What substance reduces surface tension in the fluid lining the alveoli?

Surfactant is a substance produced in the lungs that plays a vital role in maintaining proper respiratory function. It is a mixture of lipids and proteins, primarily secreted by type II alveolar cells. The primary function of surfactant is to reduce surface tension in the fluid lining the alveoli, the tiny air sacs where gas exchange occurs. By lowering surface tension, surfactant helps to prevent the alveoli from collapsing during exhalation, ensuring that they remain open and available for the next inhalation. This is crucial for efficient gas exchange, as collapsed alveoli would hinder the ability of oxygen to enter the blood and carbon dioxide to be expelled.

In contrast, substances like albumin, mucus, and interstitial fluid do not serve the specific function of reducing surface tension in the alveoli. Albumin is primarily a plasma protein involved in maintaining osmotic pressure within blood vessels. Mucus serves to trap particles and pathogens in the respiratory tract but does not directly affect surface tension in the alveoli. Interstitial fluid, which fills the spaces between cells, also does not have a function related to surface tension in the alveoli. Thus, surfactant is the substance specifically designed to address the challenge of surface tension in the lungs