Final answer:
Bronchodilators are drugs that relax the smooth muscles of the bronchial passages by stimulating the beta-2 adrenergic receptors, leading to an increase in airway size and improved airflow, particularly useful in treating asthma.
Step-by-step explanation:
A drug that relaxes the smooth muscles of the bronchial passages and is often used to treat conditions like asthma is known as a bronchodilator. These drugs work by stimulating the beta-2 adrenergic receptors, which induce bronchodilation, the increase in the size of the bronchiole due to relaxation of the muscular wall. The activation of these receptors can lead to the relaxation of the smooth muscles in the bronchi and bronchioles, consequently opening up the airways and improving airflow.
In response to the activation of the sympathetic nervous system, certain neurotransmitters are released that increase heart rate, heart contractility, and stimulate bronchodilation. This results in the dilation of the bronchi of the lungs, thereby increasing air volume. Medications that mimic this effect can be highly beneficial for patients with respiratory conditions that involve bronchoconstriction, such as asthma.
The nervous innervation of the bronchi also plays a significant role in the process of regulating airway diameter. The sympathetic nervous system promotes bronchodilation, while the parasympathetic nervous system causes bronchoconstriction. Drugs that enhance the sympathetic activity or inhibit the parasympathetic activity can therefore act as bronchodilators and assist in relaxing bronchial smooth muscles.