11 1 Air Breathers Animal life evolved in water air breathers evolved from water breathers Although oxygen is much more plentiful in air than water and diffuses much faster terrestrial animals face other challenges such as the danger of their respiratory surface drying out Just like in cutaneous respirers where the skin must stay moist this would prevent O2 from dissolving and thus diffusing into the blood Water loss by evaporation has driven selection for respiratory surfaces that are internalized like lungs If we compare animal lineages that colonized land to their marine ancestors we see similar respiratory systems that have adapted to either water or air as a respiratory medium Arthropods are one of the major lineages that colonized land and the structure of their respiratory systems reflect their lineage For example terrestrial crabs have similar respiratory systems to their marine ancestors They have retained their internal gills but they are stiff which prevents them from collapsing in air They also use a gill bailer to ventilate their internal cavity but this propels air through their body instead of water Terrestrial spiders have book lungs that are derived from the book gills of their marine ancestors These lungs comprise a series of thin lamellae that project into an internal cavity which is connected to the outside by a hole called a spiracle Insects have extensive networks of tubes called tracheae which are connected to the external air by spiracles The tracheae branch into tracheoles which can be as narrow as 0 2 x 10 6 m and are filled with hemolymph at the ends These moist blind ends are where the O2 from the air dissolves before diffusing into the surrounding tissues Many insects actively ventilate their tracheal systems by contracting their abdominal muscles or moving their thorax Vertebrates are a second lineage that extensively colonized land after diversifying in the water The ability to breathe air is thought to have evolved multiple times within fishes creating a group of lungfishes in different parts of the world The buccal cavities of these water to land fishes are ventilated in the same way as in water where the floor of the cavity lowers to create a vacuum that draws the respiratory medium into the mouth Instead of being passed back to the gills the air is passed to a diversity of air breathing organs Amphibians begin their lives as aquatic animals before metamorphosing into terrestrial animals As larvae amphibians have external gills and perform cutaneous respiration As they become adults their lungs become more developed and their gills are resorbed into their bodies Many amphibians have simple lungs that are essentially two thin walled sacs However terrestrial frogs and toads often have highly folded layers inside their lungs that increase the surface area for gas exchange The lungs take over most of the O2 absorption but cutaneous respiration is still responsible for most of CO2 excretion These changes during ontogeny are shown in the graph below Similar to that of air breathing fish and water breathing teleost fish before them amphibians ventilate their lungs by lowering the floor of their buccal cavity While the glottis the flap that covers the channel to the lungs is closed the buccal cavity increases in volume When volume increases pressure decreases Boyle s Law so pressure in the buccal cavity becomes negative when it expands This pressure gradient draws air in through the nares or mouth but because the glottis is closed it cannot yet enter the lungs The frog holds the oxygenated air in a pocket in the buccal cavity then opens its glottis When the glottis opens the chest wall compresses the lungs and forces deoxygenated air out through the nares By sequestering the oxygenated air in a pocket little mixing with the deoxygenated air occurs during exhalation Then the nares close and the floor of the buccal cavity raises pushing air down into the lungs For this reason frogs are referred to as positive pressure breathers Other vertebrate lineages are negative pressure breathers Instead of pushing air into the lungs by reducing the volume of the chamber it is in most air breathers expand the volume of the chest cavity This creates negative pressure in the chest which draws air in Because negative pressure breathers do not need to move parts of their mouth to force air into their lungs this allows the muscles for feeding to be separate from the muscles for ventilation Instead of passing directly into the lung from the mouth other vertebrates have a specialized tube called a trachea that connects the mouth nares to the respiratory surface The respiratory cycle of negative pressure breathers is clearly divided into inhalation when air is sucked in to relieve negative pressure and exhalation when the volume of the chest cavity decreases and air is pushed out Reptiles vary in the muscles used to change the volume of their chest cavity and breathe as shown in the figures below For example in turtles the rib cage is fused to the shell so the intercostal muscles between the ribs cannot be used for this purpose Instead they have a sheet of abdominal muscle that expands and contracts the lungs Birds have stiff lungs that do not expand and contract but they still breathe via negative pressure Instead of the lung expanding and contracting there are two series of flexible air sacs anterior and posterior that are associated with the lungs in the chest cavity When air is drawn in through the nares and mouth it goes through the trachea which branches into two primary bronchi one for each lung These branch further into progressively smaller tubes called secondary bronchi then parabronchi Recall that most air breathing vertebrates ventilate their lungs tidally oxygenated air enters a passageway is conducted to the respiratory surface then returns on the same path This is less efficient than unidirectional breathing because oxygenated and deoxygenated air will mix The respiratory system of birds has adaptively evolved to be extremely efficient allowing them to live at high altitudes and fly at altitudes above where any other animals can survive One of these adaptations is that the respiratory system forms a circuit the parabronchi of each lung come back together forming secondary bronchi which come back together at the primary bronchus The parabronchi are vascularized by the capillaries of the pulmonary system and are the site of gas exchange Thus air flows in only
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