Lecture 24/25: Mechanical Properties of the Lung

Question 1

What is the driving force for convection in the conducting airways?

Answer 1

Difference between barometric and alveolar pressure

Question 2

Describe the muscles used for quiet/forced inspiration & expiration

Answer 2

quiet inspiration: diaphragm, external intercostals forced inspiration: also scalene, sternocleidomastoid, upper respiratory tract muscles quiet expiration: passive forced expiration: internal intercostals, rectus abdominus, external oblique muscle

Question 3

Describe how lung compliance is created

Answer 3

Due to the elastin and collagen fibers in the parenchyma, tissue around the alveoli. Creates elastic recoil

Question 4

Describe the hysteresis loop of inspiration and expiration

Answer 4

takes much more pressure to open airways during inhalation than exhalation because of pressure needed to combat surface tension and elastic recoil

Question 5

How does emphysema and fibrosis affect compliance?

Answer 5

emphysema: increased compliance due to loss of elastin fibers. increased effort needed to exhale b/c of less recoil fibrosis: decreased compliance because of increased collagen deposition. inspiration more difficult

Question 6

What is the formula of LaPlace’s Law?

Answer 6

P=2T/r

Question 7

How do alveoli prevent collapse (2 methods). what is surfactant secreted as?

Answer 7

1) mechanical tethering 2) surfactant reduces surface tension - reduces water density at air-water interface, as alveoli expands, surfactant density decreases, so surface tension increases, which acts as a natural negative feedback loop. this insures alveoli expand at same rate secreted as lamellar bodies then forms tubular myelin. surfactant increases SA, keeps alveoli moist

Question 8

Describe the changes in volume, transpulmonary/interpleural/alveolar pressure, and flow

Answer 8

Question 9

What does transpulmonary pressure indicate? What does alveolar pressure control? How does interpleural pressure change?

Answer 9

P_TA = P_A + P_IP

transpulmonary pressure always positive, equal and opposite to elastic recoil, determined by lung volume

alveolar pressure controls flow

intrapleural pressure changed by change in thoracic lung volume

Question 10

Describe the work done in breathing

Question 11

What is Poiseuille’s Law? What is Reynold’s number?

Answer 11

Poiseuille’s Law states that

Airway resistance = 8*viscosity*length/pi*r^4

Reynold’s number is a descriptor of either laminar (2000-3000) or turbulent (>3000) flow

Question 12

List the 7 factors affecting airway resistance?

Answer 12

1) Lung volume (decreased lung volume = increased resistance)
2) Mucus - decreases radius
3) Edema - crushes airways
4) Density - increase turbulence
5) Smooth muscle contraction: Parasymp acts via Ach on M2 receptors, uses Gi pathway, Symp acts via NE on beta-2 and Gs pathway. PKA phosphorylates phospholamban which disinhibits SERCA so greater uptake of Ca = vasodilation. Histamine binds via H1 receptor and acts via Gq pathway for vasoconstriction
6) local effects of CO2 - receptors in upper airway leads to increased ventilation, decreased resistance
7) Cold and hypoxia