What is a common feature of smooth muscle in the respiratory system?

Smooth muscle in the respiratory system is characterized by being non-striated and involuntary. This type of muscle does not have the striations seen in skeletal muscle and is under the control of the autonomic nervous system rather than voluntary control, which means it functions automatically without conscious effort.

Smooth muscle plays a vital role in regulating the diameter of airways, particularly in smaller passages such as bronchioles. This regulation is essential for maintaining adequate airflow and responding to various stimuli, such as the presence of irritants or allergens. The ability to contract and relax helps to manage resistance in the airways, affecting airflow and gas exchange in the lungs.

Other options reference features that do not describe smooth muscle accurately for the respiratory context. For example, smooth muscle is not arranged in parallel fibers like skeletal muscle; instead, it is typically arranged in layers that can contract and relax as needed. Similarly, it is not found exclusively in the bronchioles, as smooth muscle can also be present in other areas of the respiratory tract, such as the trachea and larger bronchi.