Lungs are the typical breathing organs of the higher vertebrates. Physiologically they represent an apparatus interposed between the two parts of a double heart and in which air and blood are brought together. Morphologically they consist of a much elaborated respiratory surface of maximum exposure within a minimum space, together with a system of non-collapsible passage-ways for admitting air from the outside, that passes over these respiratory surfaces in intimate juxtaposition with capillaries.
The passage-ways to the lungs begin either with the nasal chamber or oral cavity, leading through the pharynx to the trachea, bronchi, and bronchioles, and eventually reaching the innumerable terminal alveolar sacs that constitute the true respiratory area where the gaseous exchange of respiration is effected.
Gills have one place for the intake of the oxygen-containing water (the mouth) and another for the outgo (gill slits), so that the respiratory procession is continuous. In animals with lungs the same part is employed for the entry and departure of oxygen-containing air, with the result that inspiration and expiration become alternating processes.
It is estimated that in man the respiratory alveolar surface enmeshed in capillaries makes a total expanse of a hundred times the area of the entire skin, or, if inflated into a single sac, one that would form a balloon ten feet in diameter, yet this extensive structure is packed away in a relatively small space, the contour of which is determined largely by neighboring organs and general form of the body.
Alveoli in contact with capillaries are lined with thin pavement epithelium, but the trachea, bronchi, and in part the bronchioles leading to the alveoli, are lined with ciliated cells, the activity of which, so long as they remain moist, tends to keep the air passages free from dust and other foreign intrusions.
The lungs as a whole are highly elastic and, although encapsuled in a double pleural sac in the higher vertebrates, are freely movable within the sac, except at the point of their attachment near the base of the bronchi (Fig. 336). Here they are joined by the trachea, a stalk-like tube connecting with the pharynx, and are penetrated by the pulmonary artery and vein which hook up with the heart and the circulatory apparatus. Otherwise the lungs are free and unattached, thus enabling them to glide easily over the inner surface of the thorax with every breath while filling all the available spaces.
The whole apparatus, which somewhat resembles in form a compound sebaceous gland, might be compared to a luxuriant tree, entirely hollow in all its parts, that has been pulled up by the roots and crowded top first into a bag. The root region corresponds to the nasal chamber, the oral cavity, and the pharynx; the main trunk of the tree to the trachea; the larger branches to the bronchi that subdivide into lesser branches and twigs, or bronchioles, terminating in the leafy foliage, or alveoli, that are crowded together in such a way as to occupy all available space within the enveloping sac, or pleural envelope.
Nasal Chamber and Pharynx
The entrance to the pulmonary system is usually through the air-conditioning nasal chamber, although in higher vertebrates the oral cavity may also serve in an emergency as an entrance. The nasal chamber is lined with a mucous membrane, known as the Schneiderian membrane. The tissue beneath this lining layer is richly supplied with capillaries, thus providing moisture and a certain degree of warmth for incoming air. A moistened surface is further insured by the fact that the lacrimal ducts, from the constantly operating tear glands in the bony orbits of the eyes, drain into this chamber. The ciliated cells of the Schneiderian membrane maintain a continuous flow of mucus, and adhering particles of foreign matter, from the nasal cavity into the oropharynx.
The walls of the nasal chamber are variously enlarged in different vertebrates by scroll-like turbinate bones, which not only increase the moist vascular surface, but also prevent an easy entrance of undesirable objects by making the passage-way tortuous. This latter purpose is also furthered by a forest, more or less dense, of outward-projecting hairs guarding the entrances, or nostrils, of the nasal chambers. This part of the air passages is cleared of undesirable accumulations by “blowing the nose.” Man is the only mammal that performs this feat in any way acceptable to his neighbors.
From the nasal cavity air passes back through the nasopharynx to the oropharynx, where it crosses the food route on the way to the esophagus, thence entering the trachea through a slit-like opening, the glottis.
The trachea, or “windpipe,” is a rigid tube, very short in frogs and toads whose lungs are far anterior in the body cavity. It is somewhat longer but still insignificant in the urodeles. In lizards it is relatively shorter than in other reptiles, although unmistakably present, while in turtles and crocodiles it is frequently so long that it becomes convoluted or even spiral in form. The lengthened trachea in the chelonians is an accommodation to the accordion-like movements of the head and neck.
Birds with long necks are of course provided with a long trachea, but frequently the trachea is even longer than the neck itself, so that it cannot remain straight but loops about. In swans these extra loops are stored within the hollow breastbone (Fig. 337), while in some birds they lie coiled under the skin, or may even extend into the body cavity. Extra long tracheae make it possible for their possessors to stretch out the neck without pulling the lungs out “by the roots.”
Usually the windpipe is nearly cylindrical, but sometimes, as in the little vocal wall lizards, or “geckos,” and also certain ducks, it may show a bulblike enlargement that acts as a resonance sac when air is expelled.
In mammals the trachea is practically straight, with a length directly dependent upon that of the neck, except in the three-toed sloth, Bradypus, whose trachea is so elongated that it extends down as far as the diaphragm and back before entering the lungs. It will be remembered that the upside-down sloth, while hanging from the limbs of tropical trees, feeds lazily upon leaves without scrambling about. It therefore has a very stretchable neck.
At all times the elastic walls of the trachea are kept mechanically distended for the passage of air by encircling rings of cartilage, resembling the metal rings embedded in a garden hose to give it flexibility and durability, and at the same time to keep it uncollapsed and open. In the case of mammals these skeletal tracheal rings are usually incomplete on the dorsal side, that is, on the side liable to press against the esophagus that lies parallel to it, thus minimizing the “corduroy road” effect that might otherwise be encountered by a bolus of food when swallowed.
Among reptiles, birds, and pinniped mammals, the tracheal rings are entire, while cetaceans present the unusual case of tracheal rings incomplete on the ventral rather than the dorsal side.
Camels and giraffes are noteworthy in having upwards of one hundred separate tracheal rings, and whales and sea-cows in having these skeletal structures spirally arranged.
Although usually of hyaline cartilage, tracheal rings become bony in the python Agama, and also in many birds.
The trachea usually branches into two bronchi, that resemble it in structure with the exception of being smaller in size and in having weaker skeletal rings.
There are three bronchi in certain ruminants, pigs, and whales, but in most snakes, with the degeneration of one lung as an accommodation to the extraordinarily elongated shape of the body, there remains only one bronchus.
Bronchioles, which continue and multiply the air passages from the bronchi, have only limited cartilaginous supports which are in the form of rings. These supports become progressively smaller, and the mucous cells of the linings of the bronchioles fewer, until both are completely absent from terminal bronchioles. The latter serve simply as ducts leading the way into the ultimate air chambers, or alveoli, in which respiration occurs.
In mammals generally the bronchioles arise like the twigs of a tree and diverge from each other, but in crocodiles and birds they run together, forming intercommunicating loops instead of terminal twigs, from the sides of which alveoli are given off.
The alveolar sacs, or the respiratory part of the whole system of air passage-ways, are hemispherical enlargements at the ends of the bronchioles. They have exceedingly thin delicate highly elastic walls over the outside of which, like vines over a trellis, extends a closely woven maze of capillaries (Fig. 338). It is estimated that in a pair of human lungs there may be more than six million of these tiny chambers, all in ultimate communication with the outside atmosphere through the air-passages which unite in the trachea.
The amount of air admitted to the alveoli is automatically regulated by means of nerve endings, the headquarters of which are located in the medulla of the brain. These nerve endings are inserted into tiny cuffs of circular muscle fibers that encircle the walls of the innumerable bronchioles, causing them to constrict or relax, as occasion demands.
The phylogeny of the lungs is a story of internal modification for the increase and efficiency of the respiratory surface, and also for adaptation to the shape of the body. The storing of lungs, for example, within the body of a squat toad, a lithe cat, a capacious cow, a box turtle, or an elongated snake, presents in each instance a different problem.
A transition between the swim bladder and true lungs is found in the dipnoans which, although not ancestral to the land vertebrates, show many of the features probably possessed by the ancestors of the latter. During the aestivation of these lungfishes the gills are not used for respiratory purposes, but instead branches of the sixth aortic loops, the pulmonary arteries, bring venous or “impure” blood to the swim bladder, which then functions as a lung. Pulmonary veins return the blood to the left side of the auricle. Polypterus and Amia are the only other fishes with similar pulmonary arteries but their veins send the blood into the sinus venosus.
In Neoceratodus, the lung or swim bladder is a single wide sac, resembling the swim bladder of physostomous fishes, but in Protopterus and Lepidosiren, the sac is bilobed, its inner surface being increased somewhat by its coarse spongy alveolar structure.
Amphibians in general carry on the pulmonary plan of the dipnoans although the primitive lungs of perennibranchs are less elaborated than those of the lungfishes. The lungs of Necturus (Fig. 339), for instance, are two' long simple sacs, enmeshed on the outside by arterial and venous capillaries and opening directly through a slitlike glottis without the intervention of either trachea or bronchi. The inner surface is not increased by folds. The whole apparatus, resembling a pair of enlarged elongated alveolar sacs, is probably more hydrostatic than respiratory in function.
Amphiuma goes a step further, in that the proximal half of each lung has the inside surface considerably increased by the elaboration of folds.
Owing to the form of their bodies, frogs and toads have more spherical lungs than salamanders. The folds within the frog’s lungs extend from the inner walls in such a way as to divide the entire cavity into marginal stall-like spaces or compartments, all opening freely into a common central cavity. The double walls of the “stalls,” formed by the invagination of the outside wall, each cany a capillary network which increases the available blood supply over the surface of the lungs (Fig. 340). In toads the stalls become partially shut off from the central cavity of the lung by right angle additions along the inner margin of the partition walls, so that a secondary internal wall is formed that is perforated on all sides with openings between the central cavity and the air chambers, or stalls, which resemble semiprivate luncheon niches around the margin of a common dining room.
The very primitive tracheal and bronchial tubes of amphibians enter the lungs at the extreme anterior end. With an increased development of the anterior part of the lungs, the bronchi acquire a more lateral entrance in higher forms.
As a result of a migratory invasion into the body cavity, the lungs of amphibians become invested on the outside by a single layer of peritoneum, which is pushed ahead of them into the body cavity during development. They do not, therefore, have a separate pleural cavity of their own, but instead lie freely in a common body cavity.
Usually the left lung in amphibians is larger than the right one, but in tropical legless forms (Apoda) the reverse is true, as the left lung is rudimentary. Some salamanders, for example Eurycea and Salamandrina, are lungless, respiration being accomplished through the integument and the buccopharyngeal epithelium.
Integumental breathing is eliminated in reptiles, whose lungs, also abdominal in position, are much sacculated within and whose trachea and bronchi are developed into definite structures. The air-transferring passage-ways and the air-absorbing mechanism of the lungs are distinctly differentiated in reptiles.
The primitive New Zealand lizard, Sphenodon, has spongy lungs that might be compared to a cluster of lungs like those of a toad, opening into a common passage, or atrium (Fig. 340), while the lungs of crocodiles go a step further in elaboration, corresponding to a bouquet of Sphenodon lungs placed together, the atria of which now open into a common bronchus.
In snakes the left lung usually becomes aborted, only the right lung remaining to occupy the narrow quarters that are available. Boa constrictors and pythons are the most ancestral in this respect, having both lungs present with the left one somewhat shorter than the right.
The different levels of the long single lung of the snake are unequally elaborated, recapitulating from distal tip to the entrance at the glottis the early phylogeny of vertebrate lungs. Thus, the distal tip is a smooth sac like the lung of Necturus, but advancing toward the proximal or anterior end there develops a gradual evolution of internal folds, at first resembling the lungs of Amphiuma, and later the chambered structure of the lungs of frogs and toads. Finally, at the base, these chambers become compounded and open into an atrium, suggesting the degree of complexity arrived at in the lung of Sphenodon.
Certain lizards, particularly Chameleon (Fig. 341), have peculiar lungs with saclike diverticula, which enable them to swell up to a certain extent, a device used perhaps to frighten their enemies. The inflated lungs of sea turtles, on the other hand, probably serve as floats, or life preservers, in maintaining a position at the surface of the water.
The lungs of all modern birds are highly modified by the presence of supplementary air sacs, cellulae aereae, which facilitate the circulation of air through the lungs, but in themselves are not directly respiratory in function, as shown by the paucity of capillaries over their surfaces.
The bronchioles, instead of ending blindly in alveolar sacs, form a system of communicating loops, and open eventually into the reservoir-like air sacs (Fig. 342). Surrounding the smallest bronchioles and opening into them are elaborate meshworks of minute air tubules surrounded by capillaries so that their walls serve as the actual respiratory surfaces (Fig. 343). It is possible, therefore, for the air to be drawn back and forth entirely through the air tubules of the lungs, with gaseous exchange taking place both on the way through the lungs to the air sacs as well as on the return. The air in the lungs of other animals is never entirely renewed with each respiration, as in birds, since some of it regularly remains stagnated in the alveolar terminals.
Embryonically air sacs sprout out from the lungs at various points and extend into the body cavity, occupying spaces between the viscera; beneath the skin (in pelicans) ; between the muscles, supporting and connective tissues; between and around the joints of the cervical vertebrae; and penetrating even into the pneumatic cavities of the hollow bones (Fig. 344).
The primitive Apteryx of New Zealand alone has much reduced air sacs that do not enter the bones or penetrate the transverse septum to invade the body cavity.
Although the air sacs of the bird’s lung are not supplied to any great extent with a capillary network, and consequently are not directly respiratory in function, yet they have several different uses, acting as bellows, balloons, ballast, friction pads, heat regulators, reservoirs, and resonance aids to the voice.
On account of the elasticity of the bird’s lung, which is hampered by being anchored fast to the dorsal wall of the thoracic basket, some mechanical aid for effecting an efficient circulation of air through the lungs becomes all the more necessary. Such a mechanism is supplied by air sacs acting as bellows, enabling the air to be forced back and forth entirely through the lungs proper. In the capacity of balloons, the air sacs when inflated cause the specific gravity of the bird to be lessened owing to the intake and retention of heated air. Without this storage of warmed air it would require considerably more muscular effort to sustain a body heavier than air in suspension for considerable periods of time. Possibly inflated air sacs may also by their turgor aid mechanically in maintaining the wings in an extended position during soaring or volplaning.
As ballast the arrangement of the air sacs is such that a proper center of gravity may be established for balanced flight, and equilibrium easily maintained by shifting the air content of the sacs from one part of the body to another.
The insertion of air sacs like pads between the muscles lessens friction, thereby giving flexibility and grace to the aerial movements of birds. Because they are filled with warm moist air the air sacs help to maintain and regulate the body temperature, for the skin of birds in the absence of evaporating sweat is of little service for this purpose.
As containers of reserve air, the air sacs are undoubtedly useful. The muscular mechanism by which a resting bird causes air to enter the lungs, like that of mammals, involves the alternate elevation and depression of the breastbone through the activity of the intercostal muscles. It is necessary, however, during flight for the breastbone and the entire thoracic basket to remain firm, in order to insure substantial anchorage of the powerful flying muscles. To do this the intercostal respiratory muscles are held in tension and are for the time being not pumping fresh air into the lungs. Therefore, an internal reservoir of air is indispensable, while the flying muscles which ventilate the lungs by acting upon air sacs as bellows also control respiratory movements during flight. The more rapid the flight, the greater the automatic supply of air drawn through the lungs to and from the pneumatic chambers by the flight muscles. Violent action in mammals interferes with respiration, but with birds it enhances it. This is why fast flying birds do not “get out of breath,” or probably suffer from “mountain sickness” in the air of high altitudes because the necessary increased wing strokes bring in a compensatory supply of rarefied air. The frigate bird, Fregata, that easily maintains a rate of one hundred miles an hour, has about the best development of air sacs to be found in any bird.
The pneumatic diverticula of the lungs of Chameleon, already mentioned (Fig. 341), and those of certain other lizards may perhaps be regarded as prophetic of the air sacs of birds.
The lungs of mammals are usually characterized in two ways: first, by being subdivided externally into two lobes; and secondly, by showing some degree of asymmetry in accommodation to surrounding organs. When asymmetrical the lobes are more numerous on the right than on the left side.
Thus, in man (Fig. 345), there are three lobes in the right lung and two in the left. The uppermost odd lobe of the right lung lies behind the right pulmonary artery, while the absence of a corresponding lobe on the left side permits the presence of the large left aortic arch.
Certain mammals, as for example Cetacea, Sirenia, Proboscidea, Hyracoidea, and most Perissodactyla, resemble other vertebrates in the absence of pulmonary lobes, while Monotremata are transitional, since they possess lobes only in the right lung.
The lungs of whales, which are located rather posteriorly in the hulls of these seagoing leviathans, are probably hydrostatic as well as respiratory in function. Whales have a unique breathing apparatus that enables them, during a plunge into the ocean depths, to imprison air in the capacious nasal chamber which is several times larger than the brain case. In fact the nasal chamber occupies the major part of a whale’s head and is capable of storing a generous supply of air that would otherwise be forced out of the lungs by the enormous pressure of the water. The apertures leading from the nasal passages to the lungs can be shut off by two plugs of tissue which function like the stopper in a bathtub. As the air in the lungs becomes stale the plugs open long enough to exchange the stale air with pure air in the nasal reservoir, and when the whale finally comes to the surface the reservoir is emptied with considerable violence accompanied by confined water vapor, causing the excited whalers who witness the performance to shout, “Thar she blows!”
The lungs of higher vertebrates are enclosed in compartments called pleural cavities, separated from the abdominal cavity which is the storehouse of most of the internal organs.
The establishment in mammals of exclusive chambers for the lungs has been a gradual evolutionary process. The primitive lungs of amphibians push down into the general body cavity, carrying with them a thin covering of serosa continuous with the peritoneum that lines this common cavity but without the formation of independent pleural chambers.
In reptiles, along with the formation of a transverse septum formed by the invasion of peritoneal folds and the assurance of privacy for the heart by the partitioning off of a pericardial chamber, there is formed around the lungs a second envelope, also derived from the peritoneal serosa, that constitutes the outer, or parietal, wall of the pleural cavity (Fig. 336). The inner, or visceral, wall is the original derivative of the peritoneum already mentioned, and this intimately invests the lungs like a tight-fitting garment.
The space between the parietal and visceral walls, that is, the pleural cavity, is filled with a serous lubricating fluid which allows freedom of movement on the part of the extensible lungs within the pleural space.
Origin of the Lungs
The lungs, like the swim bladder, probably come from rudimentary gill pouches. While the swim bladder is usually a dorsal outgrowth from the floor of the foregut, the first evidence of lungs in man, which may be seen about the third week of fetal life, is a ventral groove on the floor of the same region from which the swim bladder sprouts out dorsally. As this groove pushes deeper down, it forms a single bud that soon becomes a bilobed sac representing the future lungs (Fig. 346). Soon after, a common stem or duct is formed by a further outgrowth of the lung sacs. This is the trachea, whose appearance is followed by the branching bronchi, and last of all by the elaboration of subdivisions within the lung sacs and the establishment of the alveoli.
During the differentiation of the lung sacs the endodermal lining invades the surrounding mesoderm, as shown by Moser in a series of illuminating diagrams (Fig. 347), with the end result that a maximum surface of respiratory endodermal tissue is brought into intimate contact, back to back, with vascular mesodermal tissue carrying blood-filled capillaries.
Human lungs assume definite shape before the end of the third month, although they do not take on their respiratory function as long as the embryonic placenta is active and are not entirely inflated for three or four days after birth. The alveoli are laid down by the seventh month and thereafter merely undergo enlargement.