How long are all your blood vessels?

Almost every source repeats that the human circulatory system is 100,000 km long — enough to wrap the equator two and a half times. That figure traces back to early 20th-century capillary density estimates and does not survive scrutiny: more careful calculations from measured capillary densities in muscle, gut and skin put the total nearer 9,000 to 20,000 km. Capillaries are essentially all of it either way; the aorta and the named arteries contribute a few metres. This entry is here mainly because it is a good example of a striking number that got repeated until it became a fact.

How the figure is arrived at

Capillary density per gram of tissue, from histological sections, multiplied by tissue mass and by mean capillary length of about 0.5 to 1 mm. The result is exquisitely sensitive to the density figure, and density differs by an order of magnitude between muscle and fat — using a muscle figure for the whole body is how you arrive at 100,000 km.

How confident it is

Low estimate

9,000

High estimate

100,000

Spread

11.1× between the bounds

Included with its provenance problem stated, because the 100,000 km figure is what people will have read elsewhere and it is very likely too high by a factor of five.

Other the human body counts

• How many bones are in the human body? 206 in an adult — about 300 at birth
• How many cells are in the human body? about 30 trillion human cells, plus 38 trillion bacteria
• How many neurons are in the human brain? about 86 billion
• How many muscles are in the human body? about 600 skeletal muscles
• How many hairs are on your head? about 100,000
• How many taste buds do you have? about 2,000 to 8,000
• How many base pairs are in human DNA? about 3.1 billion per copy — 6.2 billion in most cells