What you feel at your wrist is not your blood

One beat, two journeys

Each press sends a white wave out of the figure's heart. Worked out from the proportions of the body, the path from heart to wrist at your height is about 0.78 m. The wave covers it in 0.11 seconds if it travels at 7 metres a second.

The red dot is the blood your first beat pushed out. Look where it is when the wave has already reached your hand. Then press again.

Blood moves about a hand's width per beat

With each beat, an adult heart pushes on average 91 millilitres of blood into the aorta in men and 73 in women. In its first stretch, the ascending aorta is about 2.6–2.7 centimetres across. Divided by its cross-section, that volume moves the column of blood forward by 14–16 centimetres: about a hand's width.

Even if it kept its pace from the heart all the way, the blood pushed out by your first beat would need 5–6 beats to reach your wrist. The wave makes the same journey in a tenth of a second. The aorta widens during the beat and blood moves in surges, so this is an average advance, not the journey of one particular drop.

The wave runs along the wall; the blood falls behind

The wave does not bring the new blood with it. When the heart pushes, the elastic wall of the aorta bulges, springs back and pushes on the next stretch; that bulge travels along the arteries, all the way to your fingertips. What you feel at your wrist is it going past.

The wave travels faster in a stiffer, thicker wall and slower in a wider vessel or denser blood, a relation Adriaan Moens and Diederik Korteweg found independently in 1878. At its fastest, blood leaving through the aortic valve reaches 1.2–1.3 metres a second. A wave at 7 metres a second is more than five times faster than the blood at its fastest instant, and, at a resting 70 beats a minute, about 40 times faster than its average advance.

Decade by decade, the wave gets faster

A collection of reference values published in 2010 gathered measurements from 1,455 healthy people with normal blood pressure and no other risk factors. The median speed of the wave in the aorta was 6.1 metres a second under 30 and 9.7 at 60–69. The title's 7 metres a second is almost exactly the median for people in their forties: 6.9.

Type your age above and your group lights up on the chart.

It is mostly the aorta that stiffens. In the Framingham study of 521 people free of cardiovascular disease, hypertension, diabetes, smoking, high cholesterol and obesity, the wave was slower in the aorta than on the path from neck to arm until about 50, and overtook it after that; on the neck-to-arm path, the rise with age was smaller. These are averages of groups of people, not measurements of your arteries.

Why one metre a second matters

A meta-analysis of 17 studies following 15,877 people for 7.7 years on average found that each extra metre a second of aortic wave speed was associated with a 14% higher risk of cardiovascular events and a 15% higher risk of death from any cause, after adjusting for age, sex and other risk factors. The studies also included people who were already ill, with hypertension, coronary disease or kidney disease. A 2012 expert consensus proposes 10 metres a second as the value above which the aorta counts as stiff; more than half of healthy people aged 70 and over are already above it.

This speed is measured in a clinic, with sensors on the artery in the neck and the one in the groin. Counting your pulse at home cannot tell you it.

Tonight

Find your pulse at your neck, under the angle of the jaw. Then at your wrist. Then on the top of your foot, between the first two bones. It is the same beat, arriving later the further it is from your heart.

The blood it pushed is still in your chest.