Devices for Securing Air
The process of breathing demands more than a mechanism that simply allows blood and air to get together within osmotic distance of each other, since there must also be present means for securing a continuous circulation of fresh air across the respiratory surfaces.
In the case of submerged fishes, water charged with various gases, including the essential oxygen, enters either the mouth or the spiracles and passes out through the gill slits. It is forwarded and directed in its course, not only by muscular movements which alternately expand and contract the walls of the orobranchial chamber, but also by a system of valves that prevents the water from going the wrong way.
The anterior set of these valves are collapsible folds along the inner edge of the mouth opening, those of the upper edge being called maxillary, and of the lower, mandibular. They reach their greatest differentiation in teleost fishes which have a well-developed opercular apparatus. The posterior, or branchiostegal, set of valves are membranes along the free margins of the opercular flaps (Fig. 348). A freely moving current of water is produced in the following way. First, the mouth remains open as a narrow slit, while the anterior maxillary and mandibular valves lie flat, or open, and the posterior branchiostegal valves close. Next the walls of the orobranchial chamber spread apart by muscular action, thus pulling water into the mouth to occupy the increased space. Then the valves reverse, that is the anterior ones close the slit-like mouth aperture and the posterior ones open the opercular slit, while the walls of the orobranchial cavity squeeze together, forcing the water backward over the gills and out of the opercular openings.
In some teleosts, particularly those that feed on microscopic plankton, the branchiostegal valves play the major role in this process, but in others, for example the Percidae and the Sciaeridae, the opercular flaps take the most prominent part.
Amphibians never breathe through the open mouth but instead inspire air through the nostrils and the choanae, as the newly-established passage-ways into the mouth cavity are called. They are not even able to exercise emergency breathing through the mouth, as mammals do, for as long as the mouth remains open there is no way to compel the air to enter the lungs.
Necturus and other perennibranchiate urodeles sometimes come to the surface of the water and gulp air through the mouth, which may soon be seen escaping in the form of bubbles through the gill slits, although it is doubtful whether much of it reaches the lungs. This occasional air-gulping behavior does not furnish fresh air for the external gills that hang outside the gill slits, since by means of muscles that cause them to wave back and forth, these animals obtain their supply of oxygen dissolved in the water.
The intake of air in the frog, which may be taken as a representative of the anurans, is accomplished by a combination of pump-like throat muscles and nostril valves (Fig. 349). It will be seen that when the nostril valves open in the manner of lids and the throat muscles draw down, the oral cavity within is enlarged and air is necessarily inhaled. With the closure of the nostril valves and the contraction of the throat muscles, the lungs automatically become filled by the mouthful of air that is forced backward. The expiration of air alternates with inhalation and is accomplished by means of the contraction of body muscles.
The problem of getting air into the lungs of reptiles is much like that in the case of amphibians, although some improvement is seen since ribs and rib muscles furnish a mechanical means for admitting air that is not present in the practically ribless amphibians. This improvement, however, is ineffective in turtles, whose ribs form a boxlike armor of uncompromising rigidity. These animals still resort to the amphibian method of utilizing throat muscles and nostril valves, swallowing the air by “working the throat.” No doubt the in-and-out movements of the turtle’s head and neck aid in pumping air into the lungs while the pectoral muscles, which are inside of the ribs in these bizarre reptiles instead of outside as in other vertebrates, are aided by the abdominal muscles in bringing about the expulsion of air from the lungs. The usefulness of rib muscles in pumping air in and out of the lungs is very apparent in the panting snakes, lizards, and alligators.
The latter have an exceptionally elongated nasal passage-way with a curtain, or velum, that closes off the inner choanal openings from the mouth cavity. This device makes it possible for the alligator to breathe with the mouth open under water while holding the drowning prey between the cavernous jaws, only the tip of the snout with the openings of the external nares being above the water line (Fig. 350).
So long as a bird is not in flight it breathes by means of its rib muscles after the typical reptilian manner. When a bird flies, however, as already explained, the powerful pectoral muscles, on which flight depends, require and secure anchorage upon a rigid thoracic basket that does not change shape with every breath. The bellows-like air sacs, which are filled and emptied by the action of the flying muscles rather than the rib muscles, furnish an effective means for irrigating the lungs of a flying bird with air, while the inactive rib muscles remain temporarily fixed and rigid.
In mammals both nasal and oral breathing are made possible by the backward migration of the glottis to a position in the posterior region in the throat. Nasal breathing, however, with the greater facilities thus provided for warming and moistening the inhaled air, and the added advantage of testing its quality by means of its passage over the sensitive olfactory surfaces in the nasal chamber, is the better and more favored method among mammals generally.
The outstanding advance in the breathing mechanism of mammals is furnished by the muscular diaphragm, which in lower vertebrates is foreshadowed by the transverse septum that separates pleural and pericardial chambers from the body cavity. The diaphragm consists of a central tendinous component, from which extend radiating muscle fibers derived from mid-cervical myotomes. The diaphragm when relaxed is shaped somewhat like an arched vault (Figs. 351), and is perforated by the dorsal aorta, esophagus, azygos vein, thoracic duct, postcava, and vagus nerve. As its radiating fibers shorten by contraction, the vault of the cuplike diaphragm lowers or flattens, thus increasing the space within the thoracic cavity. Consequently the atmospheric pressure from without forces air into the lungs. At the same time the viscera within the body cavity are crowded down so that the abdominal wall bulges out. The muscular opponents of the diaphragm are the strong walls of the abdomen.
In addition to abdominal diaphragmic breathing, mammals also utilize the reptilian method of rib muscles to enlarge the thoracic cavity when inspiring air. The ribs are bent, like jointed levers at an oblique angle to the vertebral column, and if acted upon by the intercostal muscles the movable sternum, to which they are attached ventrally, moves farther away from the relatively stationary backbone, thus enlarging the thoracic cavity in which the lungs are located. So it comes about that inspiration is effected not only by the depression of the diaphragm but also by the elevation of the ribs, both efforts calling for muscular activity.
Expiration, on the other hand, is to a large extent automatic through the elasticity of the stretched body walls, the taut cartilaginous ends of the bent ribs, and the tensity of the expanded lung tissues.
In big heavy animals, abdominal or diaphragmic breathing predominates over rib breathing. Jumping animals, like kangaroos and monkeys, utilize rib muscles rather more than the diaphragm in respiration.
Breathing by means of the ribs is also more pronounced in human females than in males in whom abdominal breathing predominates. The reason for the sexual difference in the respiratory mechanism may be an evolutionary adaptation brought about in connection with pregnancy, during which period the presence of a growing fetus interferes somewhat with freedom of movement of the diaphragm.
The amount of air required daily by a human being varies within wide limits but may average over 10,000 liters, including over 2000 liters of oxygen. The lungs of an adult may have an average capacity of 4000 cc of which some 500 cc, called tidal air, is renewed with every breath. Of the remainder, about 2500 cc is complemental air that can be inspired on occasion by deep breathing at the end of a normal inspiration of tidal air. The other 1000 cc is supplemental air, or the amount which can theoretically be forcibly expired after a normal expiration. After the deepest possible expiration there is still about a liter of air left in the lungs and respiratory tract. This residual air is not included in the data for lung capacity, as given above.