There are two types of alveolar cells. The type I alveolar cells which lines the alveoli and much more abundant than type II alveolar cells. Then the type II alveolar cells lesser in number but functions differently than type I alveolar cells. Type II alveolar cells secrete surfactant (composed of lecitin and sphingomyelin) for the lungs. Surfactant reduces the surface tension inside the alveoli, preventing alveolar consolidation. This is important as babies with dysfunctional type II alveolar cells (usually preterm babies less than 32 weeks age of gestation) will have respiratory distress syndrome in the absence of surfactant.
The ocean around the globe gets heated up due to the heat around the sun. This hot water is then passed through to the colder regions of the globe by the method of convection. This is a continuous process.
Due to their function, nerve fibers which carry afferent impulses are known as afferent nerves or sensory nerves, and those nerve fibers which carry efferent impulses are known as efferent nerves or motor nerves.
