On this page
Before William Harvey, most doctors did not picture blood travelling round the body in a closed loop. Harvey, arguing against the followers of the ancient physician Galen, replaced the old picture with the one in your Class 11 biology book: a pump, a closed loop of vessels and one-way valves. This post explains how he made the case, redoes his central calculation with modern numbers, and draws out what a student can learn from how he reasoned.
Who was William Harvey?
According to the AIM25 archive record for Harvey's papers, he was born in Folkestone, Kent, on 1 April 1578. He studied at Gonville and Caius College, Cambridge, then travelled to Padua, then the most renowned medical school in Europe, where he learned anatomy under Fabricius of Aquapendente. He graduated there on 25 April 1602.
Back in London he became physician at St Bartholomew's Hospital and a Fellow of the Royal College of Physicians, where he was elected Lumleian Lecturer in 1615 and gave anatomy lectures for decades. He later served as physician to James I and Charles I. In 1628 he published, at Frankfurt, a short book usually called De Motu Cordis ("On the Motion of the Heart and Blood in Animals"), in seventeen chapters. The same record notes that he reached the full idea of circulation in stages over his first twelve years of lectures. He died on 3 June 1657.
How did Harvey show that blood moves in a circle?
He combined three kinds of evidence. You can read all of them in the English translation of De Motu Cordis by Robert Willis on Project Gutenberg.
Watching living hearts. He observed hearts in many animals, including colder-blooded ones such as eels and frogs. He concluded that the heart's active phase is contraction, when it becomes hard and pale and pushes blood out, rather than expansion.
Valves in the veins. With a band tied round the arm so the veins stood out, he pressed a vein with one finger and stroked the blood upward with another. The stretch beyond the next valve stayed empty: blood could flow towards the heart but not away from it. He concluded that these valves, like the valves at the start of the aorta and pulmonary artery, exist "to prevent all reflux of the blood".
Arithmetic. This was the decisive step, and it needs a section of its own.
What was Harvey's calculation?
Harvey did not need precise numbers. He deliberately chose low estimates. In the Willis translation he writes that the left ventricle holds "upwards of two ounces" in the dead body, and supposes that only a quarter to an eighth of that is pushed into the aorta at each beat: half an ounce or less. He then notes that in half an hour the heart makes "more than one thousand beats".
His version. 0.5 ounce per beat x 1,000 beats = 500 ounces in half an hour. At 16 ounces to the pound, that is about 31 pounds, roughly 14 kg of blood in thirty minutes. Harvey's conclusion was that this is "a larger quantity in every case than is contained in the whole body". No diet could replace blood that fast. The only sensible answer is that the same blood returns to the heart.
The modern version. The NCERT Class 11 chapter on body fluids and circulation gives a stroke volume of about 70 mL and an average heart rate of 72 beats per minute.
Cardiac output = stroke volume x heart rate = 70 mL x 72 = 5,040 mL per minute, which NCERT rounds to about 5 litres.
In one hour: 5.04 L x 60 = about 302 litres.
In one day: 302 L x 24 = about 7,260 litres.
That is several thousand litres a day from one ventricle. Harvey's logic still holds with better numbers: no organ could make that much new blood from food, so it must circulate.
What did Harvey not know?
Harvey could not see how blood passed from the finest arteries into the finest veins. He inferred that a connection had to exist because the numbers left no alternative. The introduction to the same Gutenberg edition notes that it was only in 1661, four years after Harvey's death, that the Italian anatomist Malpighi, with an improved microscope, observed the capillaries joining arteries to veins. Harvey's argument is a good example of reasoning to a conclusion that the tools of the day could not yet confirm directly.
Common mistakes when students revise this topic
Calling every artery "oxygenated". The pulmonary artery carries deoxygenated blood to the lungs. Arteries are defined by direction (away from the heart), not oxygen content.
Mixing up stroke volume and cardiac output. Stroke volume is per beat (mL). Cardiac output is per minute (mL/min or L/min). Check your units before writing an answer.
Forgetting that both ventricles pump the same volume. In a closed double circulation, the right ventricle must send as much blood per minute to the lungs as the left sends to the body.
Our guide to body fluids and circulation works through the cardiac cycle and double circulation in more detail. If the path through the four chambers is still hazy, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing, so you can look up each step on its own.
What can a student learn from how Harvey worked?
Test an idea with a rough calculation. Harvey picked deliberately low numbers and still got an answer too big to fit the old theory. A quick order-of-magnitude check can settle an argument that description alone cannot.
Use your own body as a lab. The tied-arm demonstration needs nothing but a band and two fingers.
Combine evidence. Observation, a simple experiment and arithmetic each gave a partial case. Together they were convincing.
Practice routine for this week: compute cardiac output for three cases (rest, a stroke volume of 90 mL at 120 beats per minute, and your own pulse counted for one minute with 70 mL assumed), then explain in two sentences why each answer supports circulation.
Next step: count your pulse for 30 seconds tonight and redo Harvey's half-hour calculation with your own number.
