The diaphragm contracts during the phase of respiration called:

A. Bicarbonate is a buffer in the blood that helps maintain pH balance, but it is not a gas and does not directly regulate breathing.
B. Carbon dioxide (CO₂) is the primary regulator of respiration. Elevated CO₂ levels in the blood lower pH (making it more acidic), which stimulates chemoreceptors to increase the rate and depth of breathing to expel more CO₂ and restore balance.
C. Nitrogen makes up the majority of the air we breathe, but it is inert and plays no active role in regulating respiration.
D. Oxygen does influence breathing, especially at very low levels, but it is not the primary driver. In healthy individuals, CO₂ levels are the main factor controlling respiratory rate.

A. Asthma is commonly associated with allergic responses, especially allergic asthma, which is triggered by allergens like pollen, dust mites, or pet dander.
B. Allergic rhinitis is an allergic reaction that affects the nasal passages, typically caused by airborne allergens.
C. Hay fever is another term for seasonal allergic rhinitis, directly linked to allergens such as pollen.
D. Acute coryza, also known as the common cold, is caused by a viral infection (most commonly rhinovirus), not an allergic reaction. While symptoms may be similar to allergic rhinitis, the underlying cause is infectious, not immunologic hypersensitivity.