Final answer:
The three primary sources of friction in airway resistance are the diameter of the airways, surface tension within the alveoli, and lung compliance. Surface tension is reduced by pulmonary surfactant, whereas lung compliance is related to the elasticity of the lung tissue.
Step-by-step explanation:
The main sources of friction in airway resistance include the diameter of the airways, the surface tension of the alveoli, and the compliance of the lung tissue. A smaller diameter in the airways results in more collisions of air molecules with the walls, increasing resistance.
Surface tension within the alveoli, caused by the water present in the lining also contributes to resistance as it opposes the expansion of the alveoli, although this is mitigated by pulmonary surfactant which reduces surface tension. Finally lung compliance the ability of the lungs to stretch, has an inverse relationship with resistance - the more compliant the lungs, the less resistance there is to air flow.