Final answer:
A decrease in airway radius increases resistance, making it difficult to breathe out because expiration is a passive process that relies on the natural recoil of lungs and does not involve active muscle contraction like inspiration does. Surface tension in the alveoli, exacerbated by the narrowed airways, further hinders the passive outflow of air.
Step-by-step explanation:
During breathing, changes in airway resistance can affect both inspiration and expiration. A decrease in airway radius leads to difficulty in breathing out because of increased resistance. When the radius of the airway decreases, the resistance inside the airway increases, making it harder for air to flow through during exhalation.
The process of normal expiration is passive, which relies on the recoil of the lung tissue and the relaxation of the diaphragm and intercostal muscles. In contrast, inspiration is an active process requiring muscle contraction to create a negative pressure that draws air in.
The resistance during exhalation becomes much more significant as the air is not actively pushed out but has to flow past the narrowed airways due to their reduced diameter.
Furthermore, the principle described by the formula F = ΔP/R (where F is the flow rate, ΔP is the pressure difference, and R is the resistance) explains that as the resistance increases (due to a smaller radius of the airway), the flow rate of the exhaled air decreases, thus requiring more effort to breathe out.
The surface tension within the alveoli also plays a role during exhalation. If the airways are narrowed, the surface tension can further impede the outflow of air, making it more difficult to empty the lungs fully without active muscular effort, which can often be compromised in various lung conditions.