The respiratory control system consists of the cerebral cortex, respiratory center in the medulla, spinal cord, respiratory muscles, lung and chest wall, alveolar-capillary barrier, blood perfusion, mechano-receptors, and chemo-receptors. The system operates through a series of nerve impulses and includes the relationships between the respiratory control center and the muscles of respiration. The respiratory center neurons in the medulla exhibit spontaneous cyclic activity but are influenced by stimuli descending from the cerebral cortex (volitional control) as well as from two sensory loops: the mechanoreceptor and chemoreceptor pathways. Ventilation and perfusion occur together near the end of the cycle, and their output determines the partial pressures of arterial and alveolar carbon dioxide (PCO2) and oxygen (PO2), and partially, arterial hydrogen ion concentration (pH). These outputs provide feedback to the respiratory center via chemoreceptor and mechanoreceptor sensory pathways.

The major muscles involved in respiration are the diaphragm, external intercostal muscles, and scalene muscles, all of which are skeletal muscles. These skeletal muscles generate the driving force for ventilation, with the force of contraction increasing when they are stretched and decreasing when they are shortened. The contraction of the diaphragm, which is the primary muscle of respiration, causes the downward and forward movement of the abdominal contents. This action increases the vertical dimension of the chest cavity and creates a pressure difference between the thorax and abdomen. The diaphragm can generate airway pressures of up to 150 to 200 cm H2O during maximal inspiratory effort in adults. While the diaphragm moves approximately 1 cm during quiet breathing, it can move as much as 10 cm during deep-breathing maneuvers. The diaphragm is innervated by the right and left phrenic nerves, originating from the third to fifth cervical segments of the spinal cord (C3 to C5).

The external intercostal muscles play a significant role in inspiration by pulling the ribs upward and forward, increasing the lateral and anteroposterior diameters of the thorax. These muscles are innervated by intercostal nerves arising from the same level of the spinal cord (T1 and T2). Although paralysis of these muscles does not have a significant effect on respiration due to the primary reliance on the diaphragm, individuals with injuries to the lower spinal cord can still breathe independently. Only when the injury is above C3 does an individual become completely dependent on a ventilator.

During normal breathing, the accessory muscles of inspiration, such as the scalene muscles (elevating the sternocleidomastoid muscles) and the alae nasi (causing nasal flaring), do not contract. However, during exercise or significant airway obstruction, these muscles contract vigorously and actively pull up the rib cage. They serve to anchor the sternum and upper ribs during normal breathing. The pharyngeal wall muscles, including the genioglossus and arytenoid, are also considered muscles of inspiration as they help maintain upper airway patency during inspiration. All the muscles involved in rib cage movement are voluntary muscles supplied by intercostal arteries and veins and innervated by motor and sensory intercostal nerves.
Cerebral cortex Respiratory center, medulla Spinal cord Respiratory muscles Lung and chest wall Alveolar-capillary barrier BloodPerfusion Mechano-receptors Chemo-receptors PCO2, PO2, pH Diffusion Ventilation Nerve impulses Nerve impulses Force displacement •Fig. 20.9 Block Diagram of the Respiratory Control System, Demonstrating Relationships Between the Respiratory Control Center and Muscles of Respiration. Therespiratorycenterneurons,dispersedintoseveralgroupsinthemedulla,demonstratespontaneouscyclicactivitybutarestronglyinfluencedbystimulidescendingfromthecerebralcortex(volitionalcontrol)andfromtwosensoryloops:mechanoreceptorandchemoreceptorpathways.Ventilationandperfusionoccurtogetherneartheendofthecycle,andtheiroutputdeterminespartialpressuresofarterialandalveolarcarbondioxide(PCO2)andoxygen(PO2)and,inpart,arterialhydrogenionconcentration(pH).Theseoutputsfeedbacktotherespiratorycenterviachemoreceptorandmechanoreceptorsensorypathways.
Muscles of Respiration
The major muscles of respiration include the diaphragm, the external intercostal muscles, and the scalene muscles, all of which are skeletal muscles. Skeletal muscles provide the driving force for ventilation; the force of contraction increases when they are stretched and decreases when they are shortened. The force of contraction of respiratory muscles increases at larger lung volumes.
The diaphragm is the major muscle of respiration, and it divides the thoracic cavity from the abdominal cavity (
Fig.
20.10). Contraction of the diaphragm forces the abdominal contents downward and forward. This increases the vertical dimension of the chest cavity and creates a pressure difference between the thorax and abdomen. In adults, the diaphragm can generate airway pressures of up to 150 to 200 cm H2O during maximal inspiratory effort. During quiet breathing (tidal breathing), the diaphragm moves approximately 1 cm; however, during deep-breathing maneuvers (vital capacity), the diaphragm can move as much as 10 cm. The diaphragm
CHAPTER 20 Introduction to the Respiratory System is innervated by the right and left phrenic nerves, whose origins are at the third to fifth cervical segments of the spinal cord (C3 to C5).
The other important muscles of inspiration are the external intercostal muscles, which pull the ribs upward and forward during inspiration (see
Fig. 20.10 ). This causes an increase in both the lateral and anteroposterior diameters of the thorax. Innervation of the external intercostal muscles originates from intercostal nerves that arise from the same level of the spinal cord (T1 and T2). Paralysis of these muscles has no significant effect on respiration because respiration is dependent primarily on the diaphragm. This is why individuals with injuries to the lower spinal cord can breathe on their own; it is only when the injury is above C3 that an individual is completely dependent on a ventilator.
Accessory muscles of inspiration (the scalene muscles, which elevate the sternocleidomastoid muscles; the alae nasi, which cause nasal flaring; and small muscles in the neck and head) do not contract during normal breathing. However, they do contract vigorously during exercise, and when airway obstruction is significant, they actively pull up the rib cage. During normal breathing, they anchor the sternum and upper ribs. Because the upper airway must remain patent during inspiration, the pharyngeal wall muscles (genioglossus and arytenoid) are also considered muscles of inspiration. All the rib cage muscles are voluntary muscles that are supplied by intercostal arteries and veins and innervated by motor and sensory intercostal nerves.