Ventilation

The respiratory system, along with cardiovascular structures, operates as part of an intricate organization controlled by the central nervous system (CNS) to ensure optimal cell performance, providing sufficient oxygen to meet metabolic requirements and removing enough carbon dioxide so that cell function is not impaired by excessive changes in hydrogen ion concentration. The major function of the respiratory system is to maintain the arterial tension of oxygen (PaO2) and carbon dioxide (PaCO2) within acceptable limits in the face of changing metabolic needs and environmental conditions. To achieve this, the system is equipped with multiple sensors that monitor changes in blood chemistry (chemoreceptors) and changes in the mechanical properties of the lung and chest wall (mechanoreceptors). The chemoreceptors and mechanoreceptors allow ventilation to be continuously readjusted in accordance with metabolic needs, despite changes in body posture that alter the mechanical advantage or movement of the respiratory muscles. In addition, these receptors coordinate the contraction and relaxation of the respiratory muscles, so that adequate gas exchange is carried out with minimum expenditure of energy.
In addition, the respiratory chemoreceptors and mechanoreceptors participate in a protective network that adjusts the pattern of breathing and the mechanical conditions of the airways to minimize the deleterious effects on the lung of inhaled, noxious material.
Ventilation, unlike blood pressure and cardiac output, can be controlled consciously (voluntarily) as well as automatically (involuntarily). Indeed, the pathways for voluntary and automatic control of the respiratory muscles are anatomically separate. Voluntary as well as automatic control is essential for using the respiratory muscles in speech. In humans, afferent information continuously fed back to the CNS by mechanoreceptors in the airways, lungs, and chest wall allows the force of contraction of the respiratory muscles to be coordinated smoothly in volitional acts.
Besides inputs from respiratory system sensors, ventilation is influenced by projections from the vasomotor neurons to respiratory neurons and by signals received from thermoreceptors and vascular receptors. The multiplicity of inputs to the respiratory neurons ensures that ventilation is maintained when disease affects one or more afferent pathways or when the perception of some sensory cue is blunted by a depressed state of consciousness (e.g., sleep or anesthesia). However, conflicting demands and signals from different receptors may be responsible for dyspnea, a common symptom in respiratory disease.