The Organ You Have Never Thought About

This article assumes you know nothing about any of it. If you already know what an endothelium is, start with the next article instead.

The Loop

Blood goes round. That is the whole design, and everything else is detail.

The left side of your heart pushes blood out into the aorta, the body's largest artery, about the width of a garden hose. The aorta branches, and its branches branch, and those branch again, perhaps twenty times over, each generation of vessel narrower than the last. Arteries become arterioles, and arterioles feed into capillaries — the smallest vessels, so fine that red blood cells pass through them in more or less single file.

The capillaries are the point of the exercise. Everything else is plumbing that exists to get blood to them. It is in the capillaries that oxygen leaves the blood and enters your tissues, that glucose and amino acids are delivered, and that carbon dioxide and waste are picked up for removal. No exchange happens in the big arteries at all. They are transport.

From the capillaries, blood collects into venules, venules join into veins, veins join into larger veins, and everything returns to the right side of the heart. The right side then sends it to the lungs to be reloaded with oxygen, and the lungs return it to the left side to start again.

So it is two loops joined at the heart: a short one to the lungs and back, and a long one round the body and back. A red blood cell completes the full circuit in roughly a minute at rest, which is arithmetic rather than a marvel — you hold about five litres of blood, and a resting heart pumps about five litres a minute.

What a Blood Vessel Is Made Of

Picture an artery in cross-section, as three layers wrapped around a hole. The hole has a name: the lumen, the space the blood actually travels through.

That last point is worth pausing on. The vessels where all the useful work happens are made of the lining and almost nothing but the lining. It has to be that thin: oxygen has to cross it by diffusion, and diffusion only works over very short distances.

The Lining Is Not Wallpaper

For most of the history of medicine, the endothelium was understood as a smooth non-stick surface — something to stop blood clotting against a rough wall, and no more interesting than the glaze on a pipe.

It turned out to be an organ. It senses, it decides, and it manufactures. Here is some of what that single cell layer is doing while you read this:

• It sets the width of your arteries. The muscle layer does the contracting, but the endothelium gives the order, largely by releasing nitric oxide — a gas it makes on demand that tells the surrounding muscle to relax. That is vasodilation; the opposite is vasoconstriction.
• It feels the blood going past. Endothelial cells detect the drag of flowing blood — shear stress — and respond to it. Brisk, smooth flow is the main signal telling them to keep producing nitric oxide, which is a large part of why movement matters so much.
• It keeps blood liquid. A healthy lining actively discourages platelets from sticking and clots from forming. Blood does not stay fluid by default; it stays fluid because the surface it touches is working to keep it that way.
• It controls what crosses. It is a selective barrier, deciding which molecules and which immune cells get out of the bloodstream and into the tissue.
• It manages inflammation. When it needs to let white blood cells through, it puts out sticky proteins for them to grab. That is a normal, necessary function — and, when it runs continuously, a damaging one.

It also wears a coat. The blood-facing surface is covered by the glycocalyx, a fuzzy gel-like layer of sugars and proteins that acts as both a filter and a cushion, and takes much of the wear.

Two Numbers You Have Probably Been Told

Any article on this subject reaches for scale, and there are two figures that come up every time. Both are worth looking at properly, because the story of how they went wrong is quite instructive.

"Sixty thousand miles of blood vessels"

You will have met this one, usually with the flourish that it would wrap around the Earth two or three times. It traces to a single calculation by the Danish physiologist August Krogh, published in 1929, which produced a figure of 100,000 km of capillaries.

Krogh was a Nobel laureate and a brilliant experimentalist, and the number is still wrong. A 2020 review of his work sets out why. He assumed a person with 50 kg of pure skeletal muscle — which, if muscle is around 35% of body mass, describes someone weighing about 143 kg. He also used a capillary density of 2,000 per square millimetre, where modern measurements of human quadriceps give roughly 300. Both assumptions inflate the total, and they multiply together.

Krogh himself spotted that something was off. His figure implied that the muscle capillaries alone would need about a third of all the blood in the body just to stay filled, which he rightly judged impossible — and that problem is what led him to propose his famous "capillary recruitment" theory, the idea that most capillaries sit closed at rest and open during exercise. That theory has also not survived: it is now clear that most capillaries carry blood at rest, and that increased delivery during exercise comes mainly from blood moving faster through vessels that were already open.

"A kilogram of endothelium covering seven thousand square metres"

This one has an even tidier trail. It appears in a widely cited 2005 review, which drew it from a 1980 paper — and in 2021 a letter in the Journal of Thrombosis and Haemostasis pointed out that the numbers do not match the 1980 source, and do not even agree with each other.

The internal contradiction is the neat part. If the endothelium is about a micrometre thick and roughly as dense as water, then 4,000–7,000 m² of it would weigh four to seven kilograms, not one. Both figures cannot be right. Recalculating from the original source gives 270–720 m², weighing 0.27–0.72 kg — around a tenth of the number in circulation.

The honest version is less quotable and quite sufficient. Your endothelium weighs something in the region of a large orange and, spread flat, would cover somewhere between one and three tennis courts. A doubles court is 260 m², so you can check that against the corrected range yourself. Nobody has a trustworthy figure for the total length of the network, and the sensible thing is to say so.

Why This Hub Is About the Lining

Three things follow from the anatomy, and between them they explain everything that comes next.

It is exposed. The endothelium is the only tissue in the body in permanent contact with whatever is in your blood. Every rise in blood sugar, every circulating fat, every inflammatory signal, every chemical from a cigarette reaches it first and reaches it undiluted. It is the interface between what you consume and what you are made of.

It is thin. One cell thick, and in capillaries as little as a tenth of a micrometre. There is no reserve layer and no redundancy.

It has no nerves. Endothelial cells cannot report pain, because there is nothing there to report it with. Damage to the lining produces no sensation whatsoever — which is why the early years of every vascular disease are silent, and why the first symptom is so often the serious event itself.

Put those together and you get the pattern this hub keeps returning to. The lining is damaged first, because it is what the damage touches. It degrades quietly, because it cannot complain. And by the time anything is felt — chest pain, a wound that will not heal, changes in vision — the process has usually been running for years.

Where To Go From Here

The articles that follow take this in order. The next one looks at how the lining actually widens a blood vessel, and why the strongest signal for doing so is physical movement. After that comes what goes wrong — how a protective response becomes a destructive one, and how a plaque is built.

Then the awkward question of why none of it can be felt, what genuinely repairs the lining once you strip out the marketing, and an honest assessment of two compounds currently sold hard for the purpose. The hub ends where the damage ends up: the warning sign that arrives first, and the eyes, kidneys and feet.

You do not need to read them in order, but they were written in one.