Kurzgesagt: We Fell for the Oldest Lie on the Internet [video]

Edit: GPT4o gave this answer when probed for the source:

The estimate that a human’s blood vessels could stretch to about 100,000 kilometers comes primarily from anatomical studies and scaling measurements of microvascular networks, particularly capillaries, which are vast in number and length. The specific figures are generally derived from a combination of cadaver studies, statistical modeling, and anatomical measurements of blood vessels at various scales in the body.

One influential source for this estimation includes physiological textbooks and studies such as Gray’s Anatomy and Textbook of Medical Physiology by Guyton and Hall, which both compile data from multiple studies across vascular biology. Additionally, scientific articles on the vascular network, especially those focused on microcirculation research, have informed these cumulative estimates by scaling up the capillary data across average human body sizes.