Marcello Malpighi and Capillaries

Marcello Malpighi.

When discussing distances and sizes in Chapter 1 of Intermediate Physics for Medicine and Biology, Russ Hobbie and I write

Blood flows through a branching network of vessels (Sect. 1.19), the smallest of which are capillaries. Each capillary has a diameter of about 8 μm, meaning that the red blood cells can barely pass through it single-file.

Eight microns is pretty small; too small to see with your naked eye. So how did we learn about capillaries? The first person to observe capillaries was the Italian microbiologist Marcello Malpighi (1628–1694). He was one of the early users of a microscope.

Malpighi received a medical degree from the University of Bologna, and eventually became the private physician for Pope Innocent XII. He favored rational medicine, based on empirical evidence, during a time when much medicine was still based on ancient authorities with little experimental support.

English physician William Harvey had previously discovered the circulation of blood, showing that the heart pumped blood to the body through the arteries and then returned it to the heart through the veins. Harvey’s theory implied there must be some connection between the arteries and veins so blood could flow in a continuous circuit, but he never identified that connection. Even the great anatomist Andreas Vesalius had wrong ideas about this issue. Malpighi’s greatest discovery was using the microscope to observe capillaries, the tiny blood vessels joining an artery to a vein. He was one of the first scientists to observe red blood cells, and also studied the clotting of blood. He described this research in his book De Polypo Cordis, published in 1666. In addition, he examined the structure of small insects, such as the silkworm, and discovered that they do not use lungs to breathe but instead small holes in their skin called tracheae (Homework Problem 42 in Chapter 4 of IPMB discusses this issue further). He also observed the microstructure of plants and the development of chick embryos. 

Early microscopists like Malpighi, as well as Robert Hooke and Antonie van Leeuwenhoek, demonstrate why Russ and I encourage readers to think about distances and sizes in the very first section of IPMB. Having an intuitive feel for distance scales—what can be seen with the naked eye, what requires a light microscope, and what is too small to see even with a microscope—is so important for understanding biology as well as the history of biology. Early microscopists explored with world of life at a scale from one to one hundred microns; the realm of protozoans, cells, and bacteria. Below that scale, diffraction blurs images, and studies of viruses and macromolecules had to await more advanced technologies such as the electron microscope. 

Malpighi died in 1694, at the age of 66, from a stroke. He is often considered the father of histology and embryology.

Under the Microscope – Objectivity 102

https://www.youtube.com/watch?v=9QcJkgDa1dk