Surfactant in newborn lung. A section through part of an air sac (alveolus) in the lung of a newborn lamb viewed with a transmission electron microscope (TEM). At the bottom left is one of the cells which line the alveolus. The cell contains sacs of material which will go on to form surfactant, a substance that helps to expand the air spaces in the lungs to allow breathing, here seen top right. Pulmonary surfactant is a mixture of lipids and proteins which is secreted into the alveolar space by epithelial type II cells. The main function of surfactant is to lower the surface tension at the air/liquid interface within the alveoli of the lung.