Which of the following best explains the role of surfactant in the lungs?

A. It helps in the transportation of oxygen across the alveolar membrane: Oxygen transport across the alveolar membrane depends on diffusion driven by concentration gradients, not directly on surfactant.

B. It increases the rate of oxygen absorption in the lungs: Surfactant does not directly increase oxygen absorption; it facilitates gas exchange indirectly by maintaining alveolar stability.

C. It reduces surface tension to prevent alveolar collapse: Surfactant is a lipid-protein substance that lines alveoli, lowering surface tension. This prevents alveolar collapse during exhalation and allows easier expansion during inhalation, improving overall lung function.

D. It supports the structural integrity of the bronchial walls: Bronchial walls are supported by cartilage and smooth muscle, not surfactant, which primarily functions within the alveoli.

A. Thyroid hormone: Thyroid hormone regulates metabolism, growth, and development but has only a minor role in calcium homeostasis compared to parathyroid hormone and calcitonin.

B. Insulin: Insulin controls blood glucose by facilitating cellular uptake of glucose and promoting glycogen synthesis. It does not significantly affect calcium levels in the blood.

C. Adrenaline: Adrenaline, produced by the adrenal medulla, mediates the fight-or-flight response, increasing heart rate and blood pressure, without direct regulation of calcium levels.

D. Parathyroid hormone: Parathyroid hormone, secreted by the parathyroid glands, increases blood calcium levels by stimulating calcium release from bones, enhancing calcium absorption in the intestines, and reducing calcium excretion by the kidneys.