Resp Mechanics

Question 1

Describe the main and accessory muscles of inspiration

Answer 1

Diaphragm via phrenic nerves C3-5, abdominal contents move down, increase length and transverse diameter of chest cavity
External intercostals - ribs move up and forwards
Accessory: scalene (elevate 2 ribs) sternomastoids (elevate sternum)
Minor: alae nasi (flare nostrils), small muscles of neck and back

Question 2

Describe the muscles of expiration

Answer 2

Passive in quiet breathing - lungs + chest wall elastic

Abdominal wall muscles (all 4) contract = increase intra-abdominal pressure = pushes diaphragm upward
Internal intercostals - ribs down and inwards
Neck and back muscles

Question 3

Describe the shape of a normal pressure-volume curve of the lung

Answer 3

Non-linear - steepest in the middle = most compliant
Less change in volume/pressure at extremities
Hysteresis - harder to open a previously closed airway
Note - minimal residual volume in lung

Question 4

Compliance is reduced by (4) and increases with (2)

Answer 4

Reductions:

1. Pulmonary fibrosis
2. Interstitial fluid
3. Local collapse of alveoli
4. High surface tension

Increased:

1. Aging
2. Emphysema

Question 5

Describe the components of surfactant and which pneumocyte secretes it

Answer 5

Alveolar type 2 cell: (lamellar body appearance)

Mainly (62%) DPPC dipalmitoylphosphotidylcholine - charged Phos and Choline heads
Surfactant proteins (8%)
SP-A and SP-D large proteins - water soluble and innate immune fx
SP-B and SP-C lipid soluble speeds formation of monolayer

SP-A and SP-B forms tubular myelin = meshwork to keep surfactant in an even layer

Question 6

Describe 3 advantages of surfactant in the lung

Answer 6

1. Lower surface tension = increase compliance = decrease WoB
2. Stabilise alveoli - reduce tendency of small alveoli to empty into big ones
3. Keep alveoli dry - otherwise transudate comes out of capillary

Question 7

Small alveoli tend to collapse into larger ones - name the law of LaPlace

Answer 7

Pressure = 2 x Surface tension/radius

Alveoli (except at periphery) stabilised by interdependence. If one shrinks, others oppose

Question 8

Give the formulas for lung + chest wall pressure and compliance

Answer 8

Ptotal = Plung + Pchestwall

1/Ctotal = 1/Clung + 1/Cchestwall

Typically, Ctotal = 0.1 L/cm water

Question 9

Give the formula for airway resistance, where its mainly regulated and factors impacting (5)

Answer 9

R = 8nl/pi x r^4

Mainly, small bronchioles with lots of sm muscle/less cartilage

1. Lung volume
2. Bronchoconstriction
3. Secretions/inflammation
4. Density of gas
5. Dynamic compression

Question 10

Typical pressure oscillations in

1. Alveolar pressure
2. Intrapleural pressure
3. Transpulmonary pressure

Answer 10

1. Alveolar -1 to +1
2. Intrapleural -5 to -8
3. Transpulmonary = Palv - Pip