Once you have selected a body system, follow these steps to narrow your topic:
1. Break down your body system by thinking about parts of that body system.
For example: Parts of the Respiratory System include: lungs, larynx, throat, trachea etc.
2. Then, list diseases related to the body system:
Example: COPD, Bronchitis, Asthma, Emphysema etc.
3. Finally, narrow down your topic to what you are most interested in. (pick a topic)
This narrowed topic may be the only keyword(s) you need for your search. Example - Asthma.
Or before you start searching - think of synonyms or other names for what you chose. These may help you if need to try additional keywords searches to find a good article on your topic.