August Krogh (pronounced Krawg) won the 1920 Nobel Prize for physiology or medicine for the discoveries he made concerning human respiration. Krogh showed that most capillaries (the smallest blood vessels) of the body's organs and tissues are closed when they are at rest, but when there is activity and the need for oxygen increases, more capillaries will open. Krogh's explanations of respiration and capillary action were of major significance for the understanding of the physiology of the human pulmonary and circulatory systems. His research into respiration and circulation also had practical applications for the development of modern medical science.
Schack August Steenberg Krogh was born in Grenaa, Jutland. His father was a brewer, shipbuilder, and a newspaper editor. His mother, Marie Drechmann Krogh, was of gypsy ancestry. Throughout his life Krogh was active in both zoology and human physiology, accomplishing his major discoveries in the physiology of respiration. In 1910, he was able to settle an important biological controversy by establishing that, in human lungs, the absorption of oxygen and the elimination of carbon dioxide takes place by diffusion rather than secretion. From his work on human respiration, Krogh went on to describe the operation of the capillary system and the mechanisms that regulate it.
Krogh's interest in various areas of science can be traced back to his childhood. His father, who had been trained as a naval architect, influenced Krogh's interest in ships and the sea; the young Krogh also spent time in his youth studying the insects he found in the fields near his home. When he was fourteen he left school to serve on a Danish patrol boat assigned to protect fisheries in Iceland. Instead of pursuing a naval career, however, Krogh decided to complete his education. His love for the sea, nonetheless, remained with him for the rest of his life.
Krogh returned to school at the Gymnasium at Aarhus, and then went on to the University of Copenhagen in 1893. He first entered Copenhagen with the intention of studying physics and medicine, but under the influence of zoologist William Sørensen, he changed to the study of zoology and physiology. So slash rensen had advised Krogh to attend the lectures of Christian Bohr, an expert in circulatory and respiratory physiology. After attending Bohr's lectures at Copenhagen, Krogh began to work in Bohr's laboratory in 1897, and after receiving his master of science degree in 1899, he became Bohr's laboratory assistant.
One of Krogh's earliest achievements was his invention of a microtonometer--an instrument that measures gas pressure in fluids--which he developed to help in his research with a marine organism named Corethra. As a student, Krogh had done research on the larvae of Corethra to determine how its air bladders operated (he found that they worked like the diving tanks of submarines). Traveling in 1902 to Greenland, Krogh studied the amounts of oxygen and carbon dioxide dissolved in fresh and seawater. His research cast a new understanding on the role of the oceans in carbon dioxide regulation and at the same time he was able to improve his techniques for measuring gas pressures in fluids.
In 1903, Krogh received a Ph.D. in zoology from the University of Copenhagen, where, in his doctoral dissertation, he demonstrated the difference between the skin and lung respiration of the frog. Whereas the frog's skin respiration was constant and regular, Krogh found that the frog's lung respiration varied and was controlled by the autonomic system through the mechanism of the vagus nerve. Oxygen passed from the air sacs (alveoli) of the lung through a membrane to the capillaries and then to the blood stream where it formed carbon dioxide after it was used by the different tissues in the body. The process then reversed when the blood carried carbon dioxide to the alveoli of the lungs, where it was exhaled. Respiration would then vary according to the organism's need for oxygen.
Krogh was married in 1905 to Marie Jørgensen, a physiologist who also worked in Bohr's laboratory. (The couple would eventually have three daughters and one son, with the son and the youngest daughter becoming physiologists.) In 1906, the first of Krogh's papers to receive international recognition, a work that showed that nitrogen is not involved in animal metabolism, was awarded the Seegen Prize from the Vienna Academy of Sciences. In 1907, Krogh received further international attention at Heidelberg, Germany, when he discussed his findings on the diffusion of pulmonary gases at the International Congress of Physiology.
In 1908, Krogh made another trip to Greenland with his wife to study the Eskimo's meat-eating dietary habits and the effects it had on their respiration and metabolism. He was also given an associate professorship of zoo physiology at the University of Copenhagen that year. Two years later Krogh and his wife were given a laboratory at Ny Vestergade for physiological research; the couple worked there together for a number of years. Krogh then became a full professor at Copenhagen in 1916.
From 1908 to 1912, Krogh was engaged in research to resolve the question of how oxygen was transferred in the lungs to the blood. Bohr and John Burdon Sanderson Haldane (who was well-known for his own research into the mechanics of respiration), along with other scientists, believed that the lung acted as a gland in the alveolar transfer of oxygen to the blood; in other words, the lung secreted the oxygen. Krogh, in 1912, convincingly delivered the fatal blow to the secretion theory by first showing that in fishes there is no secretion of oxygen into the air sacs, and then by demonstrating that the amount of oxygen in the blood always equaled the amount that should be provided by his diffusion theory. The development of Krogh's microtonometer proved to be critical for verifying the results of these demonstrations.
It was not until 1916, however, that Krogh accomplished the work that would, in 1920, earn him the Nobel Prize for physiology or medicine. He showed that muscle tension was always slightly lower than the tensions in the capillaries, even when the muscle was at work. Noting that there were few open capillaries when a muscle was at rest, Krogh demonstrated that as soon as the muscle became active many capillaries began to open up. He was also able to show that blood did not enter the capillaries through the pressure of the blood vessels but from the relaxed tonus (partial contraction) of the active muscle. The relaxation of the muscle allowed the field of capillaries to open and the blood to flow in, thus providing more oxygen to the muscle, organ, or tissue.
Krogh's discoveries relating to gas exchanges in the lung and to the operation of the capillary system helped to develop medical techniques for breathing through the trachea. His work also improved surgical methods for open heart surgery, such as the procedure for reducing body temperature to below normal levels to slow down the rate of gaseous exchange.
In 1922 Krogh became interested in insulin (which had been discovered by Frederick G. Banting and John James Rickard Macleod the year before), partly because his own wife had diabetes. Besides being active in insulin research, Krogh helped to promote manufacturing facilities in Denmark for its production. Krogh also maintained his interest in zoology, writing about insects and becoming particularly attentive to theories about the way honey bees communicate.
During World War II, Krogh lived in Sweden, having been forced to flee Nazi-occupied Denmark because of his open opposition to Nazism. Krogh returned to Denmark in 1945, and died in Copenhagen in 1949. His research in his last years was performed at the laboratories of the Carlsberg and Scandinavian Insulin Foundations.
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