How Long are our Blood Vessels?

How long are our blood vessels? This intriguing question has captured the attention of both scientists and the public. The claim that if all the blood vessels in a human body were laid end to end, they would stretch approximately 100,000 kilometers (about 60,000 miles)—enough to circle the Earth more than twice—has circulated widely. However, inconsistencies in various articles prompted Kurzgesagt to investigate the validity of this claim more thoroughly.

On October 29, 2024, Kurzgesagt released a video stating that while they had presented this assertion in numerous videos, their viewers desired more substantiated evidence. As they began their research, they noted discrepancies in the literature regarding the types of blood vessels referenced and the calculations behind the figure. Different sources attributed the measurement to varying categories of blood vessels—some referring to arteries, while others focused on capillaries or blood vessels in general.

To delve deeper, Kurzgesagt utilized the PubMed database, where they initially struggled to find relevant scientific papers despite employing various keywords. After several attempts, they finally discovered two significant biology books: Vital Circuits and Looking at the Body (By. Dr. Suzuki).

Dr. Suzuki, the author of Looking at the Body, who had a PhD in Zoology and had written 50+ books, was contacted by Kurzgesagt for clarification about the claim. He replied that he had no recollection of any references regarding blood vessel length. This left Kurzgesagt seeking further information. In their effort, they found a citation in Vital Circuits that claimed a total length of blood vessels at 100,000 kilometers. Unfortunately, the reference provided was not accessible.

In a fortunate turn of events, Kurzgesagt decided to read the book backward, and this unconventional approach led them to a critical reference: an article published in January 1959 in Scientific American by Zweifach B. W. This article reiterated that the total length of capillaries in the body was nearly 60,000 miles. However, this finding still did not pinpoint the original source of the claim.

The investigation persisted and ultimately uncovered a reference in the 1929 book The Anatomy and Physiology of Capillaries, written by August Krogh, a Nobel Prize-winning scientist for medicine. This work detailed calculations based on a hypothetical scenario involving a man weighing 50 kilograms and having an extraordinarily high density of capillaries, positing that the total length of all these capillaries would amount to approximately 100,000 kilometers.

However, Krogh's assumptions were flawed, as he based his calculations on an idealized model of a muscular individual rather than the average human. Despite the inaccuracies, Krogh's work was influential and widely cited for decades due to his prestigious status as a Nobel laureate, leading to the perpetuation of the inflated blood vessel length claim.

In reality, new, more up-to-date research indicates that the total length of blood vessels in an average person is estimated to be between 9,000 and 19,000 kilometers. This figure more accurately reflects the network of arteries, veins, and capillaries within a typical human body.

References:

Kurzgesagt, (2013). 
"We fell for the oldest lie on the internet."
- 2024

Vital Circuits.

Doctor. Suzuki. Looking at the Body.

Zweifach, B. W. (1959). The Microcirculation of the Blood. Scientific American, 200(1), 54-60.

Krogh, A. (1929). The Anatomy and Physiology of Capillaries. Yale University Press.