Lungs, gills, and tracheas

The anatomy of a respiratory organ—gills, tracheas, or lungs—determines the way in which the organ functions. The design of the organ is specifically related to the way in which the animal functions in the world. Design refers to whether the structure is internal or external, large or small, and concerns some aspects of how it works. To understand the function of most organs and organ systems, it is necessary to know the structure.

The structure, or anatomy, can be considered from several different levels of organization. Gross structure is the size, shape, and position of the whole organ within the animal. Fine structure, on the other hand, refers to the microscopic level of organization. The microscopic structure includes the type and thickness of cells, the number of cells, and a description of the cell surface.

Common Features of Respiratory Organs

There are a few common features among gills, tracheas, and lungs in terms of both the gross structure and the fine structure. All three are organs of gas exchange and are therefore designed to permit oxygen and carbon dioxide gas to diffuse passively across this specific part of the body wall. Because one of the factors that directly affects diffusion is the total surface area, respiratory surfaces are greatly increased to maximize the movement of gas. Thus, the largest part of the total surface of an animal is the respiratory surface, no matter whether it is a lung, gill, or trachea. All three types of respiratory organs have openings to permit air or water to flow in (and out), tubes for conducting air or water to the exchange surface, the exchange surface itself, and the necessary support to hold the surface in place. The exceptions are those animals that have external gills with no enclosing cover.

The commonality of gas exchange function also imparts similar features to the microscopic level of structure. The epithelia of all three types of organs are thin and made of cells that are individually thin, providing less of a barrier to gas movement (although there is an exception to this in gill structure).

Gills

Gills are the most diverse and varied of all respiratory organs. Gills are found in aquatic animals, both vertebrate (animals with internal skeletons) and invertebrate (animals with external skeletons or no hard skeleton). With few exceptions, all animals with gills live in and breathe water, even if the volume of water is small and only one life stage of the animal breathes water. Most people are familiar with several types of animals that breathe water and use gills: fish, crabs, lobster, shrimp, crayfish, clams, aquatic insects, many worms (but not all), and numerous other animals. The form and function of gills have been variously modified in different animals through the process of natural selection.

The gills of many marine worms and of the marine sea hare and sea slug (both are related to snails) are thin, blind-ending tubules representing extensions of the body wall that form an elaborate treelike structure. For the most part, these are not complicated either in the pattern of blood flowing through them on the inside or in the way water is passed over them on the outside. Worms and sea slugs usually extend the gills into the water above and let the water’s own movement bring the oxygen-filled water to the gills. Sea hares, however, cover the gills with a flap of skin, an extension of the body wall, and pump water over the gills by the beating action of numerous tiny cilia.

The gills of crabs are formed from a central structure, called an arch, and platelike extensions of the gill arch, called lamellae. Each lamella has a thin layer of epithelium, but because it is formed as an extension of the outside layer of the body, the epithelium is covered with a very thin layer of shell, called chitin. This is true of crabs, lobster, shrimp, and all the crustaceans. Nevertheless, the layer still permits the exchange of gases between the blood on the inside and water on the outside.

Each lamella is exposed to the water on both sides and has numerous supporting cells on the inside that keep the two sides apart so that blood may flow between. These support cells are called pillar cells, and if an observer could look down between the sides, the inside would look something like a forest. Blood comes into each lamella from a single channel, an artery, located at the top, and flows out through a vein located at the bottom of the lamella.

Not all lamellae are alike, even in the same crab, much less in crabs from habitats as different as the deep ocean and a tropical forest. One of the major differences is that some epithelia are specialized to transport salts actively across the gill in much the same way as the kidney does in a mammal. These gill lamellae are thicker, with cells that are modified to transport sodium and chloride. Blue crabs, land crabs, and crayfish (which have podia instead of lamellae) are examples of animals with this type of specialization.

Not all crustaceans have gills that are lamellar in form; the crayfish and lobster have fingerlike projections of the gill arch. These projections are called podia and are more like the blind-ending tubules found in some worms than they are like the lamellae of crabs. The inside of the podia is divided, however, so that blood flows to the tip of the tube on one side and back to the animal on the other side of the podia.

Fish gills are not unlike those of a crab in many ways, but those of fish are more complex. Both are composed of flattened extensions of a trunklike support structure that contains the blood vessels. Gills of most teleost fish (bony fish, such as perch and bass) have further, or secondary, extensions from these, which are oriented perpendicular to the main lamella. The result is that the primary lamellae form a flat surface in a horizontal plane, and the secondary lamellae form the largest surface area, extending in a vertical plane. Some primitive fish, or those with adaptations to special conditions, have reduced secondary lamellae.

Some of the major differences between vertebrate and invertebrate gills are that fish gills are thinner, so that the layer separating the blood from the water is not as thick in fish. Fish-gill epithelia are usually about 20 percent as thick as those of the invertebrates described. Many fish have two different types of lamellae on the gill arch: primary and secondary lamellae. Many fish gills also contain specialized cells to transport salts across the border between the animal and the water, as happens in blue crabs.

Tracheas

Tracheas, found in insects, spiders, and related invertebrates, are long, thin tubes that extend from the outside of an animal to the inside, similar to the way a trachea leads to the lung in humans. The tracheal system, however, is made up entirely of branching tubules, each somewhat smaller than the one leading to it. The major tubes are called tracheas and the smaller ones are tracheoles; the smallest ones are as thin as hairs and reach within a very short distance of the cells inside the body. The outside opening to each trachea is controlled by a special structure, a spiracle, that can open and close the trachea.

Lungs

Lungs are the simplest type of respiratory system to describe in terms of animal type, distribution, and variations on the common form. Lungs are found only in air-breathing vertebrates, are always internalized, and have a single opening through which air flows both in and out. A single tube, the trachea, is the conducting passage for inspired and expired air. This tube branches into two smaller tubes, the bronchi, that lead to each of the individual sides, or lobes, of the lung. Tracheas and bronchi are usually, if not always, reinforced with cartilage rings for structural support to prevent collapse. Thus, animals with lungs actually have two, one on each side.

Tracheas and bronchi are not designed for gas exchange but are conducting tubes that also serve to “condition” the air before it reaches the lungs. Both tubes are lined with special cells that have hairlike projections called cilia on the surface. The cilia continually sweep small particles (such as dust) toward the outside to protect the lungs. Other cells in tracheas and bronchi secrete mucus to keep the surface moist and maintain high humidity so that air will be 100 percent humid when it reaches the alveoli.

At the end of the tubes of the lungs are the sac structures that form the gas-exchange surface. These very thin structures are the alveoli and are the site of gas movement into and out of the body. On the inside of each alveolus is an entire network of tiny blood vessels, called capillaries, that supply the blood, which is the source of the carbon dioxide that is exhaled and is the sink for the oxygen taken up at the alveoli.

Studying Respiratory Organs

The anatomy of gas-exchange organs is studied in whole living animals, in preserved specimens, and in sections prepared for microscopic examination. Both light microscopy and electron microscopy are used to study these organs. Light microscopy is simply the use of a microscope that requires standard lighting with the type of lens systems that have been used for decades; there are also some much more sophisticated systems that use specialized optics and lighting techniques. Electron microscopy uses beams of electrons to enable magnification of more than 100,000 times and the visualization of cells and their component parts. In some cases, miniaturized laser optics are used to examine the relatively large organs, such as lungs, while they are functioning. This application is often used in health-related studies and diagnosis of disease.

Studying the gross morphology of gills, tracheas, and lungs involves the dissection of a whole animal, either preserved or freshly deceased. Each option offers advantages, and both methods will be employed in a complete examination. Part of the dissection will be to determine qualitative features such as attachment points, spatial relations, and general appearance. A thorough study of the anatomy of an organ also includes measurements of features such as surface areas, number of parts, distances, and volumes. These are the quantitative measures, and they are applicable to all the organs. These numerical values are necessary to quantify the functions.

In the case of small animals, including shrews, insects, and fish the size of minnows, a dissection of the whole animal may have to be carried out with additional magnification, using a microscope designed for such work. These microscopes are called dissection microscopes and have enough distance between the lens and the object to allow dissection with small instruments by hand. In a few cases, the material is so small that microdissecting techniques developed by neurobiologists must be used. Traditional histological examination of respiratory organs has also revealed much about the design and nature of these structures. Histological examination (examining tissue structure, usually under a microscope) reveals the thickness of the epithelium, the number of layers, and some information about the nature of the material in the layers.

One of the purposes for studying the anatomy of all respiratory structures, and lungs in particular, is to improve medical applications. In veterinary medicine, knowledge of the general structure of lungs is crucial in treating diseased lungs and other respiratory problems. The use of animals in research has always been a controversial topic, but if such research is to be conducted properly, then accurate knowledge of the anatomy of the experimental animals is necessary. In this way, the animals can be kept healthy and monitored properly so that the intended purpose of the research may be realized.

Comparing Respiratory Structures

Evolutionary and taxonomic relationships are revealed in comparisons of respiratory structures in some groups of animals. Other relationships are evident in making comparisons between very different groups. There are only subtle differences in lung structure among the air-breathing vertebrates with lungs: amphibians, reptiles, and mammals. These differences occur more in the gross structure than at the cellular level, indicating how evolutionary pressures create successful adaptations.

One such adaptation is noted in the difference between whales and dolphins, or marine mammals, and land mammals. Though both groups of mammals have lungs, the abilities of marine mammals to hold their breath for extended periods of timehaemoglobin and myoglobin, which can store oxygen. Most whales can remain underwater for about sixty minutes, while dolphins typically surface every four to five minutes. Both mammals breath through blowholes on the top of their heads. While it was previously believed that marine mammals could not breathe through their mouths, in 2014, researchers noticed a Hector's dolphin off the coast of Christchurch, New Zealand, that was surfacing oddly due to a faulty or blocked blowhole. The scientists determined that the dolphin had adapted to breathe through its mouth, demonstrating the potential for evolutionary adjustments.

The differences in the gill structures of crabs, worms, and fish from various habitats give valuable insight into evolutionary changes. Land crabs and air-breathing fish have fewer gills, fewer lamellae with thicker surfaces, and special structures to hold them in position. These latter are needed in air because without the support offered by water, the gill lamellae would otherwise adhere to one another and not function. Similarly, some fish that live in water that contains too little oxygen must depend on air breathing and show these same adaptations.

One of the most widely studied aspects of comparing respiratory structures in different animals is the evolutionary transition from breathing water to breathing air. The transition from water to land has always interested scientists, and this level of anatomy is part of that interest. Biologists have examined the differences between air and water breathers in many groups of animals generally, and in groups that have both, such as the crabs and the amphibians, in particular. The features of interest include the surface area, thickness, position in the body, and pattern of circulation.

Few groups of animals display such a clear example of the evolution of structure and function in the tracheal system as insects. Insects have a respiratory system, the tracheas, that reaches nearly to the cells themselves, bringing them oxygen-laden air and removing the carbon dioxide produced by the cells. It would seem, then, that there would be little need for a circulatory system to serve that function, and indeed there is but a rudimentary one in insects.

Anatomy of respiratory structures is also studied in relation to other functions and organ systems. The circulatory system provides the blood that exchanges gases with the water or air on the outside of the animal. Thus, the anatomy of the blood vessels is an important part of the anatomy. Similarly, the control of the flow of blood to the lungs or gills is critical for respiratory system function.

The fine structure of several types of respiratory systems, but primarily gills, is part of the study of other, nonrespiratory functions. The gills of some aquatic animals play an important role in regulating the animal’s salt balance. In these animals, the fine structure of the gills shows where this process takes place and what special modifications are needed.

Principal Terms

Bronchus (pl. bronchi): An individual tube that is part of a lung and leads to one of the smaller lung parts

Diffusion: The passive movement of molecules from an area of high concentration (or pressure) to an area of lower concentration, often across a distance or a barrier

Epithelium: A thin membrane that lines or coats a surface; in respiratory organs, the layer of cells that covers or lines the surface is called the respiratory epithelium

Gill: An extension or outpocketing of the body wall of an animal that creates a limblike structure, much as the fingers of a glove extend from the palm; gills are found almost exclusively in animals that breathe water

Lamella (pl. lamellae): Lamella means platelike, and an individual lamella can be any one of several structures in the context of gas-exchange organs; these are usually found in gills

Lung: A concave inpocketing of the body wall of an animal (in contrast to a gill); found in air-breathing animals

Trachea (pl. tracheas): A tubular inpocketing of the body found mostly in insects and the spiders (arachnids), both of which are air-breathers

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