Pulmonary Surfactant. The force of surface tension in the lungs is so great that without something to reduce the surface tension, the airways would collapse after exhalation, making re-inflation during inhalation much more difficult and less effective. Collapse of the lungs is called alectasis.
Rules for Using Semicolons. A semicolon is most commonly used to link (in a single sentence) two independent clauses that are closely related in thought. When a semicolon is used to join two or more ideas (parts) in a sentence, those ideas are then given equal position or rank. Some people write with a word processor; others write with a pen or pencil. Use a semicolon between two independent clauses that are connected by conjunctive adverbs or transitional phrases.
The good news about the semicolon is that it is relatively straightforward. It has only one use for joining clauses and it is simply this: both clauses have to be able to stand alone as independent sentences, as if they had a period between them. Your second clause does not pass this test.