[Carl Ludwig and his place in the history of kidney physiology]

Only very little was known about the physiology of the kidneys until the middle of the 19th century. The turning point came in 1842 when the famous German physiologist and physician Carl Ludwig (1816-1895) presented a theory about a two-step process (filtration and reabsorption) leading to the excretion of urine. This paper turns the highlights on Ludwig and the period in which he lived. It also tells the story about the anatomical studies in the 17th century and the chemical approaches to urine and kidney physiology in the 18th century preceding the research of Carl Ludwig. The theory of filtration and reabsorption was opposed by another theory (the secretion theory) regarding the kidney as a gland, like for instance the salivary glands. The origin of this theory can be dated back to Marcello Malpighi (1628-1694) in the Renaissance, but it was carried further on by William Bowman (1816-1892) and especially Rudolph Heidenhain (1834-1897) in the 19th century. The research in the 1920's and 1930's marked the end of the fight between the two theories. It proofed that Ludwig was right and it gave him posthumously the recognition that he all the time had deserved.