Respiratory Muscles

• Diaphragm

  • Primary muscle for breathing.
  • It is a large muscle that separates the thoracic and abdominal cavities.
  • During relaxation, it compresses the lungs. Therefore, the volume of the lungs decreases.
  • The diaphragm is being innervated by the phrenic nerve.
• External Intercostals

  • Contraction of these muscles raises and rotates the ribs, which results in the increased volume of the thoracic cavity.
  • Involved in quiet and forced respiration.
  • Innervated by the intercostal nerves.
• Internal Intercostals

  • Contractions of these muscles pull the ribs downward and inward, decrease the volume of the thoracic cavity and assists forced respiration.
  • Innervated by the intercostal nerves.
• INternal intercostals = INward and downward

  EXternal intercostals = EXpand
• Abdominals

  • Mainly for forced respiration
• Accessory Muscles of Respiration

  • Sternocleidomastoid
  • Scalene
  • Pectoralis
  • Serratus anterior
  • Trapezius
• How do we breathe

  Thoracic cavity volume increases, and creates a pressure gradient for airflow -> Air flows into the lungs resulting in a change in volume -> Lung volume decreases, and creates an increase in alveolar pressure -> Air flows out of the lungs until alveolar pressure equals atmospheric
• Alveolar pressure (PA) is the pressure in the alveoli. It is also referred to as intrapulmonary pressure.
• Transpulmonary pressure (PTP) is the pressure between the alveoli and the pleural space. It is also referred to as recoil pressure or trans-lung pressure.
• Intrapleural pressure (PIP) is the pressure in the pleural space.
• • Trans-chest wall pressure (PW) = PIP - Patm

  ○ PW is the elastic recoil created by the chest wall pulling outwards
• • Transpulmonary pressure (PTP) = PA - PIP

  ○ PTP is the elastic pressure of the lungs pulling inwards.
• • In the resting position, the atmospheric pressure and the pressure inside the lungs is equal.
• The pressure that is inside the pleural space that ensures that air is remaining inside the lungs at the end of forced expiration is called residual volume.
• • the volume of air inside the lungs is known as the functional residual capacity.
• If you exhale the volume left in your lungs, there is no airflow. Which means that there is no pressure gradient between the atmosphere and the lungs.
• In the resting position:

  • PW = PIP - Patm

  PW = -5 - 0

  PW = -5

  • PTP = PA - PIP

  PTP = 0 - (-5)

  PTP = 5

  • PRS = PA - Patm

  PRS = 0 - 0

  PRS = 0
• Cyclical Changes in Pressure

  • During inspiration, PIP decreases from -5 to -7.5 cmH2O, which drives PA down to -1 cmH2O.
  • Therefore, the atmospheric pressure is greater than the pressure inside the lungs.
• • During expiration, the volume of thoracic cavity decreases, which increases PIP from -7.5 to -5 cmH2O, and forces PA up to +1 cmH2O.
  • As the diaphragm compresses the thoracic cavity, the pressure inside the lungs increases. Therefore, the pressure inside the lungs is greater than the atmospheric pressure.