Chapter 24: The Respiratory System
Respiratory System: Chapter 24
by Brian Wisenden
The respiratory system serves to exchange oxygen and carbon dioxide between the external environment and the internal environment. Very small animals have a relatively high surface area to volume ratio allowing them to meet all their metabolic gas needs by diffusion through the skin. Large animals must transport gases between the internal tissues and a diffusion surface exposed to the external medium (water or air). For this reason, the evolution of the respiratory system is closely linked to the evolution of the circulatory system.
A respiratory system needs three things: (1) a large respiratory surface area (gills or lungs), (2) a method for ventilating the respiratory surface, and (3) a pump and circulatory system (heart and blood vessels) to distribute the gases. The circulatory system is described in more detail in chapters 21 and 22. In considering the evolution of the human respiratory system, I will first cover the evolution of gas exchange surfaces, then mechanisms for ventilating them.
The simplest and most ancestral respiratory surface is the skin. This is true for very small pre-vertebrates that have no specialized gas exchange surface, but it is also true for some larger vertebrates. Amphibians derive up to 100% of their gas exchange through cutaneous respiration. Fishes, reptiles and even humans achieve some direct exchange of respiratory gases through the skin. Generally, the larger the mass of the animal, the lower the proportion of the gas exchange needs met by direct diffusion through the skin.
(Kardong fig 11.6, p 405)
The complex of inter-related traits characteristic of the vertebrates include the innovation of gills. A muscular pharynx allowed animals to switch away from cilia-driven water currents for filter feeding and instead use pharyngeal pumping to drive filter feeding. Cilia are effective only when the gut lumen is relatively narrow. A muscular pharynx allowed efficient water ventilation for any body size. Larger bodies meant a wider range of potential prey items, but created a greater distance between the skin and internal tissues. The argument for cephalization, that the sense organs should be concentrated where the body first encounters the external environment, applies equally to the respiratory system. The internal blood vessels should be concentrated at the location of the best and most reliable supply of fresh oxygen. The most ready access to oxygenated water is the fresh and continuous flow of water circulating through the pharyngeal filter mechanism. Any increase in surface area due to elaboration of the arch surface would be promoted by natural selection. Blood vessels associated with the pharyngeal arches proliferated. Ultimately, gills formed as sheet-like structures extending from the arch surface, folded and refolded, richly supplied by blood vessels and providing tremendous surface area for gas exchange.
Gills are effective only in aquatic habitats. Fishes have internal gills associated with pharyngeal bars. Larval amphibians have internal gills (frog tadpoles) and external gills (salamanders and newts). External gills are ventilated by body movement and gill sweeping rather than pharyngeal pumping.
A common misconception is that fish evolved swim bladders for buoyancy control and then later co-opted swim bladders for gas exchange, In fact, it is the other way around. Oxygen concentration in air is about 18%, whereas in water, under ideal conditions, it is only about 0.001%. Oxygen is at least 18,000 times more concentrated in air than it is in water. Other factors further decrease the amount of oxygen in water. At warm temperatures the ability of water to hold oxygen declines. Also, in small water bodies organic debris often collects and decays. Decay is an oxidative process performed by microorganisms. Organic decay commonly strips the water of its dissolved oxygen. The only way to survive in such a habitat is to obtain oxygen from the air. Fish gulp at the surface when starved for oxygen because at the surface they can pull in water that is in direct contact with the air. Under these conditions, the ability to hold even a small air bubble against a vascularized surface would provide convenient and efficient access to oxygen. A number of fishes can do this. For example, the anabantoids (e.g. gouramis, betta fish) live in warm swamps in Asia. They have a labyrinth of vascularized tubes in their head for holding air bubbles. Many will drown if denied access to the surface. Other fishes have an enlarged region of the esophagus specialized for holding an air bubble. The fish swim to the surface, gulp a bubble and swallow it. The esophageal pocket is well vascularized for extracting atmospheric oxygen. Lungfishes are specialized examples of this ability.
Lungfishes have an internal nares, or nostril. The nose of most fishes is a blind loop with an incurrent and excurrent opening on the surface of the snout. Water is circulated through the loop via cilia, and chemoreceptors in the nasal sac detect water-borne odorants. In the lungfishes, the loop is twisted about so that one end of the tube opens into the anterior portion of the mouth cavity. Lungfish can surreptitiously and effortlessly raise their snout to the water surface and exchange air in and out of their lungs. Lungfishes, coincidentally, have stumpy fins pre-adapted for bearing weight. The ability to breathe air, and resist the crushing effect of gravity out of water, afforded the extinct lungfishes of the Rhipidistia an evolutionary opportunity to exploit the arthropod-rich margins of water bodies, and a refuge from water-based predators. The ability to disperse from a temporary water body as it dried up would also promote individuals with an ability to walk/crawl on land and use atmospheric oxygen. The fossil record clearly shows that from the rhipidistians came the amphibians.
Lungfish lungs extend from the ventral surface of the esophagus (Kardong fig 11.5, p404). The ventral connection is significant because it is retained throughout vertebrate evolution and explains why the trachea in humans is ventral to the esophagus. In "higher" fishes, the esophageal out pocketing is used mainly for buoyancy control and in these species the duct to the swim bladder extends from the dorsal surface of the esophagus. A dorsal swim bladder helps keep a fish's dorsal side oriented upward. A ventral lung connection may have assisted lungfishes in raising the snout to the surface for gas exchange. Another important difference between the lung of lungfishes and the swim bladder used for buoyancy is that lungs have internal divisions that increase internal surface area for gas exchange. The internal divisions in lungfish lungs are called faveoli. Faveoili are extensions of the lung wall into the lung lumen.
Amphibians retain lungfish lung anatomy but add a few more faveoli to increase surface area for gas exchange. Amphibians continue to rely on cutaneous respiration. Consequently, natural selection on lung efficiency did not produce structural changes until the reptiles.
Reptiles have a hard, water resistant skin that precludes cutaneous respiration. Internal surface area of the lungs is increased greatly by proliferation of faveoli.. The premaxillae, the maxiallae and the palatine bones extend across the mouth cavity from the jaw margin and meet at the midline to form a shelf that keeps air flow from the internal nares separate from the processing of food. This shelf is the secondary palate and reptiles evolved it in three independent lines of evolution. The secondary palate allows reptiles (and their descendant mammals) to eat and breath at the same time. Air flow in amphibians enters the anterior margin of the mouth and is blocked by large food items. Amphibians must hold their breath, or rely on cutaneous respiration while feeding.
Mammal lungs are divided internally by alveoli instead of faveoli (Liem et al. fig 18-15, p592). Faveoli are extensions of the lung wall that open to a common lumen. Alveoli are blind sacs at the end of long, branching and ever narrowing air passages that ultimately connect to the trachea. There is no common lumen. Gas exchange occurs exclusively in the alveoli. Mammal lungs have about 10 times more internal surface area than a similar-sized amphibian lung. This level of efficiency is necessary to support the metabolic rate associated with homeothermy.
Bird lungs are the most efficient of the vertebrates because they work on unidirectional flow of air across the gas exchange surface. When a bird inhales, air sacs behind the lungs draw fresh air from the trachea while air sacs anterior to the lungs remove stale air from the gas exchange surface. When a bird exhales, the posterior air sacs push their fresh air into the lungs for gas exchange while the anterior air sacs exhale their stale air to the trachea and out. There is no "dead space" in bird lungs. Bird lungs do not have alveoli.
Cilia are the most primitive mechanism for moving water across a gas exchange surface. The advantage of cilia is that they have a low metabolic cost. The disadvantages are that they are useful only in water, they are limited to surface structures and that they are for small spaces only. Protovertebrates and the jawless fishes use cilia to a greater or lesser extent.
Ram ventilation is achieved by swimming forward with an open mouth. This mechanism is used by tuna and some sharks. While most animals are in motion some or even most of the time, few are in motion all of the time. Therefore ram ventilation is not relied upon as the only mechanism for ventilating gills.
The advent of a muscular pharynx allowed vertebrates to actively pump water across the gills. A dual pump (Kardong fig 11.10, p 408) is where buccal and opercular pumps work in tandem to deliver a continuous, unidirectional flow of water across the gills. This is the common ventilation method used by fishes and larval amphibians. The buccal valve (mouth) opens and the opercular (gill flap) closes as the floor of the mouth is lowered. This draws water into the mouth (pump #1) and across the gills into the opercular chamber (pump #2). Then the mouth closes and the opercular flap opens while the floor of the mouth is raised. This forces water from the buccal cavity across the gills into the opercular cavity, and water from the opercular cavity out into the external environment.
The buccal pump (Liem et al fig 18-14 A-D, p591), also called "positive breathing", uses positive pressure created by the buccal cavity to force air into the lungs. It is derived from the dual pump method of ventilation. Air is gulped into the buccal cavity by opening the mouth, closing the throat and lowering the floor of the buccal cavity. Then the mouth is closed, the throat opened and the floor of the mouth is raised forcing air to inflate the lungs. This is the mechanism used by lung fishes and amphibians. Exhalation is passive aided by hydrostatic pressure (lung fishes) or elastic properties (amphibians). The disadvantage of this ventilation method is that they cannot eat and breath simultaneously.
The aspiration pump, or "negative breathing", relies on rib musculature and a diaphragm to create a partial vacuum or negative pressure to suck air into the lungs (Liem et al. fig 18-16, p593). This confers the fitness benefits of better gas exchange and better feeding. Reptiles are thought to be the first to use the aspiration pump method of ventilation but fossil evidence of the early labyrinthodont amphibians (first descendants of the lungfishes) had heavy scales and well developed ribs, suggesting that they may have used an aspiration pump lost secondarily in modern amphibia. Mammals, including humans of course, use the aspiration pump. The complicated air circulation patterns of birds is also driven by negative pressure of an aspiration pump. (Kardong, Fig 11.36, p427)
Kardong 2002 McGraw Hill
Liem at al. 2001 Harcourt