Mechanism of Breathing MCQ - Practice Questions with Answers

• Breathing involves two stages:

• inspiration during which atmospheric air is drawn in
• expiration by which the alveolar air is released out

• Two main mechanisms takes place to carry out Breathing 

  1) Pressure gradient 

• The movement of air into and out of the lungs is carried out by creating a pressure gradient (difference in pressure) between the lungs and the atmosphere.
• Human lungs are negative pressure lungs.
• Inspiration can occur if the pressure within the lungs (intrapulmonary pressure) is less than the atmospheric pressure, i.e., there is a negative pressure in the lungs with respect to atmospheric pressure. 
• Expiration occurs when pressure within the lungs is more than the atmospheric pressure.

• 2) Boyle's Law 

• Breathing is due to Boyle's Law
• This law states that when temperature is constant, Pressure and Volume are inversely proportional to each other and this results in movement of air

• INSPIRATION

  Muscles of Inspiration:

• Core muscles: 
  • External intercostal muscles: These contract to elevate the ribs
  • Diaphragm: It contracts to expand the thoracic cavity
• Accessory muscles:
  • Sternocleidomastoid: It contracts to elevate the sternum
  • Pectoralis minor: It contracts to pull ribs outwards

• Mechanism of Inspiration :

• The diaphragm and a specialised set of muscles – external and internal intercostals between the ribs, help in generation of such gradients.
• Inspiration is initiated by the contraction of diaphragm which increases the volume of the thoracic chamber in the anteroposterior axis. 
• The contraction of external intercostal muscles lifts up the ribs and the sternum causing an increase in the volume of the thoracic chamber in the dorso-ventral axis.
• The overall increase in the thoracic volume causes a similar increase in pulmonary volume.
• An increase in pulmonary volume decreases the intrapulmonary pressure to less than the atmospheric pressure which forces the air from outside to move into the lungs, i.e., inspiration. 

• Mechanism of Breathing: Expiration

• Expiration takes place when the intrapulmonary pressure is higher than the atmospheric pressure.
• Relaxation of the diaphragm and the intercostal muscles returns the diaphragm and sternum to their normal positions and reduce the thoracic volume and thereby the pulmonary volume.
• This leads to an increase in intrapulmonary pressure to slightly above the atmospheric pressure causing the expulsion of air from the lungs, i.e., expiration
• We have the ability to increase the strength of inspiration and expiration with the help of additional muscles in the abdomen. 
• On average, a healthy human breathes 12-16 times/minute. 
• The volume of air involved in breathing movements can be estimated by using a spirometer which helps in the clinical assessment of pulmonary functions.

Muscles of Expiration:

Core muscles:

• Internal and External  intercostal muscles: These contract to pull the ribs down.
• Diaphragm: It relaxes to reduce the thoracic cavity.

Accessory muscles:

• Abdominals: These contract to compress the abdomen.
• Quadratus lumborum: These contract to pull the ribs down.

MECHANISM