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Lecture 9

BIO2242: Textbook summary - Lecture 9

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Monash University

Respiration There are two separate but interrelated respiratory processes: cellular respirator, the oxidative processes that occur within the cells, and external respiration, the exchange of oxygen and carbon dioxide between the organism and its environment across a respiratory surface. Problems of aquatic and aerial breathing An animal’s mechanism of external respiration is determined largely by the nature of its environment. The two great arenas of animal evolution – water and land – are vastly different in their physical characteristics. The most obvious difference is that air contains far more oxygen than does water. The density and viscosity of water are approximately 800 to 50 times greater, respectively, than those of air. Furthermore, gas molecules diffuse 10,000 times more rapidly in air than in water. These differences mean that aquatic animals must have evolved very efficient ways of removing oxygen from water – yet fishes with highly efficient gills and pumping mechanisms may use as much as 20% of their energy just extracting oxygen from water. Respiratory surfaces must be thin and always kept wet with a thin film of fluid to allow diffusion of gases across an aqueous phase between the environment and the underlying circulation. This is hardly a problem for aquatic animals, immersed as they are in water, but it is a challenge for air breathers. To keep respiratory membranes moist and protected from injury, air breathers have in general developed them as invaginations of the body surface and then added pumping mechanisms to move air in and out of the respiratory region. The lung is the best example of a structure adapted for the breathing on land. In general, evaginations of the body surface, such as gills, are most suitable for aquatic respiration; invaginations, such as lungs and trachea, are best for air breathing. Respiratory organs: Gas exchange by direct diffusion Protozoa, sponges, cnidarians, and many worms respire by direct diffusion of gases between organism and environment. This kind of cutaneous respiration is not adequate when the cellular mass exceeds approximately 1mm in diameter. However, by greatly increasing the surface of the body relative to its mass, many multicellular animals can supply part or all of their oxygen requirements by direct diffusion (flatworms). Cutaneous respiration frequently supplements gill or lung breathing in larger animals such as amphibians and fishes. For example, an eel can exchange 60% of its oxygen and carbon dioxide through its highly vascular skin. During winter hibernation, frogs and even turtles exchange all respiratory gases through their skin while submerged in ponds and springs. Some lungless salamanders have larvae with gills, and gills persist in the adults of some, but adults of most species have neither lungs nor gills. Tracheal systems Insects and certain other terrestrial arthropods (centipedes, millipedes, and some spiders) have a highly specialized type of respiratory system, in many respects the simplest, most direct, and most efficient respiratory system of active animals. It is a branching system of tubes (trachea) that extends to all parts of the body. The smallest end channels are fluid-filled tracheoles, which terminate in close association with the plasma membranes of body cells. Air enters and leaves the tracheal system through valve-like openings (spiracles) that may be closed to reduce water loss. A filter may also decrease entrance of water, debris, or parasites. Some insects ventilate the tracheal system with body movement; the familiar telescoping movement of the abdomen of bees on hot summer days is an example. Respiratory pigments occur in insect blood, but because the cells have a direct pipeline to the outside, bringing oxygen in and carrying carbon dioxide out, an insect’s respiration is independent of its circulatory system. Consequently, insect blood plays a minor role in oxygen transport. Efficient exchange in water: gills Gills or branchia of various types are effective respiratory devices for life in water. Gills may be simple external extensions of the body surface, such as dermal papulae of sea stars or branchial tufts (gills) of marine worms and aquatic amphibians. The dorsal love of paddlelike appendages called parapodia also serves as an external respiratory surface for some marine polychaete worms whose blood vessels branch through the parapodia close to the surface for enhanced gas exchange. Most efficient are internal gills of fishes, molluscs, and arthropods. Fish and mollusk gills are thin filamentous structures, richly supplied with blood vessels arranged so that blood flow is opposite to the flow of water across the gills. This arrangement, called countercurrent flow, provides the greatest possible extraction of oxygen from water. In molluscs, water is moved over the gill filaments by cilia. In fish, water flows over the gills in a steady stream, pulled and pushed by an efficient, two-valved, branchial pump composed of the mouth and opercular cavities. Gill ventilation is often assisted by the fish’s forward movement through the water with its mouth open (ram ventilation). Lungs Gills are unsuitable for life in air because, when removed from the buoying water medium, gill filaments collapse, dry, and stick together; a fish out of water rapidly asphyxiates despite the abundance of oxygen around it. Consequently most air-breathing vertebrates possess highly vascularized internal cavities called lungs. Structures called lungs occur in invertebrates, but these structures are not homologous to vertebrate lungs and are usually not efficiently ventilated. Lungs that can be ventilated by muscle movements to produce a rhythmic exchange of air are characteristic of terrestrial vertebrates. Most rudimentary of vertebrate lungs are those of lungfishes, which use them to supplement, or even to replace, gill respiration during periods of drought. Although of simple construction, a lungfish lung has a capillary network in its largely unfurrowed walls, a tubelike connection to the pharynx, and a primitive ventilating system for moving air in and out of the lung. Amphibian lungs vary from simple, smooth-walled, baglike lungs of some salamanders to the subdivided lungs of frogs and toads. Total surface available for gas exchange is much increased in lungs of non-avian reptiles, which are subdivided into numerous interconnecting air sacs. Most elaborate of all are mammalian lungs containing millions of small sacs, called alveoli, each intimately associated with a rich vascular network. A large surface area is essential for the high oxygen uptake required to support the elevated metabolic rate of endothermic mammals. A disadvantage
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