Function of respiration: primarily functions in gas exchange. Inspiration: air rich in oxygen inhaled into lungs: expiration: co2 produced during oxidative process of body exhaled from lungs, both gases transported by blood --involves respiratory & circulatory systems. Function of respiratory zone: site for gas exchange between air in alveoli and blood in pulmonary capillaries, about 300 million alveoli in human lungs, each associated with up to 1000 capillaries, large surface area (tennis court) Respiratory muscles: lung tissue elastic, but unable to expand/contract by itself, air sucked into lungs, which is powered by respiratory muscles of chest wall. Principle: diaphragm: increase chest volume longitudinally through flattening out & cross- sectional area through lifting ribs when contract (bucket-handle) Innervated by phrenic nerves from cervical segments 3, 4 and 5. Its contraction also raises lower ribs bc diaphragm has vertically oriented fragments attached to costal margins: parasternal intercartilaginous muscles: elevate ribs, external intercostals: elevate ribs.