Breathing properly requires the presence of what compound that affects surface tension of alveoli in the lungs?

The presence of surfactant is crucial for proper breathing as it significantly impacts the surface tension of the alveoli in the lungs. Surfactant is a complex mixture of lipids and proteins produced by the type II alveolar cells. Its primary role is to reduce the surface tension at the air-liquid interface in the alveoli, which prevents the alveoli from collapsing during exhalation. By lowering surface tension, surfactant enhances lung compliance, making it easier for the lungs to expand during inhalation. This is particularly important because the alveoli are very small and would naturally tend to collapse without the presence of surfactant.

In contrast, potassium, plasma, and sodium chloride do not play a direct role in affecting surface tension in the alveoli. While they have important physiological functions, they do not specifically contribute to the mechanics of breathing in the way that surfactant does. Thus, surfactant is essential for maintaining the stability of the alveoli and facilitating efficient gas exchange, making it the correct answer.