Which hormone decreases threshold to carbon dioxide and causes bronchi to dilate?

• A. Estrogen
• B. hCG
• C. Progesterone
• D. Relaxin

Answer: (C)

Progesterone acts as a respiratory stimulant and bronchodilator. It lowers the central chemoreceptor threshold for carbon dioxide, so the respiratory center increases ventilation more readily in response to CO2. This means you’ll see an elevated ventilatory drive and lower arterial CO2 levels. At the same time, progesterone relaxes bronchial smooth muscle, leading to bronchodilation and easier airflow. This combination helps meet the higher metabolic demands of pregnancy. The other hormones don’t produce these respiratory changes: estrogen tends to affect mucous membranes and fluids more than central CO2 sensitivity or bronchodilation, hCG’s main role is supporting early pregnancy with no major effect on respiration, and relaxin primarily relaxes pelvic ligaments with less influence on bronchial tone.