Pulmonary respiration is the exchange of oxygen and carbon dioxide in the lungs during ventilation through a process known as bulk flow. Bulk flow occurs in our passageways due to a high and low pressure difference that occurs at each end (Powers and Howley, 2010). The two physiological occurrences that create this exchange are inspiration and expiration. The muscle mostly responsible for the inspiratory process during sedentary body functions is the diaphragm, which when contracted, reduces the intrapleural pressure causing the lungs to expand. During increased activity, such as during exercise, additional muscles are recruited to assist in the increased breathing pattern. These muscles are the external intercostals, pectoralis minor, scalene, and the sternocleidomastoid (Powers and Howley, 2012). In the expiration process of sedentary body function, the need for muscle activity is non-existent due to the elastic nature of the chest walls and lungs, which regain their equilibrium after inspiration. During exercise, muscle recruitment becomes necessary and the muscles of the rectus abdominus and internal oblique contract to help push the diaphragm upward which increases the intrapulmonary pressure to assist in expiration (Powers and Howley, 2012).
Comparing the muscles of respiration to the skeletal muscles involved in activity, our muscles are affected by continuous muscle contraction and will eventually enter the fatigue stage when a stress is placed too high for too long. Similar to skeletal muscle, our respiratory system adapts to increase training demands and this result in a decrease in respirator ventilation. Powers and Howley do not provide a specific reason for this occurrence, for which it is believed that unlike skeletal muscle, our lungs do not undergo hypertrophy or an increase in mitochondrial density that skeletal fibers have the ability to do. However, in a review by Donald Mckenzie, physiological adaptations that occur to ventilator respiration during submaximal exercise are attributed to improvements among the heart, lungs and skeletal muscle, which have an overall effect on respiratory levels. Physiological changes that occur as recorded by McKenzie are an increase in left ventricular wall and cavity size, an increase in heart mass, red cell mass, plasma volume, and mitochondrial density to name a few. Skeletal muscle adaptations that occur from consistent repetitive muscle contraction are an increase in the capillary to fiber ratio, an increase in type 1 (slow twitch) muscle fibers, and an increase in the mitochondrial oxidative enzymes. These changes result in an increase in stroke volume, cardiac output, and oxygen carrying capacity (Mckenzie, 2012). These changes can cause chronic physiological adaptations to the ventilator response system by increasing respiratory muscle endurance (such as the external intercostals, pectoralis minor, scalene, sternocleidomastoid, rectus abdominus and internal oblique).
McKenzie, D. C. (2012). Respiratory physiology: adaptations to high-level exercise. British Journal Of Sports Medicine, 46(6), 381-384.
Powers, S., & Howley, E.. (2012). Exercise Physiology: Theory and Application to Fitness and Performance. (8th ed.). New York, NY: McGraw-Hill Companies, Inc.