The Academy
A Light, and a Heartbeat
How a watch measures a pulse, and what the spacing between beats means
The wave, not the blood
The heart is a pump — two pumps, side by side, one sending blood to the lungs and one to the rest of you. Each contraction throws roughly 70 millilitres into the aorta, and that sudden push sends a pressure wave running out along every artery in the body.
The wave is not the blood. Blood in a large artery travels at perhaps 40 centimetres a second; the pressure wave travels at 5 to 10 metres a second, twenty times faster. What you feel at your wrist is the wave arriving, in the same way a wave crosses a lake without carrying the water across with it.
That speed is itself a measurement: stiffer arteries carry the wave faster, so pulse-wave velocity is used clinically as an index of arterial stiffness.
Shining a light through yourself
Put a fingertip against a bright light and it glows red — long-wavelength light passes through tissue, and haemoglobin absorbs the rest. Now consider what happens as the pressure wave arrives: for a fraction of a second there is more blood in that fingertip, so more light is absorbed and less comes back.
A small led and a small photodetector are therefore enough to watch a heartbeat. The signal is tiny — the pulsating component is around 1% of the total light — but it is periodic, and periodic signals can be pulled out of noise. The technique is called photoplethysmography, or ppg: literally 'light volume writing'. It is what the green LEDs on the back of a smartwatch are doing, and what a hospital fingertip oximeter does.
Green light is used on wrists because haemoglobin absorbs it strongly and it does not penetrate far enough to pick up motion from deeper tissue. Oximeters use red and infrared together, because oxygenated and deoxygenated haemoglobin absorb those two differently — comparing them gives blood oxygen saturation.
Reading the shape
A single ppg beat has a characteristic shape. The steep rise is systole — the ventricle contracting and the wave arriving. The peak is maximum arterial volume. Then the fall, interrupted partway down by a small notch and a secondary bump: the dicrotic notch, which is the aortic valve snapping shut and the column of blood rebounding off it.
That notch is a real mechanical event you can see in light shone through a finger, which remains one of the more remarkable facts about the human body available to anyone with a phone.
A ppg is not an ecg. An ecg measures the heart's electrical activity through electrodes on the skin, and its familiar spikes are depolarisation waves, not pressure. ppg measures the mechanical consequence, at the far end of the plumbing, some 200 milliseconds later.
The spacing is the interesting part
A healthy resting heart does not beat like a metronome. The interval between beats varies continuously — it shortens as you breathe in and lengthens as you breathe out, an effect called respiratory sinus arrhythmia, and it drifts on longer cycles too.
This is heart rate variability, and more of it is generally better. It reflects an autonomic nervous system with both hands on the controls: the sympathetic branch speeding the heart, the vagus nerve slowing it, continuously negotiating. Low variability — a heart beating with machine regularity — is associated with poor outcomes in cardiac patients, and it is one of the things intensive-care monitors watch for.
Which is why, if anything were interfering with a heartbeat, the spacing is exactly where it would show. A steady average rate can hide a great deal; the intervals cannot.
Seen in the game · Stage 5 · the heart rate monitor
Where this comes from
Enspinors is a ten-minute investigation built out of real instruments — a true compass bearing, a shortwave band that behaves like one, a time signal modelled on wwv in Fort Collins. This article is one of the Academy lessons inside it, published here in full and free.
Enspinors on the App Store · $4.99 · no ads, no in-app purchases, no tracking