Final answer:
Airway resistance is decreased by chemicals that relax the muscles around the airways such as norepinephrine and epinephrine, which are both treatments used for anaphylactic shock where they help in improving breathing by reducing resistance.
Step-by-step explanation:
Airway resistance is decreased by substances that can relax the muscles surrounding the airways, leading to dilation. Norepinephrine and epinephrine are both chemicals that can cause this effect. This is why they are used as treatments in conditions like anaphylactic shock, where reducing airway resistance is crucial to improve breathing.
For example, epinephrine is the drug of choice in anaphylactic shock situations due to its ability to quickly decrease airway resistance alongside other vital functions like increasing blood pressure. Histamine, on the other hand, is known to increase airway resistance by causing, not reducing, inflammation during allergic reactions which leads to constriction of the airways. Elevated carbon dioxide levels in the blood can indicate the need for deeper or faster breathing, but do not directly lead to a decrease in airway resistance. Therefore, histamine does not decrease resistance, and elevated carbon dioxide is not a primary factor in the direct reduction of airway resistance.
In regards to the choices provided for the question, the correct answer is b) norepinephrine and c) epinephrine.