It is pointed out in the passage that, during an attack of asthma, ----.

Asthma may be defined as a malady
characterized by attacks of breathlessness due to
paroxysmal narrowing of the small bronchi and
bronchioles. Asthma may start at any age, but
most commonly in childhood. The typical attack
starts suddenly with breathlessness and
wheezing, the difficulty being mainly in expiration.
A small amount of viscid mucus is usually
expectorated towards the end of the attack.
Attacks may occur at any time, but especially
during the night or in the early morning. Their
duration is variable, some ending in an hour or so,
others, especially if complicated by bronchitis,
continuing for days. The frequency of the attacks
varies from one or two a year to several daily.
During attacks the lungs become over-distended
with air, since the obstruction to respiration is
greater in expiration than in inspiration and the
muscles of forced inspiration, which are brought
into action, are stronger than the expiratory
muscles.