Respiratory organs of vertebrates

In most vertebrates, the organs of external respiration are thin-walled structures well supplied with blood vessels. Such structures bring blood into close association with the external medium so that the exchange of gases takes place across relatively small distances. There are three major types of respiratory structures in the vertebrates: gills, integumentary exchange areas, and lungs. The gills are totally external in a few forms (as in Necturus, a neotenic salamander), but in most they are composed of filamentous leaflets protected by bony plates (as in fish). Some fishes and numerous amphibians also use the body integument, or skin, as a gas-exchange structure. Both gills and lungs are formed from outpouchings of the gut wall during embryogenesis. Such structures have the advantage of a protected internal location, but this requires some sort of pumping mechanism to move the external gas-containing medium in and out.

The quantity of air or water passing through the lungs or gills each minute is known as the ventilation volume. The rate or depth of respiration may be altered to bring about adjustments in ventilation volume. The ventilation volume of humans at rest is approximately six litres per minute. This may increase to more than 100 litres per minute with increases in the rate of respiration and the quantity of air breathed in during each respiratory cycle (tidal volume). Certain portions of the airways (trachea, bronchi, bronchioles) do not participate in respiratory exchange, and the gas that fills these structures occupies an anatomical dead space of about 150 millilitres in volume. Of a tidal volume of 500 millilitres, only 350 millilitres ventilate the gas-exchange sites.

The maximum capacity of human lungs is about six litres. During normal quiet respiration, a tidal volume of about 500 millilitres is inspired and expired during every respiratory cycle. The lungs are not collapsed at the close of expiration; a certain volume of gas remains within them. At the close of the expiratory act, a normal subject may, by additional effort, expel another 1,200 millilitres of gas. Even after the most forceful expiratory effort, however, there remains a residual volume of approximately 1,200 millilitres. By the same token, at the end of a normal inspiration, further effort may succeed in drawing into the lungs an additional 3,000 millilitres.

The gills

The gills of fishes are supported by a series of gill arches encased within a chamber formed by bony plates (the operculum). A pair of gill filaments projects from each arch; between the dorsal (upper) and ventral (lower) surfaces of the filaments, there is a series of secondary folds, the lamellae, where the gas exchange takes place. The blood vessels passing through the gill arches branch into the filaments and then into still smaller vessels (capillaries) in the lamellae. Deoxygenated blood from the heart flows in the lamellae in a direction counter to that of the water flow across the exchange surfaces. In a number of fishes the water-to-blood distance across which gases must diffuse is 0.0003 to 0.003 millimetre, or about the same distance as the air-to-blood pathway in the mammalian lung.

The countercurrent flow of blood through the lamellae in relation to external water flow has much to do with the efficiency of gas exchange. Laboratory experiments in which the direction of water flow across fish gills was reversed showed that about 80 percent of the oxygen was extracted in the normal situation, while only 10 percent was extracted when water flow was reversed. The uptake of oxygen from water to blood is thus facilitated by countercurrent flow; in this way, greater efficiency of oxygen uptake is achieved by an anatomical arrangement that is free of energy expenditure by the organism. Countercurrent flow is a feature of elasmobranchs (sharks, skates) and cyclostomes (hagfishes, lampreys) as well as bony fishes.

A number of vertebrates use externalized gill structures. Some larval fishes have external gills that are lost with the appearance of the adult structures. A curious example of external gills is found in the male lungfish (Lepidosiren). At the time the male begins to care for the nest, a mass of vascular filaments (a system of blood vessels) develops as an outgrowth of the pelvic fins. The fish meets its own needs by refilling its lungs with air during periodic excursions to the water surface. When it returns to the nest, its pelvic-gill filaments are perfused with well-oxygenated blood, providing an oxygen supply for the eggs, which are more or less enveloped by the gill filaments.

It is theoretically possible for a skin that is well supplied with blood vessels to serve as a major or even the only respiratory surface. This requires a thin, moist, and heavily vascularized skin, which increases the animal’s vulnerability to enemies. In terrestrial animals a moist integument also provides a major avenue of water loss. A number of fishes and amphibians rely on the skin for much of their respiratory exchange; hibernating frogs utilize the skin for practically all their gas exchanges.

The lung

The lungs of vertebrates range from simple saclike structures found in the Dipnoi (lungfishes) to the complexly subdivided organs of mammals and birds. An increasing subdivision of the airways and the development of greater surface area at the exchange surfaces appear to be the general evolutionary trend among the higher vertebrates.

In the embryo, lungs develop as an outgrowth of the forward portion of the gut. The lung proper is connected to the outside through a series of tubes; the main tube, known as the trachea (windpipe), exits in the throat through a controllable orifice, the glottis. At the other end the trachea subdivides into secondary tubes (bronchi), in varying degree among different vertebrate groups.

The trachea of amphibians is not divided into secondary tubes but ends abruptly at the lungs. The relatively simple lungs of frogs are subdivided by incomplete walls (septa), and between the larger septa are secondary septa that surround the air spaces where gas exchange occurs. The diameter of these air spaces (alveoli) in lower vertebrates is larger than in mammals: The alveolus in the frog is about 10 times the diameter of the human alveolus. The smaller alveoli in mammals are associated with a greater surface for gas exchange: although the respiratory surface of the frog (Rana) is about 20 square centimetres per cubic centimetre (50.8 square inches per one cubic inch) of air, that of humans is about 300 square centimetres.

An important characteristic of the lungs is their elasticity. An elastic material is one that tends to return to its initial state after the removal of a deforming force. Elastic tissues behave like springs. As the lungs are inflated, there is an accompanying increase in the energy stored within the elastic tissues of the lungs, just as energy is stored in a stretched rubber band. The conversion of this stored, or potential, energy into kinetic, or active, energy during the deflation process supplies part of the force needed for the expulsion of gases. A portion of the energy put into expansion is thus recovered during deflation. The elastic properties of the lungs have been studied by inflating them with air or liquid and measuring the resulting pressures. Muscular effort supplies the motive power for expanding the lungs, and this is translated into the pressure required to produce lung inflation. It must be great enough to overcome (1) the elasticity of the lung and its surface lining; (2) the frictional resistance of the lungs; (3) the elasticity of the thorax or thoracoabdominal cavity; (4) frictional resistance in the body-wall structures; (5) resistance inherent in the contracting muscles; and (6) the airway resistance. The laboured breathing of the asthmatic is an example of the added muscular effort necessary to achieve adequate lung inflation when airway resistance is high, owing to narrowing of the tubes of the airways.

Studies of the pressure-volume relationship of lungs filled with salt solution or air have shown that the pressure required to inflate the lungs to a given volume is less when the lungs are filled with liquid than when they are filled with air. The differences in the two circumstances have been thought to result from the nature of the environment-alveolar interface, that interface being liquid-liquid in the fluid-filled lung and gas-liquid in the air-filled lung. In the case of the latter, the pressure-volume relationship represents the combined effects of the elastic properties of the lung wall plus the surface tension of the film, or surface coating, lining the lungs. Surface tension is the property, resulting from molecular forces, that exists in the surface film of all liquids and tends to contract the volume into a form with the least surface area; the particles in the surface are inwardly attracted, thus resulting in tension. Surface tension is nearly zero in the fluid-filled lung.

The alveoli of the lungs are elastic bodies of nonuniform size. If their surfaces had a uniform surface tension, small alveoli would tend to collapse into large ones. The result in the lungs would be an unstable condition in which some alveoli would collapse and others would overexpand. This does not normally occur in the lung because of the properties of its surface coating (surfactant), a complex substance composed of lipid and protein. Surfactant causes the surface tension to change in a nonlinear way with changes in surface area. As a result, when the lungs fill with air, the surface tensions of the inflated alveoli are less than those of the relatively undistended alveoli. This results in a stabilization of alveoli of differing sizes and prevents the emptying of small alveoli into larger ones. It has been suggested that compression wrinkles of the surface coating and attractive forces between adjacent wrinkles inhibit expansion. Surfactants have been reported to be present in the lungs of birds, reptiles, and amphibians.