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In Class 11 you learn that capillaries are where the blood actually does its job: oxygen, nutrients and wastes cross their thin walls. But a working muscle needs far more oxygen than a resting one. How does the body deliver it? Speeding up the blood is part of the answer. August Krogh showed that the bigger part is opening capillaries that were shut. This post covers what he did, the numbers he found and a short calculation you can do yourself.
Who was August Krogh?
According to his Nobel Prize facts page, Krogh was born on 15 November 1874 in Grenå, Denmark, and died on 13 September 1949 in Copenhagen. He won the 1920 Nobel Prize in Physiology or Medicine "for his discovery of the capillary motor regulating mechanism".
His Nobel biography says he began medicine at the University of Copenhagen, soon moved to zoology, and in 1897 joined the laboratory of the physiologist Christian Bohr. Copenhagen created a post in zoophysiology for him in 1908 and made it a full chair eight years later; he held it until 1945. Much of his work was done with his wife, Marie Krogh. Krogh built very small, precise instruments for measuring gases, and with Marie he showed in 1910 that diffusion alone explains how oxygen passes from the lungs into the blood, against the views of Bohr and J. S. Haldane.
What puzzle did Krogh set out to solve?
In his Nobel lecture, Krogh explains that by 1915, comparing studies of how much oxygen muscles take from the blood, he had reached an awkward conclusion. Either those studies had serious errors, or blood flowed through capillaries in a way nobody pictured.
The usual picture was that all capillaries are open and about equally wide, with the small arteries setting how fast blood runs through them. Krogh concluded instead that in resting muscle blood flows through only a small percentage of capillaries, spread fairly evenly through the tissue. That meant capillaries were not passive tubes. They had to be able to contract on their own, and he needed experiments to test it.
How did he count open capillaries?
First he counted all the capillaries. Filling blood vessels with a coloured material that hardens let him count them in thin cross-sections. In a guinea pig he found roughly 3,000 per square millimetre, an area little larger than a pin's head.
Then he counted the open ones. He put fine Indian ink particles into the blood of a living animal. Only capillaries with blood flowing through them at that moment filled with ink. In one guinea-pig muscle he saw only about 200 open capillaries per square millimetre. In a breathing muscle that had been working right up to the end, he found 2,500, nearly all of them. Open capillaries were always fairly evenly spread, and some in resting muscle were only 2 to 3 thousandths of a millimetre wide, narrower than a red blood cell, which squeezed through by folding or stretching.
His earlier count is in The number and distribution of capillaries in muscles (1919), which also calculates the oxygen pressure needed to supply the tissue.
A worked example: how far must oxygen travel?
Krogh's way of thinking gives a quick estimate. Imagine each open capillary supplying a small circle of muscle around it, and ask for the radius of that circle.
With n open capillaries per square millimetre, each one supplies an area of 1/n mm².
For a circle, πR² = 1/n, so R = √(1/(πn)).
Few open, n = 200: R = √(1/628) = √0.00159 = 0.040 mm, about 40 micrometres.
Working, n = 2,500: R = √(1/7,854) = √0.000127 = 0.011 mm, about 11 micrometres.
Opening 12.5 times as many capillaries cuts the longest diffusion distance by about √12.5 ≈ 3.5 times, and it greatly increases the wall area oxygen can cross. That is Krogh's point: the extra surface does as much work as the extra flow.
Common mistakes to avoid:
Thinking Krogh discovered capillaries. Marcello Malpighi first saw them, as our post on Malpighi and the capillaries explains. Krogh's discovery was how they are opened and closed.
Treating capillaries as passive pipes whose width depends only on blood pressure. Krogh's frog-tongue experiments showed a contracted capillary can stay shut even against arterial pressure.
Forgetting the square root in the estimate. The distance falls with the square root of the number of open capillaries, not in direct proportion.
What did he leave unanswered?
Krogh was open about the gaps. In the lecture he lists the questions still being worked on: whether capillaries are controlled by nerves, what makes them contract, and whether contractile cells sit in their walls. He describes a colleague observing branched cells around capillaries, first described by Rouget, and says no definitive results had yet been reached. He compared these questions to the heads of the Hydra: cut one off and several more grow.
His working method also became a principle. The biochemist Hans Krebs later named it the August Krogh principle, stated in the title of his 1975 paper as "For many problems there is an animal on which it can be most conveniently studied". Krogh chose the frog's tongue because it is thin, see-through and full of capillaries.
What can a student take from Krogh's way of working?
Krogh said his discoveries came from "nothing more than patient observation under the microscope and simple probing with fine needles". He noticed that published numbers did not fit together, stated the only explanation that would make them fit, and then went and counted.
You can use the same habit in biology. When two facts in your textbook seem to clash, write down what would have to be true for both to hold, then look for the evidence. If diffusion distances and surface area feel abstract, the Fermi Simulation Lab has interactive simulations for ideas that are easier to see than read. For the circulation chapter itself, see our guide to body fluids and circulation.
Next step: redo the worked example for 1,000 open capillaries per square millimetre and check that your radius falls between the resting and working values.
