Respiratory Solutions in Terrestrial Animals
A few land animals (like spiders) have evolved highly modified gill like respiratory structures that function in air. But the hazards of desiccation for such evaginated surfaces are considerable, and major structural problems are associated with an array of filaments or a branched structure both sufficiently strong to maintain its shape against surface tension and gravity and sufficiently thin-walled to allow easy passage of gases. It is not surprising, therefore, that most terrestrial animals have evolved invaginated respiratory systems. These systems are of two principal types, lungs and tracheae. In both, the air inside the system, when viewed under a microscope such as teaching microscopes, is kept moist, and a film of water in which gases can dissolve covers the cells of the exchange surface. Thus the process of gas exchange has re¬mained essentially aquatic in laid animals, as it has in the leaf, as evident when examined under a microscope.
LUNGS
Lungs, which are invaginated gas-exchange organs limited to a par¬ticular region of the animal and dependent on a blood transport system, are most typical of two unrelated animal groups, the land snails and the higher vertebrates, including some fish, most amphibians, and all reptiles, birds, and mammals. In their simplest form, lungs are little more than chambers with a slightly increased blood supply and with some sort of passageway leading to the outside. From such a rudimentary beginning, the evolution of the lung has tended toward a greatly increased surface area, by subdivision of its inner surface into many small pockets or folds, and toward an increased blood supply to its exchange surface.
Let us look at the human respiratory system in some detail, as an example of the mammalian type. Air is drawn in through the external hares, or nostrils, and enters the nasal cavities, which function in warming and moistening the air, filtering out dust parti¬cles, and smelling. It then passes into the pharynx (throat). You will recall that the pharynx also functions as a part of the digestive system; the air and food passages cross here. During inhalation, air leaves the pharynx via a ventral opening, the glottis, and enters the larynx. (In humans, the term “ventral” refers to the front of the body.) When seen under a medical microscope, the epiglottis looks like a flap of tissue that covers the glottis during swallowing, thus preventing food from entering the larynx.
The larynx is a chamber surrounded, when studied under teaching microscopes, by a complex of cartilages (commonly called the Adam’s apple). In many animals, including humans, the larynx functions as a voice box. It contains a pair of vocal cords-elastic ridges stretched across the laryngeal cavity that vibrate when air currents pass between them; changes in the tension of the cords result in changes in the pitch of the sounds emitted.
The trachea is an air duct leading from the larynx into the thoracic cavity. Its epithelial lining, when viewed under teaching microscopes, is ciliated; the cilia beat in waves that carry foreign particles and mucus up the trachea away from the lungs. A series of C-shaped rings of cartilage are embedded in the walls of the trachea and prevent it from collapsing upon inhalation. At its lower end, it divides into two bronchi, tubes that lead toward the two lungs. Each bronchus branches and rebranches, and the bronchioles thus formed branch repeatedly in their turn, forming smaller and smaller ducts that ultimately terminate in tiny air pockets, each of which has a series of small chamberlike bulges in its walls termed alveoli. The total alveolar surface is enormous: about 100 square meters-many times greater than the total area of the skin.
The walls of the alveoli are exceedingly thin being usually only one cell thick, and a dense bed of blood capillaries surrounds each alveolus, as seen under a microscope such as a medical microscope. The alveoli are the site of the actual gas exchange and may therefore be regarded as the primary functional units of the lungs. Oxygen entering an alveolus dissolves in the film of water on its wall and then moves by either simple diffusion or facilitated diffusion across the intervening cells to the blood. Experiments have demon¬strated, with the use of specific medical microscopes, that both this movement and the reverse movement of carbon dioxide are cases of diffusion; no active transport across the cell barriers is involved.
Air is drawn into and expelled from the lungs by the mechanical process called breathing. In mammals this process generally involves muscular contractions of two regions, the rib cage and the diaphragm. The latter is a muscular partition separating the thoracic and abdom¬inal cavities. Inhalation, or inspiration, occurs when¬ever the volume of the thoracic cavity, in which the lungs lie, is increased; such an increase reduces the air pressure within the chest below the atmospheric pressure and draws air into the lungs. The increase in thoracic volume is accomplished by contractions of the rib muscles that draw the rib cage up and out, and by contraction, or downward pull, of the normally upward-arched diaphragm; the first mechanism is popularly called chest breathing, while the second is called abdominal breathing. Normal exhalation, or expiration, is a passive process; the muscles relax, allowing the rib cage to fall back to its resting position and the diaphragm to arch upward. This reduction of thoracic volume, combined with the elastic recoil of the lungs themselves, causes a rise in the pressure inside the lungs to a level above that of the outside atmosphere and drives out the air.
The pattern of airflow in the respiratory system of birds differs fundamentally from that of mammals when both specimens are examined under a microscope. In addition to paired lungs, birds possess several (most commonly eight or nine) thin-walled air sacs that occupy much of the body cavity and even penetrate into the interior of some of the bones. The air sacs are poorly supplied with blood vessels and they themselves do not absorb oxygen or release carbon dioxide. Their arrangement and bellowslike action, however, make possi¬ble continuous unidirectional flow of air through the lungs.
Birds are far more efficient than mammals in extracting oxygen from air, both because of the continuous unidirectional flow of air through their lungs, and because blood in the capillaries associated with the gas exchange surface in the lung flows at an angle to the flow of air and thereby provides some of the same benefits as the countercurrent ex¬change system of fish gills, as seen under the microscope. This superior efficiency enables birds to fly actively at high altitudes, where there is less oxygen in the atmosphere.
The mammalian and avian method of breathing is known as nega¬tive-pressure breathing, by contrast with positive pressure breathing, where air is forced into the lungs rather than drawn in. Adult frogs use both these methods. With the mouth closed and nostrils open, the frog lowers the floor of the mouth, thereby sucking air into the mouth cavity (negative-pressure method). Then it closes the nos¬trils and raises the mouth floor; this reduction in the volume of the mouth cavity exerts pressure on the imprisoned air and forces it into the lungs (positive-pressure method). (We should note in passing that a frog is an excellent example of an animal that uses a variety of gas-exchange mechanisms. The lungs are only occasionally filled, much exchange surface being provided by the thin membrane of the mouth cavity and by the soft moist skin).
