Salbutamol relaxes bronchial smooth muscle to widen airways

Steps:

• Asthma narrows air passages due to bronchoconstriction from smooth muscle tightening.
• Salbutamol is a beta-2 agonist that targets receptors in bronchial smooth muscle.
• It stimulates these receptors to increase cyclic AMP, leading to muscle relaxation.
• Relaxed muscles dilate bronchi and bronchioles, improving airflow to lungs.

Why D is correct:

• Salbutamol acts as a bronchodilator by binding to beta-2 adrenergic receptors, relaxing smooth muscle via the second messenger cyclic AMP, as defined in pharmacology.

Why the others are wrong:

• A: Salbutamol does not affect mucus secretion; it targets muscle tone, not glandular activity in alveoli.
• B: Alveolar surface area is unchanged; salbutamol acts on airways, not lung parenchyma.
• C: Elastic recoil aids expiration but is not salbutamol's action; it prevents constriction, not recoil.

Final answer: D