Many popular science sources say that the total length of blood vessels in the human body is two and a half times the length of the equator. We checked whether this statement is correct.

Users write about the incredible total length of blood vessels services questions and answers, Reddit And Instagram* dedicated to donation publics on VKontakte, private clinics and Russian regional Media. The statement is found in educational presentations on the websites of medical colleges and universities, on portal "Education in Russia" and recommendations for preparing biology lessons for the 8th grade.

Vessels are tubular structures, through which blood and lymph circulate throughout the human body. Although vessels are often referred to exclusively as components of the circulatory system, there are also lymphatic vessels. But, since in most viral publications we are talking about the total length of blood vessels, further in the text this term will be used in this meaning.

Blood vessels form a system of channels closed to the heart. First of all, they ensure the delivery of oxygen and nutrients to organs and tissues, as well as the removal of metabolic products. Basic types of vessels: arteries (carry blood away from the heart), veins (carry blood towards the heart) and capillaries (connect arteries and veins). Due to the ability to narrow and dilate, blood vessels also participate in thermoregulation. More The total number of vessels is located in tissues and organs with high metabolic activity and oxygen demand (for example, in muscles, lungs, liver, kidneys and brain), the least in avascular structures where metabolism is low (such as cartilage, the cornea of the eye, the lens, the epidermis of the skin and some tendons).

Beginning the study of blood vessels put Roman physician Galen, who lived in the 2nd–3rd centuries. True, he believed that blood is formed in the liver and does not circulate, but slowly, progressively moves through the veins in the lower part of the body (arteries do not play any role in this case). With some reservations, this point of view prevailed until the 16th century, when the Flemish physician and founder of modern anatomy Andreas Vesalius performed the first systematic dissections of human bodies and refuted many of Galen’s claims. And in 1628, the English physician William Harvey published the work Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus (“Anatomical study of the movement of the heart and blood in animals”), where he experimentally and logically proved that blood moves in a vicious circle thanks to the work of the heart. His discoveries laid the foundation for modern cardiology and vascular physiology.

The Danish physiologist August Krogh made a significant contribution to the study of blood vessels - for his research in this area he in 1920 received Nobel Prize in Medicine and Physiology. It was Krogh and calculated the length of all blood vessels in the human body is approximately 100,000 km. True, the average person from those calculations was completely different from the average person in reality - his weight was 143 kg, of which 50 kg were muscles (for comparison, about the same amount at the peak of his sports career weighed boxer Nikolai Valuev). According to 2005 data, the average weight of a person amounted to 62 kg, that is, more than half the size of Krogh's reference man. At the same time, the ratio of his muscle mass to body weight looks quite realistic: for example, depending on age, in men the muscle have to from 31% to 44% of the total weight, and in women - from 26% to 33%. Approximately in the same range as show Some studies show this indicator in athletes involved in American football, powerlifting, sumo and shot put.

In 2022, a group of American and Japanese scientists counted the length of blood vessels in the human body, trying to eliminate the shortcomings of research a century ago. They assumed the weight of an average person to be 70 kg and assumed that muscles accounted for 35% of the weight. The scientists also took into account that Krogh used an incorrect assessment of vascularization (capillary density in the muscles), and took updated modern data. However, such calculations will also have limitations - vascularization varies depending on the tissue: in the heart it higherthan in skeletal muscles, and in back muscles higher, than in the gluteal. As a result, scientists came to the conclusion that the total length of blood vessels in the human body (with all averaging and assumptions) ranges from 9,000 to 19,000 km. Although this figure does not exceed two and a half times the length of the equator, it is still impressive - for comparison, the path along the highway from Kaliningrad to Vladivostok amounts to 10,337 km.

True, these data should be treated with caution. Firstly, the length of the capillaries is not fixed - they constantly grow and disappear. This process called neoangiogenesis and is one of the main mechanisms general exchange to maintain body temperature. Secondly, it is important to remember that the circulatory system represents is a fractal and has a fractal dimension. Because of this, the correctness of any statement about the total length of all vessels depends, among other things, on the scale chosen by the researcher and the degree of detail of the object being studied.

Thus, the estimate of the length of all human vessels at 100,000 km is a completely scientific calculation, which was carried out by Nobel laureate August Krogh back in the 1920s. However, firstly, in his model the scientist used a real giant instead of an average person, and secondly, modern data showed that he greatly overestimated the density of capillaries in the muscles. According to current calculations, the total length of blood vessels in a human body weighing 70 kg ranges from 9,000 to 19,000 km.

Cover image: Image by Arek Socha from Pixabay

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