How did anyone work out that blood goes round and round?
― It came down to a single multiplication on paper
Today, schoolchildren learn that blood circulates around the body. But we've only known this for about 400 years. And the clincher wasn't a new dissection or a microscope. Just multiplying out how much the heart pumps was enough to overturn an idea that had stood for 1,400 years.
Gently press two fingers of one hand against the inside of your other wrist. You'll feel a steady beat: thump, thump.
That beat never stops, even while you sleep. It happens roughly 100,000 times a day.
So where does all that blood go? People long ago thought it was simply used up and consumed in every corner of the body.
An idea believed for 1,400 years ― and wrong
The ancient Roman physician Galen taught that blood was made in the liver and consumed as nourishment at the body's extremities: a one-way trip with no return. For the next 1,400 years, this idea remained the foundation of European medicine.
The man who overturned it was the English physician William Harvey. In a slim book published in 1628, he demonstrated two things.
Multiply out how much the heart pumps, and in one hour it comes to many times the total blood in the body. There's no way the liver could make that much every hour.
Inside the veins of the arm are tiny doors that only open toward the heart. If blood simply flowed away and never returned, these doors would serve no purpose.
Put the two together, and only one answer remains. Blood isn't disappearing — the same blood is going round in a loop and coming back.
Reason 1: the sheer volume gave away the answer
Each heartbeat is thought to push out about 70 millilitres of blood — roughly half a small paper cup. That happens about 70 times a minute.
Multiply it out: about 4.9 litres a minute. In an hour, that's 294 litres. But the body's entire blood supply totals only about 5 litres. Look at Figure 1: counting an hour's worth in 5‑litre buckets comes to 59 of them.
It's simply not plausible that the body manufactures — and destroys — that much blood every single hour. Harvey laid out this estimate in his book and concluded: "Therefore the blood must circulate." He used no special instruments at all.
Reason 2: the one-way doors inside your arm
Harvey also demonstrated a second experiment anyone can try. Tie a light band around the arm to make the veins stand out, then stroke a vein with your finger, moving away from the heart. The vein flattens, and no blood flows back into that stretch.
But stroke the very same vein toward the heart, and the blood moves through easily. That's because the vein contains tiny doors that open in only one direction. Today we call them venous valves. Figure 2 shows how this works.
Every one of these doors faces toward the heart. If blood simply vanished at the far end of the body, there'd be no reason to line the return route with doors. The structure itself revealed the direction blood was travelling.
The thin vessels connecting the outgoing and returning routes were invisible with the instruments of the time. Harvey wrote that they "must exist," even unseen, and moved on. Capillaries were finally observed four years after his death.
Summary
What proved blood circulates wasn't a new observation, but an estimate of volume that anyone could check. Once you counted, a one-way trip simply didn't add up.
A number that doesn't add up
is stronger evidence than any authority.
For how blood in the legs makes it back to the heart, see Why do your feet swell when you stand for a long time? For how the heart itself keeps beating on its own, see Why can the heart keep beating on its own?
- Let your hand or forearm hang down, and clench and open it gently to make the blue veins stand out. No need to tie anything off.
- Press one spot on a raised vein lightly with a fingertip, and with another finger, slowly stroke toward the elbow (toward the heart). The stroked stretch will look paler.
- Keep your pressing finger in place, and blood won't quickly return to that stretch. Lift the finger, and blood fills back in smoothly from the hand side.
Don't press hard or for too long. Veins are easier to see if your skin is thin, and harder to see when your hands are cold. Count your pulse for 15 seconds at the inner wrist and multiply by four to get your beats per minute.
Want to know more? ― Terms, formulas, and links to the textbookWe've labelled which level each part belongs to, from middle-school science to university specialist courses
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school "Biology Basics / Physics Basics"
- High school+Advanced high-school content, or a textbook sidebar topic
- UniversityNot covered in high school — university specialist content (fluid dynamics, physiology)
- ResearchNot even taught as settled fact at university — an active research question
Middle schoolTerms: this phenomenon has names
- Stroke volume: the volume of blood pushed out by one heartbeat. About 70 millilitres at rest in an adult.
- Cardiac output: the volume of blood the heart pumps per minute. Found by multiplying stroke volume by heart rate.
- Venous valve: a thin flap inside a vein that opens in only one direction, preventing blood from flowing backward.
- Systemic and pulmonary circulation: the route around the whole body, and the route through the lungs. Blood passes through both loops in sequence.
Middle schoolHigh schoolChecking with a formula: how many times the body's blood is pumped in an hour
Let's retrace the same estimate Harvey made in his book, using modern figures.
| In symbols | Q = V × f |
| In words | Volume pumped per minute = volume per beat × beats per minute |
| Where this formula comes from | It follows from conservation of volume. A pump repeats the same volume per stroke, so as long as blood isn't being created or destroyed along the way, this relationship holds. |
| Stroke volume (symbol V, unit millilitres) | About 70 millilitres (adult at rest) |
| Heart rate (symbol f, unit beats per minute) | About 70 beats per minute (adult at rest) |
| Total blood volume in the body (unit litres) | About 5 litres (roughly 1/13 of body weight) |
| Volume pumped per minute (millilitres) | 70 × 70 = 4900 |
| Convert to litres | 4900 ÷ 1000 = 4.9 |
| Volume pumped per hour (litres) | 4.9 × 60 = 294 |
| How many times the body's blood volume | 294 ÷ 5 = 58.8 |
The heart pumps out roughly 59 times the body's total blood volume every hour. Over a day, that's about 7,000 litres — more than 30 household bathtubs. There's simply no way the body could manufacture and discard that much every day.
High schoolHigh school+What determines blood pressure?
High schoolBlood flow can be organized much like an electric circuit. There's a pressure difference driving the flow, a resistance that hinders it, and the resulting flow rate.
High school+Formally: the difference in mean blood pressure = cardiac output × total peripheral resistance. Narrower blood vessels raise resistance, requiring higher pressure to push the same volume through. Most blood pressure medications act on some part of this equation.
UniversityHow hard it is for fluid to flow through a narrow tube
A viscous fluid flowing slowly through a narrow tube obeys the Hagen–Poiseuille equation (Hagen–Poiseuille flow). The flow rate is proportional to the fourth power of the tube's radius. Halve the radius, and the flow drops to one-sixteenth. This fourth-power relationship is why even a slight narrowing of a blood vessel can dramatically change circulation. Poiseuille himself was also a physician, and arrived at this relationship while studying blood flow.
📖 For the derivation and further reading: Hagen–Poiseuille flow ／ Cardiac output
ResearchWhat's still not fully understood
- Flow through the narrowest passages Red blood cells fold themselves to pass through vessels narrower than their own size. Flow there can't be described by simple tube equations, and is still studied through both computation and experiment.
- What controls how blood is distributed Blood shifts toward muscles during exercise and toward the digestive tract after a meal, changing moment to moment. Exactly which signals act, and in what order, isn't fully known.
- How new blood vessels grow The body extends vessels to wherever they're needed. Understanding this control is tied to research into wound healing and disease.
In other words, everything in this article is "the best explanation we have right now." The figures are rough guides at rest, and vary with body size and age.
Links to the school curriculum (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science / structure and function of the animal body | Systemic and pulmonary circulation, direction of venous valves |
| High school | Biology Basics / maintaining the internal environment | Multiplying stroke volume by cardiac output |
| High school+ | Biology / regulation of the circulatory system | The relationship between blood pressure and resistance |
| University | Fluid dynamics / physiology | The Hagen–Poiseuille equation and the fourth power of radius |
| Research | Microcirculation / hemodynamics | Flow through narrow passages, control of blood distribution |
| ― | Everyday connections | Counting your pulse, reading blood pressure figures |
- William Harvey, Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus, 1628 (Japanese translation: 『心臓と血液の運動について』, "On the Motion of the Heart and Blood")
- Ganong's Review of Medical Physiology (『ギャノング生理学』, Maruzen Publishing) ― chapter on the circulatory system (stroke volume, cardiac output, peripheral resistance)
- Cardiac output (Japanese Wikipedia)
- Hagen–Poiseuille flow (Japanese Wikipedia)
※This article is a general-audience science explainer. The figures given are rough approximations meant to aid understanding of the underlying mechanism. Please consult a doctor or other specialist for judgments about your health or test results.