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Half a millimetre a second

The sparks you see in a blue sky are white blood cells in your eye

They are white blood cells passing, one at a time, through the capillaries of your retina, in front of the cells that sense light. They move at about half a millimetre to one millimetre a second, and with a little patience you can measure them yourself.

What you see in a clear sky

Look at a cloudless blue sky. After a few seconds small bright points appear: they light up, dart along a short, winding path, and go out. They are not in the sky. They are in your eye.

Each point travels along a capillary, a blood vessel so thin that cells pass through it one at a time. These capillaries lie in front of the cells that catch the light, and the points follow their paths. Eye doctors call it the blue field entoptic phenomenon: entoptic means “from inside the eye”.

Right at the centre of vision, over a patch 500 to 600 micrometres across, there are no capillaries at all: there the retina has to see with nothing in the way. So the sparks keep to a ring around the point you look at. The network drawn here is schematic; the empty centre has its measured size.

Why they shine

A capillary around the centre of vision is 3.5 to 6 micrometres wide inside. A red blood cell is 7.8 micrometres across. To get through, it folds and travels in single file.

Red cells absorb blue light more strongly than white cells do. Against a blue sky, a capillary full of red cells lets less light through.

A white cell is bulkier and much stiffer than a red one, and it moves on with more difficulty. The red cells ahead of it pack together, and behind it a stretch of clear plasma opens up. The white cell and the plasma behind it let the blue light through: that is the bright point you see, sometimes with a short tail after it.

For you, half a millimetre a second is barely moving. For a red cell 7.8 micrometres across, it is 64 of its own lengths every second.

Measure your own blood

In 1980 Charles Riva and Benno Petrig found a way to measure the speed of these sparks. They drew moving points on a screen, and each person adjusted their speed until they looked like their own sparks. In five young people they found a speed that rose and fell with every heartbeat, between about half a millimetre and one millimetre a second.

You can do a version of their test. The screen fills with blue and you watch your sparks. Then drawn sparks appear, and you say whether they are faster or slower than yours. After a few rounds their speed closes in on yours.

Before you start

  1. Turn the screen brightness to maximum and dim the room.
  2. Look at the centre of the screen, relaxed. The sparks appear in a ring around it.
  3. Give them 20 to 30 seconds. Put a finger on your wrist: you may see the sparks speed up with every heartbeat.
How far away is the screen?

The result is a rough reading. To turn what you saw into millimetres on the retina, the page assumes how far away you hold the screen, a typical pixel size, and 0.29 millimetres of retina for each degree of your field of view. Modern cameras that photograph the retina directly have measured faster white cells: an average of 1.37 millimetres a second in six people, from 0.77 to 2.10 (Martin and Roorda, 2005), and 1.80 in the capillaries white cells favour, in one young woman (Tam and colleagues, 2011).

Why blood is slowest here

At rest your heart pumps about 5 litres of blood a minute, or 83.3 cubic centimetres a second. The same amount has to pass, every second, through every level of the tree of vessels: through the aorta, the arterioles, the capillaries. Speed is flow divided by the area it passes through: v = Q/A.

The aorta has a cross-section of 2.5 square centimetres, and blood crosses it at an average of 33 centimetres a second. Each capillary on its own is tiny. Side by side, the body's roughly 10 billion capillaries add up to 2,500 square centimetres, 1,000 times the aorta. So the blood slows 1,000 times, to about 0.3 millimetres a second.

Cross-section of all vessels of one kind, side by sideAverage speed, v = Q/A
Aorta2.5 cm²33 cm/s
Small arteries20 cm²4 cm/s
Arterioles40 cm²2 cm/s
Capillaries2,500 cm²0.33 mm/s
Venules250 cm²3.3 mm/s
Small veins80 cm²1 cm/s
Venae cavae8 cm²10 cm/s
Areas from Guyton and Hall's textbook; speeds calculated for a flow of 5 litres a minute. Both scales are logarithmic.

The numbers check each other. Divide 2,500 square centimetres among 10 billion capillaries and each gets a tube 5.6 micrometres wide, inside the textbook's 4 to 9 micrometres.

The average of a third of a millimetre a second is calculated for the whole body. Where it has been measured directly, the speed is of the same order. In the skin fold at the base of the fingernail, Bollinger and colleagues filmed red cells in 1974 at an average of 0.84 millimetres a second in the limb coming from the arteries and 0.47 in the limb leaving towards the veins. In the eye, in people who matched them, the sparks moved at about half a millimetre to one millimetre a second; cameras on the retina have clocked white cells at up to two.

The slowing has a purpose. A capillary is 0.3 to 1 millimetre long, so blood stays in it for one to three seconds. In that time oxygen and nutrients pass through its wall, half a micrometre thick, to the cells. Almost no cell in the body is more than 20 to 30 micrometres from a capillary.

Next time the sky is clear

Go outside on a clear day and look at the sky, away from the Sun. Let your eyes rest on the blue and wait. Each spark is a stiff cell squeezing through a tube barely wider than itself, pushing the red cells ahead of it at half a millimetre to a millimetre a second. Put a finger on your wrist: you may notice them quicken with each beat of your heart.

Sources and method

The average speeds along the tree of vessels are calculated from the areas in Guyton and Hall's textbook (12th edition) and a flow of 5 litres a minute. The measured speeds come from the studies cited: by matching a simulation (Riva and Petrig), from images of the retina (Martin and Roorda; Tam and colleagues) and from films of the capillaries in the skin fold at the base of the fingernail (Bollinger and colleagues).

The capillary network in the drawing is schematic; the empty centre (500 to 600 micrometres), the width of the capillaries and the size of the cells are the measured ones. The shape of the folded cells is illustrative. The drawn sparks speed up with each beat, at a pulse of 70 a minute, because the measured speed rises and falls with the heart; the size of the swing in the drawing is illustrative.

In the test, the speed of the drawn sparks is turned into speed on the retina using 0.29 millimetres for each degree of the field of view. The pixel size is assumed (a phone screen about 7 centimetres wide; otherwise 96 pixels to the inch), and the distance is the one you chose. The result is given as a range.