Pulmonary Mechanics I

Question 1

what kind of pump is the lung and what does this mean in terms of inflation and deflation?

Answer 1

negative pressure pump

• the lung inflates due to a drop in intrapleural pressure outside the lung, but inside the chest wall
• the lung deflates from the passive elastic recoil of the lung (increase in intrapleural pressure)

Question 2

what is the intrapleural pressure if respiratory muscles are relaxed?

Answer 2

pressure in the intrapleural space is less than atmospheric pressure, thus said to be -5 cm H2O

Question 3

what happens to intrapleural pressure in pneumothorax

Answer 3

intrapleural pressure rises up to atmospheric pressure, so the lung collapses

Question 4

eupnea (normal ventilation value)

Answer 4

normal (quiet) breathing (7.5 L/min)

-inspiration active (only diaphragm), but expiration passive (recoil of lung and chest wall w/o muscles)

Question 5

muscles of inspiration

Answer 5

diaphragm (primary muscle)
external intercostals (lifts ribs up and forward, increases anterior-posterior dimension of chest cavity to add tone to chest wall)
accessory (sternomastoid lifts sternum, scalene lifts upper ribs, etc.)

Question 6

muscles of expiration

Answer 6

internal intercostals (lower ribs)
4 abdominal muscles (depresses lower ribs, compresses abdominal cavity, pushes up diaphragm and other chest muscles)
only used in strenuous exercise (active breathing)

Question 7

hyperpnea (and normal ventilation value)

Answer 7

deeper and faster breathing than normal (like in exercise, up to 120 L/min)

• inspiration and expiration are both active
• inspiration uses external intercostals
• expiration uses internal intercostals

Question 8

tachypnea

Answer 8

more rapid breathing than normal (hyperventilation)

Question 9

hyperventilation and what it causes

Answer 9

breathing faster than required for O2ation leads to alveolar hypocapnia (too little CO2 in blood)
-respiratory alkalosis (increases pH b/c decreased H+)

Question 10

hypoventilation and what it causes

Answer 10

happens in muscular dystrophy or respiratory muscle paralysis, and it leads to alveolar hypoxia and hypercapnea
-respiratory acidemia (H+ high in blood, so low pH)

Question 11

lung compliance in terms of pressure

Answer 11

Cl = dV/dPl

Question 12

chest wall compliance in terms of pressure

Answer 12

Cc = dV/dPc

Question 13

total compliance in terms of pressure

Answer 13

consists of 2 compliances in series (lung and chest wall)

Ct = dV/dPt = dV/dPalv

Question 14

what does contraction of the diaphargm do on the intrapleural space?

Answer 14

it exerts an expansive force (increase volume), decreasing the intrapleural pressure, making it more negative, and acting to inflate the lung

Question 15

when is external pressure not equal to atmospheric pressure?

Answer 15

if a weight is placed on the chest, then external pressure > atm, and is called the body surface pressure

Question 16

what are the transmural pressures, and what do they set?

Answer 16

1. chest wall pressure = interpleural pressure - atmospheric pressure (sets degree of inflation of chest wall)
2. lung pressure = alveolar pressure - interpleural pressure (sets degree of inflation of lung)
3. total transmural pressure = relaxation pressure = alveolar pressure - atmospheric pressure = transmural chest pressure + transmural lung pressure (since in series; sets degree of inflation of entire system of bag w/in a bag)

Question 17

what happens if all respiratory muscles are relaxed? what things are balanced?

Answer 17

the system is at mechanical equilibrium

• total transmural rpessure = 0
• transmural lung pressure = - transmural chest pressure
• positive outward lung pressure is balanced by inward passive elastic recoil pressure of lung, and negative inward chest wall pressure is balanced by passive outward elastic recoil pressure of chest

Question 18

how is pulmonary compliance measured with a spirometer? (in terms of ressure and volume)

Answer 18

patient inspires and holds breath with glottis open, then relaxes chest muscles against weighted spirometer
-since lung PV = spirometer PV, when lung V increases, the interpleural P decreases, so spirometer V decreases and the P of spirometer (equated to alveoli) increases

the respiratory muscles must be relaxed, since it’s passive elastic recoil pressure we want to measure

Question 19

static compliance

Answer 19

determines what particular volume the lung and chest wall will assume for a given transmural pressure when the elastic vessels are at mechanical equilibrium with no air moving

Question 20

how do you normalize compliance for lung volume?

Answer 20

specific compliance (Csp) = dV/(V * dP)

Question 21

what happens in barrel-chest coal miners?

Answer 21

decreased total compliance (Ct)

• could be due to properties of lung or chest wall (Cl or Cc) so need to measure each separately
• fibrosis of lung, chest wall physique

Question 22

static compliance curves for lung, chest wall, and total system (y axis = vital capacity, x axis = pressure)

Answer 22

the curves are all nonlinear, so the compliance (slope) depends on degree of inflation, so is usually measured at FRC
-from left to right is Pc - Pl - Pt if P > 0 (and past end inspiratory volume), but Pc - Pt if P < 0

Question 23

functional residual capacity (FRC)

Answer 23

lung volume when Pt = 0, at end-expiratory position

-occurs at about 36% vital capacity, when Pc = - Pl

Question 24

what happens during forced expiration (at volumes below FRC?)

Answer 24

the expansive force of the chest wall provides dominant driving force for return of lung to FRC

Question 25

what happens to Pc if lung is resting or expanded?

Answer 25

resting: Pc = 0, only elastic force of lung opposes inspiration
expanding: Pc > 0, and elastic recoil of lungs AND chest provide passive driving force for expiration

Question 26

lung compliance in fibrotic lung disease VS emphysema

Answer 26

fibrosis: compliance decreases (dV decreases), so it’s harder to inhale
emphysema: compliance increases (dP, or recoil pressure decreases) so it’s harder to exhale

Question 27

pneumoconioses

Answer 27

related lung diseases induced by inhalation of dust asbestos, coal, silica, etc.

• form granulomatous and fibrous tissue in lungs, decreasing compliance (as in fibrosis) to cause “stiffer” lung
• inspiration hard, work of breathing increased

Question 28

what happens to FRC and TLC if compliance of lung increases?

Answer 28

they will both increase too (positive relationship)

Question 29

how does smoke cause emphysema?

Answer 29

neutrophils accumulate in lung to remove inhaled smoke particles, and release proteases

• alpha-antitrypsin in plasma usually inhibits proteases to protect lung, but smoke inhibits aa1
• lung CT is digested by proteases, which increases Cl

Question 30

effect of surfactant on lung compliance

• what happens if saline-filled lung?
• what happens if air-filled with surfactant?
• what happens if air-filled w/o surfactant?

Answer 30

compliance increases by decreasing surface tension

• increased compliance b/c surface tension eliminated
• normal compliance
• decreased compliance (very high surface tension)

Question 31

stabilization of lung by surfactant (in terms of bubbles)

Answer 31

since P = 2t/r, if 2 bubbles have the same tension, the samller bubble with have a larger internal pressure

• w/o surfactant, smaller alveoli would empty into the larger alveoli due to the surface tension
• with surfactant, the larger alveolus has pressure equal to smaller alveolus, and the two unequal alveoli are stable

Question 32

what does surface tension do when radius increases?

Answer 32

they both increase, b./c a larger area would dilute surfactant, causing increased surface tension and pressure

Question 33

how is pulmonary stability maintained?

Answer 33

elastic fibers, connective tissue, and surfactant, although surfactant is the main one, or else the lung would have collapsed