Biological Systems in Vertebrates, Vol. 1

Functional Morphology of the Vertebrate Respiratory Systems

J N Maina

• Publisher: CRC Press
• Year: 2019
• Print ISBN: 9781138417229
• eBook ISBN: 9780429530425
• Edition: 1
• Topic: Medicine
• Subtopic: Veterinary Medicine

Gives an account of the morphologies of vertebrate respiratory organs and attempts to explicate the basis of the common and different structural and functional designs and stratagems that have evolved for acquisition of molecular oxygen. The book has been written with a broad readership in mind: students of biology as well as experts in the disciplines of zoology, physiology, morphology, biological microscopy, biomedical engineering, and ecology and those that work or may contemplate working on materials and aspects concerning respiration in whole organisms will find it useful. Scientists in earth sciences with particular interest on the outcomes of past interactions between environmental factors (the physical domain) and evolution and adaptation (the biological domain), mechanisms that have set the composition, patterning, and anatomies of extant animal life, will find the book of interest.

1

Respiration—Fundamental Principles and Concepts

1 OXYGEN: A PARADOXICAL MOLECULE

The development of eukaryotic cells from prokaryotic ones (about 2 billion years ago), the realization of sexual reproduction (about 1 billion years ago), and the accretion of independent cells into a cohesive, integrated, multicellular state about 700 million to 1 billion years ago were momentous events in the evolution and progress of animal life. These quantum events culminated in the rise of vertebrates and ultimately that of the endothermic-homeotherms, the most highly metabolically active modern taxon. Few processes in biology are as ancient and as important for life as respiration. While animals can survive for weeks without food and days without water, they continually need molecular oxygen for energy production by oxidative phospholylation. Unlike metabolic substrates such as carbohydrates and fats that can be conserved in large quantities and utilized as needed, oxygen has to be unceasingly contracted from outside. In a person weighing 70 kg, at any one moment there is only about 1.55 L of oxygen in the body. Of the total amount, 370 cm³ is found in the alveoli, about 280 cm³ in the arterial blood, 600 cm³ in the capillary and venous blood, 60 cm³ dissolved in body tissues, and 240 cm³ bound to myoglobin. The quantity of oxygen dissolved in the tissues (about 0.8 cm⁻³ kg⁻¹) can support life for about 6 minutes and only for a few seconds during exercise.

Even before the discovery of oxygen by Joseph Priestley in 1771 and three years later determination of the gaseous composition of air by Antoine Lavoisier, it was well known that breathing [i.e., ventilation of the “body” (lungs) with air = pumping air in and out of the “chest” (lungs)], a mechanical process conspicuous particularly in large animals (especially birds and mammals) was essential for life. Until recently, death was loosely associated with cessation of breathing and the common method of killing was by strangulation. The familiar phrase “breath of life” bespeaks the importance of respiration, i.e., acquisition of oxygen, for life. In an adult person, about 12,000 L of air passes through the lungs every day. Respiratory efficiency connotes the speed at which an animal uses its resources to meet the demands placed on it by the environment and the lifestyle that it pursues. Energy generation, its storage, and utilization are processes central to the metabolic performances of animals. Energy drives all biological processes from molecular to ecological levels. It is imperative for maintaining the structural and functional integrity of organisms and fortifying homeostasis against external and internal perturbations. Animals that can achieve and sustain high oxygen to carbon dioxide exchange ratios in relation to their body volumes and those that can establish stable tissue-to-fluid oxygen concentrations under various environmental conditions can attain the highest levels of aerobic metabolism. They are among the most ecologically successful organisms.

Oxygen, a product of photosynthesis by plants and cyanobacteria, has been the most singularly pervasive molecular factor in setting the form, patterning, and geographic distribution of extant animal life. By the start of the Paleozoic (about 600 million years ago), the partial pressure of oxygen (PO₂) in water and air had risen to a modest level of 0.2 kPa [i.e., one-hundredth (= 0.2% oxygen by volume)] of the modern sea level pressure. When the first vertebrates (ostracoderms) appeared on Earth some 550 million years ago, the PO₂ was only 0.9 kPa. In the Devonian Period (some 300 million years ago), when amphibians ventured onto terra firma, the PO₂ had risen to 4.7 kPa. The present level of atmospheric PO₂ (about 21 kPa) was not reached until the Carboniferous Period (some 250 million year ago) when reptiles first appeared on land. The level of atmospheric oxygen shifted greatly in the Phanerozoic. During the Carboniferous Period, for example, it rose to a hyperoxic level of 35% (compared to the present atmospheric level of 20%) and then dropped sharply to a hypoxic low of 15%. These changes were duplicated in water. They had a dramatic effect on aquatic life. The abundance of oxygen during the Mid-Devonian to Carboniferous Periods supported development of exceptionally huge animals such as the giant dragonfly-like Meganeura that reached a body length of 60 cm and a width of 3 cm. The oxygen-rich atmosphere granted higher metabolic capacities to the extant animal life. It motivated radiation into diverse ecological habitats, resulting in unprecedented speciation. Paucity of molecular oxygen during most of the Precambrian is envisaged to have curtailed progress of life from simple unicellular to complex multicellular states. The so-called “Cambrian Explosion,” an event epitomized by remarkable speciation, is attributed to an upsurge of molecular oxygen at the Precambrian-Cambrian boundary.

Constituting about one-quarter of the atoms in organic matter, molecular oxygen is a fundamental building block for life. Paradoxically, due to the generation of highly reactive oxidative species, e.g. the superoxide anion radical (O₂⁻), hydrogen peroxide (H₂O₂), hydroxyl radical (OH⁻), and singlet oxygen (¹O₂), molecular oxygen is extremely toxic to carbon-based life. As part of the antioxidant defense system, life has evolved a battery of simple nonenzymatic molecules and complex enzymes that scavenge oxidative oxygen radicals. The former includes glutathione, ascorbate, urate, bilirubin, ubiquinol, p-carotene, and tacopherol, while the latter comprises superoxide dismutase, catalase, and glutathione peroxidase. Superoxide dismutase converts O₂⁻ to H₂O₂ plus O₂ and catalases and peroxidases convert H₂O₂ to water (H₂O) and oxygen (O₂). Incorporation of an injurious molecule such as oxygen in the biochemistry of energy production indicates the importance of efficient metabolic processes and energy production for animal life. The relatively small molecular weight of oxygen, its high intracellular diffusivity, and appropriate redox potential promoted its utilization as a proton acceptor in the tricarboxylic chain reaction. Moreover, water, the end product of aerobic metabolism, is both an innocuous and a necessary substance for life.

Fermentation (glycolysis) is an inefficient way of producing energy. Much of it is left secured in the chemical bonds of organic molecules such as alcohols and organic acids—harmful products that must be cleared before they accumulate to toxic levels. Through glycolysis, a molecule of glucose yields only 2 molecules of adenosinetriphosphate (ATP) that contain about 15 kCal energy compared to 36 ATP molecules (= 263 kCal energy) produced through aerobic metabolism. When it evolved, aerobic metabolism granted an efficient means of energy production, allowing animals to invest the excess in founding complex, more optimal, and adaptable states.

While there have been assertions that life can exist without oxygen in ordinary habitats, such cases can only occur in the simplest animal life. Intestinal parasites are purported to live without oxygen and intertidal molluscan facultative anaerobes to remain for days without it. In adverse conditions, some animals enter latent (ametabolic) states. Cryptobiosis, a condition wherein life practically stops, is the most extreme of such states. However, even in such conditions an infinitely small quantity of energy is necessary to maintain important processes such as protein turnover and ion flux. In certain unique habitats, life can exist without oxygen. For example, in submarine geothermal plumes occurring at depths of 3 km or more below the ocean surface (sites where oxygen is lacking since photosynthesis is not possible due to lack of sunlight), alternative pathways of energy production have evolved. Chemoautotrophic endosymbiotic bacteria break down hydrogen sulfide (commonly present in abundance), producing ATP. This particular strategy displays nature’s profound inventiveness in circumventing the many constraints that eventuate along life’s pathways. The hydrogen sulfide/sulfur-based energy production cycle that exists around volcanic submarine vents supports flourishing colonies of animals that include giant pogonophoran tube worms, crabs, shrimps, giant clams, fishes, and mussels. It is worth noting that some of these species were unknown to science 30 years ago!

A comprehensive account of the functional morphology of the vertebrate respiratory organs must delve into how different organs and organ-systems have evolved, developed, been refined, and integrated for the purpose of gas exchange. Moreover, the description must explore these states and phenomena outside the purview of the so-called “model animals”. Among the elasmobranchs, the commonly studied species are a variety of dogfish, e.g. Scyliorhinus canicula, Squalus suckleyi, Squalus acanthias, and skates. Among the bony fish (class: Pisces) studies have been made largely on the subclass Teleosti, with the most highly investigated species being the cod (Gadus morhua), eel (Anguilla anguilla), goldfish (Carassius auratus), trout (Onchorhynchus mykiss, formerly Salmo gairdneri), and the sea raven (Hemitripterus americanus). In amphibians, the common grass frog (Rana pipiens), European frog (Rana temporaria), and the marine toad (Bufo marinus), all of which are anurans, are unfortunately taken to be representative of the diverse class Amphibia. Within the class Reptilia, particular interest has been shown in painted turtles either of genus Pseudemys or Chrysemys. The laboratory white rat (Rattus rattus), mouse (Mus musculus), and the guinea pig (Carvia porcellus) have been widely used among mammals while in birds, the domestic fowl (Gallus gallus variant domesticus), pigeon (Columba livia), muscovy duck (Cairina moschata), and guinea fowl (Numida meleagris) have been preferred. These few animals, most of which were selected more for convenience and availability than for any particular morphological or physiological merit, are not representative of the large vertebrate taxa. Many contradictory views and conflicts occurring in biology (including comparative respiratory morphology) emanate from unwarranted extrapolations and generalizations based on narrow observations and findings noted for a few unrepresentative animals.

2 GENERAL PRINCIPLES IN THE ENGINEERING OF NATURAL AND BIOLOGICAL STRUCTURES

In biology, the evolution of sound designs has been attained at enormous cost. About 99.99% of all species of animals ever evolved on Earth are now extinct. Regarding their makeup for survival, for all intents and purposes the animals that succumbed may be considered failed experiments. Except for metals, nature has experimented with virtually every construction material and produced practically every device—except the wheel! Parsimonious as nature is, the rationale behind avoiding the wheel is not all that difficult to appreciate. Since moving parts are the most susceptible to failure, engineers themselves have kept them to a minimum! The forms of nature’s technological innovations are intellectually engaging and aesthetically pleasant. Complexity pervades all levels of organization of living matter. A virus, the simplest biological entity, on average contains 10⁴ atoms, a complex organic molecule contains well over a million atoms, and a cell about 10¹⁴ atoms. For all their worth, reductionism and all mechanistic approaches in biology attempt to explain natural phenomena by recognizing, isolating, analyzing, and manipulating fewer and simpler components of a complex structure that are responsive to exact simple laws of physics.

While not very often declared, expressly or tacitly, the goal of biological science is to determine the rules that control the workings of cells, organs, organ-systems, and organisms. Biologists believe that their results will eventually be explicable at cellular and molecular levels. As observed by the great Greek philosopher Aristotle (332 B.C.), every structure exists for some reason—although the actual purpose may not always be clear, especially if the structure is a biological one. Nature doesn’t yield its secrets willingly: they have to be diligently teased from it through well-planned inquiries and painstaking attention to details. In engineering design, most devices have a single purpose. However, in biology, many structures perform more than one function, either simultaneously or successfully. If choices of structural design are many in engineering, they should be more abundant in biology. The occurrence of various solutions to a given structural problem is one of the reasons why animals have evolved into such a large variety of forms. Life’s prolific tree has produced between 5 and 50 million species of animals!

The task of developing an efficient structure is not an easy one. Billions of years of natural selection have provided a cornucopia of exquisite, imperfect, or intermediate biological structures. In spite of the fact that materials found in biology are often very different from those used in engineering, the geometries of the structures in which materials can be used to support loads are fairly much the same. Certain structural states and forms best exploit the strength properties of the materials. Albeit the fact that animals have yet to invent the wheel, nature is generally more clever than engineers at developing the potential of a given structural concept. The engineering of nature is mainly an engineering of soft tissues. Soft, resilient tissues able to support existing loads and grow and evolve have been utilized to a great extent. A common cause of accidents in engineering arises from structural failure resulting from the designer’s lack of correctly anticipating the magnitude and the direction of the loads that have to be resisted. Just as in architectural designs, in biology, the structural materials must have definable physical properties such as strength, insulation, and elasticity.

The scale of adjustment that allows a biological system to cope with shifting functional loads constitutes a reserve capacity (= safety factor). Such sufficiencies may be considered “excessive constructions” over and above those essential for basic performance. In engineering schemes, a safety factor is defined as “the ratio between the load that just causes failure of a device, i.e., the component’s maximal capacity (= strength) to the maximum load that the device is anticipated to bear during operation”. Safety factors vary greatly between different tissues and organs. Biological systems change harmonically with the fluctuating strains and stresses to which they are subjected. In composite structures (such as biological tissues), theoretically there should be room for infinite creativity. Physical (constructional) and biological (phylogenetic, developmental, functional, and ecological) controls prescribe the number of feasible outcomes (= phenotypes). Enhancing the safety margin of operation exacts commitment of more resources for construction and maintenance. By natural selection acting on the phenotype, the values of safety factors in different tissues, organs, and organ-systems is aligned to particular performance needs. Excess capacities and extravagant structures occasion unnecessary costs in terms of energy consumption for infrastructural maintenance and performance.

Unlike human engineers, nature h...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Dedication
• Table of Contents
• Preface
• Acknowledgements
• 1. Respiration—Fundamental Principles and Concepts
• 2. Gills
• 3. Skin
• 4. Swim (Air) Bladder
• 5. Transitional (Bimodal) Breathers
• 6. Amphibian Lung
• 7. Reptilian Lung
• 8. Avian Lung
• 9. Mammalian Lung
• 10. Summary and Conclusions
• References and Works to Consult
• Index