The Respiratory Environment
Every living thing of which we have any knowledge exists on the planet earth at the bottom of a vast atmospheric ocean. Air envelops not only all land surfaces but extends also to the uttermost depths of every body of water, large or small, so that aquatic as well as terrestrial animals and plants find themselves bottom-dwellers with respect to the all-inclusive atmosphere.
Air not only presses on all the external surfaces of the body at approximately fourteen pounds to the square inch, but also envelops the internal surfaces of the lungs. The oxygen contained in the air forms the indispensable setting for the drama of life. Ordinarily the amount present is about 21 per cent by volume, while approximately 78 per cent is free nitrogen, an inert gas which dilutes oxygen to livable proportions.
Although the large amount of free nitrogen in the air plays no direct part in respiration, it is a most important chemical component of the protein compounds that make up living matter. It is not, however, available for* use as protoplasm-building material in its abundant free form, but must undergo a sequence of chemical combinations through the agency of plant life before it can finally be incorporated in the animal body.
The mixture of oxygen and nitrogen which we call air is essential to life. It is said that a dog can live three months without food, three days without water, but only three minutes without air. Of all known organisms only the extremely specialized group of anaerobic bacteria seem to be able to live without the oxygen of the air, and even they obtain it by the chemical break-up of their own bodily structure.
Although the atmospheric envelope is enormously extensive in every outward direction, the only part of it occupied by living things is comparatively the merest film in thickness, where the atmosphere comes in contact with the solid earth. The greatest vertical distance from the atmospheric floor reached by any organism has been attained by modern human aviators, who in order to outstrip soaring birds have had to include in their equipment a supplementary supply of stored oxygen. The exceptional altitudes gained by human pioneers of the air must be regarded as insignificant in extent when compared with the distances involved in the horizontal exploration of the earth’s surface.
Probably eighty per cent of all known animals breathe free air. This category includes mammals, birds, reptiles, many amphibians, some fishes, the great fraternity of tracheate arthropods, besides certain gastropods among the mollusks, and some annelids. Strictly, however, no animal breathes “free air,” since a water film is necessary for every respiratory surface. In the minority innumerable aquatic invertebrates, fishes, and perennibranchiate amphibians habitually breathe air that has been dissolved in water, that is, air which occupies the invisible interstices between molecules of water. Some animals, for example whales and pulmonate snails, live habitually in water but come periodically to the surface for free air, while a few exceptional land animals, such as the terrestrial isopods and land crabs, still retain at an obvious disadvantage the primitive aquatic method of taking air in water by keeping their gills moist, although they have deserted water as a medium in which to live.
Since there is considerably more oxygen in free air than is dissolved in water, free-air breathers in general exhibit more energy than aquatic forms, living as they do in a more favorable respiratory environment. As a matter of comparison sea water contains five to seven cubic centimeters of oxygen per liter; flowing fresh water, six to eight cubic centimeters; and free air over 200 cubic centimeters per liter.
The Exchange of Gases
When the Declaration of Independence was signed no one knew that life processes are due to a form of slow combustion, dependent upon a component in air called oxygen. Lavoisier made this clear for the first time in 1777. In 1794 he was guillotined by his unappreciative fellow countrymen as “one of the enemies of the country.” Thus politics dominates science.
The exchange of gases which we term “breathing” is primarily a physical rather than a biological phenomenon. The taking in of oxygen is a process of passive diffusion that ceases as soon as the oxygen within the cells concerned balances with that outside. As a result of taking in oxygen, tissues are slowly broken down, while the energy used to build them up is released, much as “stored sunshine” from plants of the Carboniferous Period is recovered in the form of heat energy whenever these fossils which we call “coal” are burned under a draft of air. In both burning coal and living body the most conspicuous product of combustion is carbon dioxide. This is given off directly, since it acts as a poison when retained. In fact the removal of carbon dioxide is so urgent a matter that no animal can “hold its breath” very long without being compelled by an imperative stimulus quite beyond its control, to resume breathing movements. This powerful stimulus, which insures the continuous working of the mechanism of respiration, is due to an excess of carbon dioxide in the blood acting upon a reflex center in the medulla of the brain.
Unlike food, neither the carbon dioxide of metabolism nor the oxygen of the air can be stored within the body. Consequently, although respiration can be reduced to a minimum during times of exceptional inactivity, it cannot entirely cease during life, a fact that distinguishes a living energy-producing organism from dead things.
Respiration, with reference to carbon dioxide, is an excretory function, for in the maintenance of life it seems to be of more importance to get rid of this deadening gas than to acquire more oxygen, although the two processes go hand in hand and are both indispensable. Aquatic animals are easily killed in carbonized water, even when oxygen is present in sufficient amount for breathing. The excretory phase of respiration is clearly demonstrated by the fact that hydrogen sulphide, injected into the blood, is eliminated through the lungs.
Physiologists distinguish between external and internal respiration. The former is concerned with the gaseous exchange of oxygen and carbon dioxide between blood and air. The latter has to do with the essential transfer between blood and other tissues, or ultimate cells of the body, that constitutes the effective part of respiration, and brings about the release of energy characteristic of life. The distinction between external and internal respiration disappears in small animals which have not elaborated a circulatory system, the transfer of gases taking place directly through the undifferentiated surface of the organism in contact with its atmospheric environment.
The Essentials for Any Respiratory Device
In order to utilize the oxygen of the air, any living mechanism that has evolved far enough to have a true circulatory system must meet the following conditions: (1) the blood that is to receive oxygen must be separated from the air by a retaining cellular wall; (2) the wall must be sufficiently permeable to permit easy osmosis of gases; (3) the wall must be kept moist in order to permit thinness and permeability without drying up upon exposure; (4) the total walls or respiratory surfaces must be extensive enough in area to insure an adequate osmosis of oxygen for the organism concerned; and (5) a current of fresh air must be made to pass repeatedly or continuously across the respiratory surface. These conditions are met in a variety of ways by different animals.
Different Kinds of Respiratory Mechanisms
In the more primitive aquatic forms, diffuse breathing through the surface of the body precedes localized breathing through specific respiratory organs, such as gills or lungs, although both methods may be employed simultaneously, as in amphibians. Diffuse breathing is a decided handicap, however, since the necessarily delicate integument in animals that employ this method is not only subject to mechanical injuries, but its possessor must remain under water in order to escape the disastrous effects of exposure to drying air.
The two most successful breathing mechanisms among terrestrial animals are tracheal tubes and lungs. Tracheal tubes, which have been elaborated by the great specialized host of insects, consist essentially of ramifying tubes of intumed integument that admit air to the immediate neighborhood of the blood within the body cavity (Fig. 315).
Lungs are air sacs in intimate proximity to vascular networks, with elaborate modifications for increasing the respiratory surface without harmful exposure to the desiccating effects of the outside air.
In addition to the gills, skin, and lungs of vertebrates, the tracheal tubes of insects, and the more primitive ectoplasmic devices of protozoans, a museum of respiratory inventions would be bound to contain a long array of devices that different members of the animal kingdom use in solving the universal biological problem of gas exchange. To cite only a few of the more familiar of these devices among invertebrates there may be mentioned the aboral tentacles of starfishes; the respiratory tree safe within the digestive cavity of the mud-inhabiting sea cucumbers; the curious respiratory bladder of rotifers; the integument of the sinuously waving annelids; the expansive mantle of mollusks; and the compact lung-books of spiders. Among vertebrates may be added the remarkable respiratory tail of the goggle-eyed Periophthalmus of the Indo-Pacific mangrove swamps, a fish that can remain for hours out on land with only its highly vascular tail submerged in water. Finally there is an Antillean frog, Hylodes martinensis, which undergoes its entire metamorphosis within the egg, likewise accomplishing breathing during this critical period by means of a broad respiratory allantois-like tail.
It must not be forgotten, moreover, that in all reptiles, birds, and mammals, the allantois is provided as a temporary breathing organ during embryonic life. This highly vascular device for gas exchange is absorbed before hatching in the case of reptiles and birds, and in mammals is lost at birth with the placenta.
The lungless salamanders which swallow air have a pharyngo-esophageal network that acts as an accessory respiratory contrivance to supplement the integument and gills, while the larva of the South American toad Xenopus (Fig. 316), possesses a kind of integumentary chin whiskers which, according to Bles, are respiratory in function.
Certain fishes, Callichthys, Hypostomus, Doras, Misgurnus, and Cobitis, breathe by means of a vascular rectum, alternately sucking in and squirting out water through the anus. Turtles in similar fashion utilize a pair of lateral cloacal sacs with capillary walls.
The air-breathing labyrinthine fishes, Polycanthus, Osphromenus, Trichogaster, Macropodus, Ophiocephalus, Clarias, and the East Indian climbing perch, Anabas (Fig. 317), have a peculiar enlargement of the gill cavity, behind the eyes and dorsal to the first and second gill arches, in which pocket-like space there is a much folded vascular structure, the labyrinth, that meets all the requirements of a respiratory organ, under the difficult conditions of enveloping mud.
The glimmering flying fishes, Exocoetes, that enliven the surface of tropical waters, may “hold their breath” for the brief intervals during which they forsake the water, since their gills do not seem to be supplemented by any peculiar additional breathing organs, although moist gills for a brief time in air would be admirable breathing organs.
The swim bladder of fishes, as well as the accessory air sacs of birds and of some reptiles, which will be more fully described later, are both special devices connected with the function of respiration.