The heart gets all the attention. Most of what goes wrong happens in the pipes — and specifically in a lining one cell thick that almost nobody has heard of.
What the circulation is actually doing, why the vessels matter more than the pump for most people, and why one thin layer of cells sits at the center of cardiovascular health.
Your heart beats somewhere around a hundred thousand times a day, without pause, for as long as you are alive. It is an extraordinary piece of engineering and it is not, for most people, where the trouble starts.
The trouble is usually in the pipes.
The systemThe heart pushes blood into arteries, thick-walled vessels built to withstand pressure. They branch into progressively smaller vessels and finally into capillaries — so thin that red blood cells pass through more or less in single file.
The capillaries are where the entire point of the exercise happens. Oxygen and nutrients leave; carbon dioxide and waste enter. Everything upstream is delivery infrastructure. Veins then return blood to the heart at much lower pressure, which is why they need valves and why leg muscles contracting helps push blood back up.
The endotheliumEvery millimeter of that circuit is lined by a single layer of cells called the endothelium. One cell thick, and if you spread it out it would cover a startling area.
It was assumed for a long time to be inert plumbing lining. It is not. It is an active tissue that continuously decides how the vessel behaves:
Atherosclerosis — the plaque process behind most heart attacks and strokes — does not begin with the heart. It begins with endothelium that is not working well, allowing particles into the artery wall and permitting the inflammatory response that follows. This is why endothelial function appears so often in this school, and why exercise, which is one of the most reliable ways to improve it, keeps turning up as the answer.
Physical activity improves endothelial function through several routes at once. Flowing blood physically drags along the vessel lining, and that shear stress is itself the main signal that prompts nitric oxide release. Move more, signal more, and the vessel becomes better at relaxing when it should.
That is the mechanism underneath a great deal of otherwise unremarkable advice. It is also the subject of the nitric oxide lesson already in this school, which puts it more sharply: the pump is the tool, the workout is the medicine.
The two lessons that follow cover the two numbers people actually get told about their cardiovascular health — blood pressure, and cholesterol — and what each one really measures.
The structure and function of the circulation, and the endothelium's role in regulating vessel tone, permeability and clotting, is established physiology.
Endothelial dysfunction precedes and predicts atherosclerotic disease. Well demonstrated in human studies.
Regular aerobic exercise improves endothelial function measurably. Shown repeatedly in controlled trials.
Shear stress from flowing blood as the principal stimulus for endothelial nitric oxide release is well characterized.
That any supplement marketed for circulation matches the effect of regular physical activity on endothelial function.
The circulation is a closed loop lined throughout by a single active layer of cells, and because that lining governs vessel width, permeability and clotting, most cardiovascular trouble starts there rather than in the heart itself.
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