Respiratory System Mechanics

Airway resistance

R_airway = P_alv– P_m / Flow

P_m = pressure at the mouth
P_alv = pressure at the alveoli

The airways are a system of dicotomous branching tubes. The combined caliber of the daughter tubes becomes increasingly larger as the airway continues to branch.

~ 40% of airway resistance is in the extra-thoracic airways
~ 40% of airway resistance is in the large inter-thoracic airways
~ 20% of airway resistance is in the small inter-thoracic airways (< 2 mm, 8^th generation out of 22-23 airway generations)
* airways are elastic structures therefore are capable of being compressed or expanded. The stronger the structure the smaller the ability to change. (larger airways have more cartilage therefore less ability to change structure)

Chronic bronchitis – hyperplasia of mucous glands in the large airways ß caliber. Inflammation thickens the airways.

Lung Volume – Determined by Transpulmonary Pressure

Transpulmonary pressure: PL = P_alv – P_Pl

Extraluminal airway pressure ~ P_Pl.
The caliber of the airways is a function of the pressure gradient between in and out. A large transpulmonary pressure will tether the airway open.

P_Pl The elasticity of the lung has a tethering effect on the airways

P_alvIn Emphysema the "springs" are less stiff so the airways tend to narrow even though there is no intrinsic defect in the airways themselves.

Transpulmonary pressure is an important factor in tethering the airways open

Things that decrease large airway caliber (epiglottitis, tumors, bronchitis) Increase airway resistance

Maximum Expiratory Flow Volume Curve – fill lungs and blow out as hard as you can.

Pulmonary Fibrosis

V_maxis less because the TLC is less
V_max50is also less.
Even without airway disease flow rates are decreased because of the relationship between the airways and the elastance of the lungs.

Emphysema – often associated with airway disease

V_max is reduced (because of narrowing of the airways)
V_max50is also reduced

Obstructions

Abnormality of the distal airways, higher flow preserved, reduced flow at smaller lung volumes
Proximal obstruction, higher flow affected. The more proximal the obstruction the higher the flow affected.

Volume changes as a Function of Time

Flow is equal to the tangent of the curve F=Vol/Time

FEV₁ = the change in volume during the 1^st second of forced expiration. (~ V_max)

FEV_25-75 = the change in volume over the time it takes to get from 25% to 75% of FVC (~ V_{max 50})

	Obstruction	Pulmonary fibrosis
FVC	ß	ß
FEV1	ß ß ß	ß
FEV1/FVC	ß ß ß	Normal ratio