Mechanics of Breathing

Question 1

Outline elastic recoil properties of 2 compartments of resp. system. specify relationship btw elastic recoil pressure and transmural pressure

Answer 1

LUNGS AND CHEST WALL

• at FRC, outward recoil of chest is equal and opposite to inward recoil of lungs
• pressure measured in cm H2O
• if glottis open at end of breath, open system to atm. so pressure in airways is 0. pressure in intrapleural cavity is 5 cm less (-5cm H2O)

Question 2

describe changes in alveolar, intra-pleural, transmural pressures, airflow, lung volume during quiet breath (and units)

Answer 2

transmural pressure= pressure across wall (inside - out)
at FRC, transpulmonary pressure (Ptp) keeps lungs open against their inward recoil tendency.. Ptp = 0–5= 5 cm H2O
at FRC, chest transmural pressure (Ptw)= -5-0= -5 cm H2O

At FRC, Ptw and Ptp are equal and opposite.
Respiratory system is at rest with zero transmural pressure (Prs= Pa-Pb=0)

Question 3

describe sequential order of changes in respiratory muscle contraction, thoracic lung volume, intra-thoracic pressure, airflow during inspiration vs expiration

Answer 3

Inspiration (volume change precedes pressure)
1. inspiratory muscles contract
2. chest wall expands, intra-thoracic pressures decrease (Ppl by 3 cm, PA by 1cm)
leads to increase in transpulmonary/distending pressure of lungs (Pa-pl)
3. air flows into lungs until Pa=PB

to inspire, we create pressure less than atm in airway,

expiration:

1. inspiratory muscles stop contracting
2. lungs recoil inward (reduce thoracic volume, increase intra-thoracic pressures) Ppl and Pa increase. transpulmonary pressure decreases
3. air flows out until Pa=Pb
   * * quiet expiration is passive. lung recoil increases alveolar pressure above atmospheric pressure driving air out.

Question 4

describe role respiratory muscles in changing thoracic dimensions during quiet and forceful breath

Answer 4

inspiration is active. muscles include diaphragm external intercostal, sternocleidomastoid, scaleneus

external muscles for active exp: internal intercostal, abdominal

Question 5

describe motor innervation respiratory muscles, predict impact of spinal cord injury at different levels.

Answer 5

Quiet inspiration: cervical spinal roots- C3,4,5. 3 CN give rise to phrenic nerve supplying diaphragm
scalene: C2-C7
sternomastoid: C2-c3, accessory CN
intercostal: T1-T11
abdominal muscles: T7-L1
quiet expiration: passive due to inward recoil of lung