We living things—trees, snails, elephants, and humans—are minor interruptions in the flow of elements. We arrive, grow by assembling all necessary building blocks, survive for a while, then go through the inevitable process of returning all our borrowed ingredients to the world around us. Not a big deal. It has been going on for the three-and-a-half billion years life has been present on Earth. It's nice to realize we are part of the planet, part of the universe, composed of star dust that originated back when the solar system began, and even earlier with the Big Bang. Thinking along these lines gets me muddled, but humble. Better consider the here and now, no matter how brief it may be.
If I look at a tiny single-celled organism under a microscope, it goes about its business doing all the essential things we do: moving, eating, excreting, reproducing. What isn't so clear is how it maintains itself minute-by-minute during times when it may be motionless, not eating, or in fact not visibly doing much of anything at all. But it is doing something, and a great deal at that.
What is going on is an exchange of vitally important elements and compounds—atoms and molecules—between its own cellular substance and the surrounding world, which in this case is only a drop of water on a glass slide. The membrane that wraps up and holds a cell together is much more than just a piece of cellophane. It has a highly complex structure that is an essential controlling part of every living cell, a sensitive gatekeeper that allows things to go in and out as needs arise within the cell. Too much of this, not enough of that.
The structure of a cell membrane need not be described here, except to say its molecular arrangement is very intricate, not passive, and constantly working. So even though I can't see what is passing in and out of an Amoeba, I know there is a highly active exchange going on at the molecular and atomic levels between its tiny body and the surrounding world.
The surface area of an Amoeba's one cell is great considering its volume, so as long as the creature remains microscopic, it never builds up a deficit or is overcome by a surfeit, but maintains a nice healthy balance.
Go up the scale of life to a somewhat larger animal, one that is composed of a number of cells. Different species of fascinating little swimmers called rotifers each have a set number of cells, no more and no less than their kind calls for. Because the creatures still are close to being microscopic, and are not heavily built into thick layers of tissues, each cell obtains essential oxygen as the Amoeba does, and gets rid of carbon dioxide the same way, but together they have a problem with other accumulating wastes, so rotifers have a simple but elegant system of tubes that conduct unwanted nitrogen-containing fluids to the outside.
Getting oxygen is of prime importance to every cell, because burning food, or cellular respiration, cannot take place without it. As animals increase in size, therefore in mass or volume, oxygen in the air or surrounding water is farther and farther away from the innermost cells. Something has to be done about that, and circulation of oxygen-carrying blood is the answer. Blood also transports wastes away from cells. But this means that somehow there must be adequate exchange of atmospheric gases.
For an earthworm, a moist skin is all the breathing surface it needs, but woe betide any worm that begins to dry out, because essential gases can only pass through moist cellular membranes. Other segmented worms living in the ocean have more elaborate structures, real gills, that provide extensive surface area for the exchange of gases dissolved in the ocean.
Insects gulp air by means of accordion-like abdomens, drawing in and exhaling gases through a system of fine tubes called tracheae. They largely forego blood vessels, because the air tubes penetrate every part of their bodies, an excellent system as long as the animals remain small. For this reason insects are never the size of squirrels.
How about larger animals living in the sea? Creatures like clams and lobsters? They must solve the same problems as their lesser cousins, and do so with elegantly designed gills. The architecture of a clam's gill is extraordinary, with intricate internal plumbing (for the passage of blood) and external vanes that greatly expand the surface area exposed to water. To keep water moving across the corrugated surface, clam gills are thickly covered with microscopic hairs that rhythmically beat and cause new layers of water to flow past. A lobster lacks these ciliary hairs, but has fan-like appendages—legs of a sort—that beat the water so vigorously you can see the intake and jets being exhaled.
An ordinary pet goldfish clearly demonstrates breathing. Look at the fish head-on and you'll find a pair of flap-like valves opening and closing in its mouth. These are synchronized with gently moving gill covers on either side of the head. The fish gulps in a mouthful of water, the oral flaps close; the fish compresses its mouth (not swallowing at the moment), and the water flows into the gill chambers, past the gills where oxygen is extracted, then out the opercular openings on the sides. Nice and steady. But excite a fish and rapid gill action intensifies visibly. In fact, an increased rate of breathing in a fish indicates stress of one sort or another, perhaps fright, perhaps an oxygen deficit.
One of the great evolutionary advances in times past was when animals left an aquatic life in marshes and crept upon the land. The earliest amphibians, large salamanders, like all sluggish amphibians today, used both skin and lungs as breathing organs. A modern frog's lungs are simple sacs with corrugated inner walls to increase the surface presented to the inhaled air. Oxygen and carbon dioxide are exchanged on a regular basis. In winter, when these cold-blooded vertebrates hibernate in a pond's bottom mud, the lungs cease working and bodily needs are taken care of by a slow exchange of gas through the skin.
As more demanding advances came to be, such as warm-bloodedness with concomitant increased activity, the lungs of land dwelling vertebrates had to provide far more absorptive area for each inhalation than a frog requires. We are as good an example as any.
When at rest, each of our breaths lasts between four and six seconds and moves about half a liter of air. If we chase down the road after an errant dog, our breathing rate more than doubles, and during a single minute of serious exercise we might inhale up to 100 liters of air. What of the lungs themselves?
Without offering an anatomy lesson, the interior of human lungs consists of between 300 and 400 million tiny air sacs, all folded up inside a compact chest. Each air sac, or alveolus, is lined by an extremely thin membrane, on the other side of which lies a mesh of tiny blood vessels ready to pick up and discharge oxygen and carbon dioxide. Laid end-to-end, these little capillaries would extend almost 2,400 kilometers! What is even more astonishing is how much surface area is exposed to the atmosphere to allow for a rapid exchange of vital gases. Should all the tiny sacs be flattened out, they would cover an area of up to 80 square meters, or about the floor space of a small house. Imagine dragging that around with you!
Comments to the author Bill Amos welcomed.
© 2000 William H. Amos
| Bill Amos, a retired biologist and frequent contributor to Micscape, is an active microscopist, naturalist and author. He lives in northern Vermont's forested hill country colloquially known as the Northeast Kingdom. His home is shown right. |
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Editor's notes: Other articles by Bill Amos are in the Micscape library (link below). Use the Library search button with the author's surname as a keyword to locate them.