Final answer:
The human body needs more than its skin as a barrier to infecting pathogens for two reasons: pathogens can enter the body through openings and mucous membranes, and skin alone does not provide complete protection.
Step-by-step explanation:
The human body needs more than its skin to function as a barrier to infecting pathogens because pathogens can enter the body through openings and mucous membranes that are not covered by skin. Skin alone does not provide complete protection, and it acts only as a chemical barrier against pathogens. The body also needs physical barriers to prevent various types of infection.
Regions of the body that are not protected by skin, such as the eyes and mucous membranes, have alternative methods of defense. Tears and mucus secretions trap and rinse away pathogens, and cilia in the nasal passages and respiratory tract push the mucus with the pathogens out of the body. Other defenses throughout the body include the low pH of the stomach, blood proteins that disrupt bacterial cell membranes, and the process of urination that flushes pathogens from the urinary tract.