Which of the following is NOT an organ system in the human body?

The vascular system is not considered a distinct organ system in the same way as the endocrine, respiratory, or skeletal systems. Instead, it is more accurately viewed as a subset of the circulatory system, which encompasses both the heart and blood vessels. The circulatory system is responsible for the transport of blood, nutrients, gases, and other substances throughout the body.

In contrast, the endocrine system involves glands that produce hormones for regulating various bodily functions, the respiratory system is responsible for gas exchange, and the skeletal system provides structure and support to the body. Each of these three systems is comprised of specific organs that work together to perform crucial life functions, whereas the vascular system's role is more integrated into the larger circulatory framework. Therefore, it is categorized differently than the other systems listed.