Knocking on Heaven's Door - Lisa Randall [41]
With technology and biological systems, the individual components of the larger systems have internal structure too. After all, computers are built from microprocessors built from transistors. And when doctors look inside human beings, they find organs and blood vessels and everything else that one encounters upon dissection that are in turn built from cells and DNA that one can see only with more advanced technology. The operation of those internal elements is nothing like what we see when we observe only the surface. The elements change at smaller scales. The best description for the rules those elements follow changes as well.
Since the history of the study of physiology is in some ways analogous to the study of physical laws, and covers some of the interesting length scales for humans, let’s take a moment to think a bit about ourselves and how some aspects of the more familiar inner workings of the body were understood before turning to physics and the external world.
The collarbone is an interesting example for which the function could only be understood upon internal dissection. It has its name because on the surface it seems like a collar. But when scientists probed inside the human body they found a key-like piece to the bone that gave it another name we often use: the clavicle.
Nor did anyone understand blood circulation or the capillary system connecting arteries and veins until the early seventeenth century when William Harvey did meticulous experiments to explore the details of hearts and blood networks in animals and humans. Harvey, though English, studied medicine at the University of Padua, where he learned quite a lot from his mentor Hieronymus Fabricius, who was interested in blood flow as well but misunderstood the role of veins and their valves.
Not only did Harvey change our picture of the actual objects involved—here we have networks of arteries and veins carrying blood in a branching network to capillaries working on smaller and smaller scales—but Harvey also discovered a process. Blood is transferred back and forth to cells in ways that no one anticipated until they actually looked. Harvey discovered more than a catalog—he discovered a whole new system.
However, Harvey did not yet have the tools to physically discover the capillary system, which Marcello Malpighi succeeded in doing only in 1661. Harvey’s suggestions had included hypotheses based on theoretical arguments that were only later validated by experiments. Although Harvey made detailed illustrations, he couldn’t achieve the same level of resolution that users of the microscope such as Leeuwenhoek would subsequently attain.
Our circulatory system contains red blood cells. Those internal elements are only seven micrometers long—roughly one hundred thousandth the size of a meter stick. That’s 100 times smaller than the thickness of a credit card—about the size of a fog droplet and about 10 times smaller than what we see with the naked eye (which is in turn a bit smaller than a human hair).
Blood flow and circulation is certainly not the only human process doctors have deciphered over time. Nor has the exploration of inner structure in human beings stopped at the micrometer scale. The discovery of entirely new elements and systems has since been repeated at successively smaller scales, in humans as much as in inanimate physical systems.
Coming down in size to about a tenth of a micron—10 million times smaller than a meter—we find DNA, the fundamental building block of living beings that encodes genetic information. That size is still about 1,000 times bigger than an atom, but is nonetheless a scale where molecular physics (that is, chemistry) plays an important role. Although still not fully understood, the