reflexes
Central regulation of breathing
The respiratory centre is located in the brainstem. Its role is to regulate the depth and frequency of breathing so that normal levels of pO2 and pCO2 are maintained.
The respiratory centre mainly receives input from chemoreceptors in many parts of the body, but also from mechanoreceptors in the lung. The brainstem processes these inputs and determines whether ventilation should be stimulated or inhibited.
The respiratory centre can also be voluntarily overridden by the cerebral cortex. This is what allows us to consciously control our breathing pattern if we want to.
The most important input of the respiratory centre is the level of pCO2, rather than the level of pO2. This means that breathing is mainly controlled by the level of pCO2 in the body, not pO2.
The respiratory centre constantly fires signals to the respiratory muscles, mainly the diaphragm via the phrenic nerve. If it determines that ventilation should be stimulated it will fire signals more often, which causes the diaphragm to contract more forcefully and more often. If it determines that ventilation should be inhibited it will fire signals less often, causing the diaphragm to contract less often and with less force.
Reflex regulation of breathing
The most important reflex involved in the regulation of breathing is the Hering-Breuer reflex. The purpose of this reflex is to prevent over-inflation of the lung.
Stretch receptors in the lung are activated when the lung is excessively stretched, such as during large inspirations. Once activated they will stimulate afferent fibres of the vagal nerve. This signal reaches the respiratory centre, which inhibits further inspiration.
43. O2 and CO2 transport in the body
45. Chemical control of respiration. Acidosis, alkalosis