How Did We Find Out That Blood Goes Round and Round the Body?
― The Clincher Was One Multiplication on Paper
Children now learn in primary school that blood flows around the body. Yet this was worked out only 400 years ago. And the decisive step was not a new dissection or a microscope. One multiplication, of how much the heart pumps, overturned a belief that had stood for 1,400 years.
Gently press two fingers of one hand on the inside of the other wrist. You feel a steady thump, thump.
That beat carries on while you sleep, too. It comes to about 100,000 beats a day.
So where does all that pumped blood go? People long ago thought it was "used up in every corner of the body and simply vanished."
A Mistake People Believed for 1,400 Years
The ancient Roman physician Galen explained that blood is made in the liver and fades away at the body's far reaches as nourishment. It was a one-way trip. For the next 1,400 years, this account was the foundation of European medicine.
The man who overturned it was the English physician William Harvey. In a slim book published in 1628, he made two points.
Multiply out what the heart pumps, and in one hour it is dozens of times the blood in the body. The liver could not possibly make that much every hour.
Inside the veins of the arm are tiny doors that open only toward the heart. If blood simply flowed away to the extremities, these doors would be pointless.
Put the two together and only one answer is left. The blood is not vanishing. The same blood goes round and comes back.
Reason 1: The Sheer Amount Gave It Away
In one beat, the heart is said to push out about 70 millilitres of blood. That is roughly half a small paper cup. It does this about 70 times a minute.
Multiply, and you get about 4.9 litres a minute. In an hour, that is 294 litres. Yet all the blood in your body adds up to only about 5 litres. Look at Figure 1. Counting one hour's worth in 5-litre buckets gives 59 buckets.
It is hard to believe that the body makes this much new blood every hour and destroys the same amount. Harvey set out this estimate in his book and concluded, "So the blood must be going round." He used no special tools.
Reason 2: One-Way Doors Inside the Arm
Harvey also described one more experiment that anyone can do. Tie the upper arm loosely so the veins stand out, then stroke a vein with a finger, moving away from the heart. The vein flattens, and no blood flows back into it.
Stroke the same vein toward the heart, though, and the blood moves along easily. That is because the vein holds a row of tiny doors that open in only one direction. Today they are called venous valves. Figure 2 shows how they work.
Every one of these doors faces toward the heart. If blood only vanished at the body's far ends, there would be no reason to line the road back to the heart with doors. The structure itself showed which way the blood goes.
The hair-thin vessels linking the outbound road to the return road were invisible with the tools of the time. Harvey wrote that they must exist though unseen, and moved on. Capillaries were actually observed four years after his death, so the story goes.
Summary
What showed that blood circulates was not a new observation but an estimate of quantity that anyone could check. When people counted, the numbers did not add up for a one-way trip.
When the numbers don't add up,
that is stronger evidence than any authority.
How blood from the legs gets back up to the heart is explained in detail in Why Do Your Legs Swell When You Stand for Too Long? For how the heart itself keeps beating on its own, see Why Can the Heart Keep Beating by Itself?
- Let your hand and forearm hang down, and gently clench and open your fist so the blue veins stand out. There is no need to tie anything.
- Press lightly on one point of a raised vein with a fingertip. With another finger, slowly stroke toward the elbow (the heart side). The stroked stretch turns paler.
- Keep the first finger in place and wait. Blood does not come straight back into that stretch. Lift the finger, and it fills quickly from the hand side.
Do not press hard or hold the pressure for long. Veins show more easily in people with thin skin, and less easily when you are cold. Count the pulse on the inside of your wrist for 15 seconds and multiply by 4, and you also get your beats per minute.
For Those Who Want More ― Terms, Formulas and Links to TextbooksEach part is labelled by level, from lower-secondary science to university courses
- Middle schoolCovered in lower-secondary school science
- High schoolCovered in upper-secondary "Basic Biology / Basic Physics"
- High school+Advanced upper-secondary material, or a textbook side-column topic
- UniversityUniversity-level material (fluid mechanics, physiology) not taught in high school
- ResearchNot taught even at university as settled fact; researchers are still working on it
Middle schoolTerms: This Phenomenon Has Names
- Stroke volume: The volume of blood the heart pushes out in one beat. For an adult at rest it is said to be about 70 millilitres.
- Cardiac output: The volume of blood the heart pumps in one minute. It equals stroke volume times heart rate.
- Venous valve: A thin flap inside a vein that opens in only one direction. It stops blood flowing backward.
- Systemic and pulmonary circulation: The route around the whole body and the route through the lungs. Blood passes through the two loops one after the other.
Middle schoolHigh schoolChecking with a Formula: How Many Times the Body's Blood Is Pumped in an Hour?
Let's follow the same estimate Harvey made in his book, using today's figures.
| In symbols | Q ＝ V × f |
| In words | Volume pumped per minute ＝ volume pushed out per beat × beats per minute |
| Where it comes from | It comes from conservation of volume. The pump pushes the same amount each beat, repeated once per beat. As long as no blood disappears or appears along the way, this relation holds. |
| Stroke volume (symbol V, unit millilitres) | About 70 millilitres (for an adult at rest) |
| Heart rate (symbol f, unit beats per minute) | About 70 beats (for an adult at rest) |
| Blood 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 | 294 ÷ 5 ＝ 58.8 |
In one hour, the heart pumps about 59 times the blood in the body. In a day that is about 7,000 litres, more than 30 household bathtubs. Making that much every day and throwing it away every day is plainly implausible.
High schoolHigh school+What Sets Blood Pressure?
High schoolBlood flow can be laid out much like electric current. There is a pressure difference that drives the flow, there is resistance that impedes it, and together they set how much flows.
High school+Formally, this is written as: difference in mean blood pressure ＝ cardiac output × total peripheral resistance. Narrower vessels raise resistance, so a higher pressure is needed to move the same amount. Many blood-pressure drugs act on one part of this equation.
UniversityHow Hard It Is for a Liquid to Flow Through a Thin Tube
For a viscous liquid flowing slowly through a narrow tube, the Hagen–Poiseuille equation (Hagen–Poiseuille flow) holds. The flow rate is proportional to the fourth power of the tube's radius. Halve the radius and the flow drops to 1/16. This fourth power is why a slight narrowing of the vessels changes circulation so much. Incidentally, Poiseuille was himself a physician, and he arrived at this relation while studying the flow of blood.
📖 Derivation of the equation and further reading: Hagen–Poiseuille flow ／ Cardiac output
ResearchWhat Is Still Not Clear
- Flow in the finest passages Red blood cells squeeze through tubes narrower than themselves by folding their bodies. Flow there cannot be described by the simple tube equation, and it is still studied by both calculation and experiment.
- What decides how blood is shared out When you exercise, blood goes to the muscles; after a meal, to the gut. Its destination changes moment by moment. Which signals act, and in what order, is not yet fully understood.
- How new vessels grow The body extends blood vessels to where they are needed. Understanding this control is tied to research on wound healing and disease.
In other words, this article too is "an explanation within what is known today." The numbers are rough guides for a person at rest, and they vary with build and age.
Links to Textbooks (by Level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science / Structure and function of animal bodies | Systemic and pulmonary circulation, direction of venous valves |
| High school | Basic Biology / Maintaining the internal environment | Multiplying stroke volume to get cardiac output |
| High school+ | Biology / Regulation of the circulatory system | Relation between blood pressure and resistance |
| University | Fluid mechanics, physiology | Hagen–Poiseuille equation and the fourth power of the radius |
| Research | Microcirculation, haemodynamics | Flow in thin passages, control of blood distribution |
| ― | Links to daily life | Counting your pulse, reading blood-pressure values |
- 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")
- 『ギャノング生理学』 (Ganong's Review of Medical Physiology, Japanese edition), Maruzen Publishing (丸善出版) ― circulation chapter (stroke volume, cardiac output, peripheral vascular resistance)
- Cardiac output (Japanese Wikipedia)
- Hagen–Poiseuille flow (Japanese Wikipedia)
※This article is a science explainer for a general audience. The figures given are rough guides to help you understand the mechanism. For judgments about your health or test results, please consult a doctor or other professional.