Maximum amount of smooth muscle, in relation to wall thickness, is present in:

The trachea and upper respiratory passages (first 7 generations) have cailage & glands in their walls but little smooth muscle. They are lined by ciliated epithelium. Intermediate-sized and small-sized bronchi and bronchioles do not contain cailage. Cilia are present till respiratory bronchioles Terminal bronchioles have maximum amount of smooth muscle.