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.