Respiratory Muscles

Quiet inspiration

Diaphragm and external intercostal muscles contract- active process

Diaphragm

Separates thoracic cavity from abdominal cavity, dome shaped at rest and flattens during contraction, increased thoracic volume = decreased pressure

External intercostal muscles

Lifts ribs up and out during contraction, increased thoracic volume = decrease pressure

Forced inspiration

Accessory inspiratory muscles = scalenes, sternocleidomastoid, trapezius, contraction of muscles increases thoracic volume and decreases pressure in addition to diaphragm and external intercostal muscle contractions

Quiet expiration

Passive, no muscle contraction, inspiratory muscles relax = decrease thoracic volume and increased pressure

Forced expiration

Active process, internal intercostal muscles and abdominal muscles active

Internal intercostal muscles

Depress ribs, decreased thoracic volume and increased pressure

Abdominal muscles

Compress abdomen, pushes diaphragm upwards, decreased thoracic volume and increased pressure

Inspiration process

No pressure difference and intrapleural pressure is subatmospheric at start, inspiratory muscles contract to increase intrathoracic volume, intrathoracic and intrapleural pressure decrease, lungs expand and increase alveolar volume, alveolar pressure decreases and becomes subatmospheric, air moves into lungs, intra alveolar pressure equals atmospheric pressure, intrapleural pressure is less than beginning

Expiration process

Intra alveolar pressure equals atmospheric pressure, intrapleural pressure is highly subatmospheric, inspiratory muscles relax and intrathoracic volume decreases, intrapleural pressure increases, lungs recoil and alveolar volume decreases, alveolar pressure increases and becomes higher than atmospheric, air moves out of lungs, intra alveolar pressure equals atmospheric pressure, intrapleural pressure is more than beginning

Compliance

Indicator of lung expandability, low compliance = greater force required for breathing

Factors that affect compliance

Elastic connective tissue around alveoli = positive effect, mobility of thoracic cage = negative effect, surface tension in alveoli = negative effect - need for surfactant production to breathe easily