Control of Respiration
- The respiratory system normally maintains the partial pressures of arterial oxygen and carbon dioxide within a relatively narrow range. Because arterial partial pressures of carbon dioxide exert a heavy influence on blood pH, the respiratory system also contributes to the fine-tuning of acid-base balance as discussed further in Respiratory Acid-Base Control. Respiratory maintenance of arterial oxygen, arterial carbon dioxide, and blood pH occur unconsciously and are controlled by a basic negative feedback control circuit which senses these parameters, integrates the information, and then coordinates a change in respiration that contributes to returning these values to their normal levels.
- Here we discuss the basic components of this control circuit and the mechanisms by which they function. It should be pointed out that this basic circuit can be overridden by the cerebral cortex, allowing for voluntary hyperventilation and hypoventilation that can lead to derangement of arterial oxygen, carbon dioxide, and pH. However, once significant, these voluntary derangements will ultimately lead to a loss of consciousness and thus reestablishment of the unconscious feedback control circuit.
- Like all negative feedback control circuits, control of respiration involves a set of sensory components which sense changes in relevant variables and send afferent stimuli to an integrative hub that analyzes the information. Once analyzed, the integrative hub then coordinates an efferent response that conrol a number of effector responses which can modulate the physiological variables originally sensed. In negative feedback control circuits these effector responses return the relevant variables to their original value, thus yielding relatively stable values for these variables across time. In the case of respiratory control the principal sensory components of the feedback loop are a variety of chemoreceptors which sense and respond to partial pressures of arterial oxygen and carbon dioxide as well as blood pH.
- A variety of lung receptors, including pulmonary stretch receptors, irritant receptors, and J receptors also send afferent information regarding the status of the lung; however, these sensory components are only activated in special circumstances and likely contribute little to normal respiratory control. Whatever the case, these sensory components send their afferent information to the brainstem respiratory centers which act as the integrative hub of the circuit. The brianstem centers then send efferent stimuli that modulate the activity of the respiratory muscles, yielding changes in respiratory drive that return arterial oxygen, arterial carbon dioxide, and blood pH back to their set points. Below we talk about the contributions of each of these components in more detail and provide an integrated picture of how this control circuit responds to changes in Integrated Respiratory Control.