Round Red Things Floating in the Blood

This page is for the parent, not the child. The child listens to the story; this page is for answering the questions they'll ask afterward. The narration script itself is available via the script button during playback.

What this episode is about

In the 1600s, when the microscope had just been invented, what did people see when they first looked at blood — and what did they get wrong?

This episode deliberately unsettles the usual question of "who discovered it first." More than one person first saw red blood cells, and Leeuwenhoek wasn't the earliest. Yet he's the one we remember — because he wrote about them more than thirty-five times, across thirty-one letters, over thirty-nine years. Discovery as duration rather than as a single moment is the spine of this episode.

What actually happened

• The microscope appeared in the late 1500s and early 1600s — first in simple forms, later as compound microscopes using multiple lenses.
• In 1656, the French physician Pierre Borel (physician-in-ordinary to Louis XIV) was the first to apply the microscope to medicine, and described a kind of "worm" found in human blood.
• In 1657, Athanasius Kircher, a Jesuit priest and scholar in Germany, examined the blood of plague victims and described "worms" of plague. By today's standards, both observations were mistaken.
• In 1661, 1664, and 1665, Marcello Malpighi of Italy discerned blood cells.
• In 1678, Jan Swammerdam of Amsterdam described red corpuscles.
• In the last quarter of the 17th century, Antoni van Leeuwenhoek of Delft produced the first complete account of red cells.
• Leeuwenhoek was not a university researcher. He sent what he observed in letters to the Royal Society and others. According to the research, he commented on red blood "globules" and arthropod hemocytes at least thirty-five times, in thirty-one letters, spanning thirty-nine years — some of the passages were extensive.
• He compared blood across many kinds of creatures, and viewed cells in different chemical and physical environments to understand their properties.
• The line quoted in the story — "round, red globules floating in a crystalline fluid" — is Leeuwenhoek's own description.

Questions your child might ask

"So who was really first?" It's hard to say cleanly. And that's the point of this episode. Several people saw them around the same time, and each saw them a little differently. In science, "first" is blurry far more often than you'd think.

"Were the people who said 'worms' stupid?" No. Microscope images were fuzzy back then, and "there are small moving things in blood" was an accurate observation as far as it went. They simply didn't know what those things were. The observation was right and the interpretation was wrong — which still happens in science today.

"What do red blood cells do?" They carry breath (oxygen) to every corner of the body. But Leeuwenhoek didn't know that. What he saw was "round red globules"; what they actually do was worked out much later.

"Can you send research in a letter?" In that era, that was one way of publishing. Letters to a learned society filled the role that journal papers fill today.

Concepts in this story

• Observation and interpretation differ — two people can see the same thing and call it different things.
• The blurriness of "first" — discoveries usually have several people layered into them.
• The power of persistence — the difference between looking once and looking for thirty-nine years.

What's evidence, and what's retold

What's confirmed in the research Borel's (1656) and Kircher's (1657) "worm" observations, Malpighi (1661, 1664, 1665), Swammerdam (1678), and the statement that "the first complete account of the red cells was made by Anthony van Leeuwenhoek of Delft in the last quarter of the 17th century" — all confirmed in the abstract of a paper on the history of blood cell discovery. The figures thirty-nine years, thirty-one letters, thirty-five times and the quoted line come from a 2022 paper cataloguing Leeuwenhoek's red blood cell observations.

What was deliberately left out The widely told story that Leeuwenhoek was a cloth merchant who ground his own lenses does not appear in the abstracts this episode is based on, so it isn't in the story. We used only what the sources confirm: that he wasn't at a university and sent his findings by letter.

What was simplified The story doesn't give a precise year (such as 1674), only "a little over three hundred years ago," because the source specifies no further than "the last quarter of the 17th century." Names are also shortened and identified mostly by country, so a child can follow along.

Sources, and what came after

Source papers

• "Round, red globules floating in a crystalline fluid" — Antoni van Leeuwenhoek's observations of red blood cells and hemocytes. PMID 35364426 (2022)
• [Discovery of blood cells in the 17th century]. PMID 11505724 (2001)

What was worked out later

• What red blood cells do — that they carry oxygen, and that a protein called hemoglobin does that job, was established much later.
• The reach of the microscope — the same tool went on to reveal white blood cells and platelets in turn (the next episodes in this series).
• The evolution of "seeing" — today we go beyond light microscopes to electron microscopes that reveal structures inside the cell. Every time the tool changed, something new became visible.

Try this together

• Keep an observation diary with a magnifying glass. Look at the same thing (one leaf, the skin on the back of a hand) once a day for three days, and write down what's newly visible each time. It makes "there's something you only see on the second look" a physical experience.
• When your child names something incorrectly, instead of just saying it's wrong, try "your observation was right — only the name is different."
• If a museum or science center has a microscope exhibit, look through it together.