Which of the following describes an example of a closed system?

A closed system is one in which there is no exchange of matter with the surroundings, allowing for controlled processes to take place. In the context of blood analysis, when blood is pipetted into a cassette for analysis, it is typically sealed and isolated from external contaminants. This containment enables precise manipulation and testing without interference from the external environment, making it a prime example of a closed system.

By contrast, blood cultured in a petri dish is an open system where the sample is exposed to the external environment, which can lead to contamination and variable outcomes. Manual processing of blood samples often involves interactions with the atmosphere, such as opening tubes or containers, exposing them to potential contaminants. Using a centrifuge to separate blood components, while it does isolate components efficiently, still entails exposure to the environment during the setup and processing stages. Thus, the characteristics of a closed system are best illustrated by the use of a cassette, where the blood sample remains enclosed and protected from external variables.