Fearsome Fauna

1
Is There Room for All Those Parasites?
A proper Victorian lady is supposed to have thought of herself as being of solid wood from the neck down. Whatever problems that vision of the human body might have caused or prevented in a previous century, I suspect that it is closer to the way most of us think of the inside of ourselves than teachers of anatomy would like to believe. Sure, we know that some people have internal parts removed or even replaced, and we know the names of a number of things that are in there somewhere and maybe even approximately where they are located. What we are much less likely to know is that there is a remarkable amount of room in our bodies that is not occupied by anything remotely close to solid and that there are inner spaces of real significance.
Why are these interior spaces of such importance? They provide most of the places where internal parasites, our fearsome fauna,live and work for most of their lives--also for most of ours. There are few genuinely vacant spaces in this body of ours, but there is a large amount of fluid-filled space in our arteries and veins and in the spaces between cells and organs. And there exists almost equally large amounts of potential space that we are continually filling with food or air. Occasionally a parasite will actually burrow through or dissolve away something we think of as solid: our cornea, our muscles, our kidney, or even our brain. A few parasites are capable of hollowing out spaces in some of our solid parts. But mostly they live contentedly in the spaces we provide for them. Like a fish in the ocean or a bird in the air, they find a suitable habitat inside us and thrive in it. If we have trouble thinking of ourselves as having internal spaces rather than being mostly solid, we will certainly not be able to mentally accommodate the multitude of things that swim and grow and reproduce in those spaces. We do literally accommodate them, though, so we might as well get used to the idea that, often without drastic cost to ourselves, we provide parasites with a good and comfortable home: food, shelter, and a quiet place to copulate.
Fluid-filled space makes up 20 to 25% of our total body volume, which means that somewhere close to 5 or 6 gallons of us forms an internal ocean where a creature of appropriate size can swim mostly unimpeded. And many of these spaces are separated from the next space by only the thinnest and most easily breached barrier. The entirely fluid portion of our blood is almost a gallon, all of it in rapid circulation to all the nooks and crannies of our body. It can carrywith it not only blood cells, dissolved food, and oxygen but any sort of creature small enough to swim or float along. A nearly parallel system of lymph vessels returns blood fluids to general circulation. Unlike in the external ocean, where the search for food and sustenance is the prime motivation for life of all sorts, our internal ocean provides a banquet of food and oxygen. It is comfortable enough to support all our own microscopic cells and anything else that finds its way into that cozy habitat--and that can survive our body's attempts to get rid of it.
The potential spaces or variable spaces are mostly associated with our gut or our lungs. While we may think of our stomach as a sort of saclike structure somewhere near the middle of us, the sides of it are actually close enough to each other to rub just a bit, unless food is inside it. We are likely to feel that rubbing sensation as hunger and do what we can to expand that nearly flat space. The rest of the digestive apparatus, many yards of cylindrical intestine, is equally collapsed if there are no contents. But for most of our lives the intestine is mostly filled with food in various stages of being turned into refuse. Intestinal contents are destined to be more than just refuse. They also become us. Barring long-term starvation, our 30 feet of gut always has something in it. Some of the most unsightly parasites live and thrive in that disgusting muck. Unsightliness is almost as much in the eye of the beholder as is beauty, but tapeworms are nobody's pinups, and hookworms are the stuff of nightmares.
Parasites that live in our gut and its branches are really not inside our body but outside.Mathematical topologists (those who study the shape and surfaces of things) tell us that we and most of our fellow animals are shaped like an extraordinarily long doughnut with a very complicated hole running through the middle of it. Just like the doughnut skin, our skin covering is continuous from what covers our elbow to what lines our gut. Just as a jelly bean placed in the center of a doughnut hole is not inside the doughnut, a parasite living in our gut and its branches is not really inside our tissues. When some creature nibbles on the inner lining of our gut, it is much like a mosquito or black fly nibbling on our outer skin; it's just less visible. The mechanisms of our body that detect and reject anything that is not us don't work particularly well in our stomach or intestine. Large volumes of bacteria and other microorganisms that would be rejected in a few days if they were under our skin live comfortably all our life in the complicated habitat of our stomach and intestines, just as bacteria, fungi, and some larger creatures live and feed on our skin.
Our lungs are, like the gut, capable of dramatic expansion and contraction, though they hold much less food potential for a parasite. And they too are really part of the outside rather than the inside: kind of a complex hole pushed into the doughnut that the topologists proclaim we are. Further, in much of the lung's space for much of the time, the winds must be nearly tornadic for tiny young parasites. For them, living in a lung must be comparable to trying to live outside on the top of a 20-story building on the Chicago lakefront. For most parasites, the lungs are only a small part of anadolescent walkabout or a temporary home on the way to more comfortable and long-term quarters. Our bloodstream carries them there, and some use it as a way of getting into our gut. We cough them up and swallow them with the rest of the phlegm.
© 1999 by W. H. Freeman and Company. All rights reserved.