Final answer:
The pH of the blood is NOT a chemical barrier that helps prevent infections, unlike urine, mucus, and tears, which act as chemical barriers in the innate immune response. Chemical barriers work by creating environments that are hostile to microbes and by containing enzymes killing pathogens.
Step-by-step explanation:
The question "Which of the following is NOT a chemical barrier that helps prevent infections?" refers to the body's innate immune response mechanisms. Among the options provided: urine, mucus, and tears are indeed chemical barriers that help prevent infections. However, the pH of the blood is not typically considered a chemical barrier in this context as it does not act directly in destroying pathogens on the body's outer surfaces or at body openings. The human body utilizes chemical barriers as part of the innate immune response in various ways. For instance, sweat, mucus, tears and saliva contain enzymes that kill pathogens.
Urine and vaginal secretions maintain an acidic environment that is hostile to many microbes. Skin secretes acids that keep the surface slightly acidic, which is unfavorable to pathogen survival. In the gastrointestinal tract stomach acid works as a chemical barrier by killing pathogens in food or water. These barriers are all integral to the body's first line of defense against infection.