1Diagram 1: The three phases of gas exchange1.Breathing1.Breathing• When an animal breathes, a large, moist internal surface is exposed to air. O2diffuses across the cells lining the lungs and into surrounding blood vessels. At the same time, CO2diffuses out of the blood and into the lungs. As the animal exhales, CO2is removed from the body2. Transport of gases by the circulatory system• The O2that diffused into the blood attaches to hemoglobin in red blood cells. The red vessels in the diagram are transporting O2-rich blood from the lungs to capillaries in the body’s tissues. CO2is also transported in blood from the tissues back to the lungs, shown here by the blue vessels3. Body cells take up O2from the blood and release CO2to the blood• O2functions in cellular respiration in the mitochondria as the final electron acceptor in the stepwise break down of fuel molecules. H2O and CO2are waste product, and ATP is produces to power cellular work2The part of an animal where gases exchange are exchanged with the environment is called the respiratory surface. Respiratory surfaces are made up of living cells whose plasma the respiratory surface. Respiratory surfaces are made up of living cells whose plasma membranes must be wet to function properly.Gas Exchange takes place by diffusion. The surface area of the respiratory surface must be extensive enough to take up sufficient O2for every cell in the body and to dispose of waste CO2.Figure 1: The earthworm is an example of gas exchange through the use of the entire outer skin. Oxygen diffuses into a dense net of thin-walled capillaries lying just beneath the skin. Earthworm's and other “skin breathers” must live in damp places or in water because their whole body surface has to stay moist. Small size or flatness provides a high ratio of respiratory surface to body volume, allowing for sufficient gas exchange for the entire bodyFigure 2: Gills have evolved in most aquatic animals. Gills are extensions, or outfoldings, of the body surface specialized for gas exchange. A fish has a set of feather-like gills on each side of its head. O2diffuses across the gill surfaces into capillaries, and CO2diffuses into the opposite direction, out of the capillaries and into the external environment.3Figure 3: The tracheal system of an insect is an extensive system of branching internal tubes with the respiratory surface found at their tips. The smallest branches exchange tubes with the respiratory surface found at their tips. The smallest branches exchange gases directly with body cells and therefore gas exchange in insects requires no assistance form the circulatory system.Figure 4: Most terrestrial vertebrates have lungs which are internal sac lined with moist epithelium. As the diagram indicates, the inner surfaces of the lungs branch extensively, forming a large respiratory surface. Gases are carried between the lungs and the body cells by the circulatory system.4Diagram 2: Structureof Fish GillsStructureof Fish GillsThere are four supporting gills arches on each side of the body. Two rows of gill filaments project from each gill arch. Each filament bears many platelike structures called lamellae (singular, lamella) which are the actual respiratory surfaces. A lamella is full of tiny capillaries that are separated from the outside by only one of or a few layers of cells. Ventilation refers to any mechanism that increases the flow of the surrounding water or air over the respiratory surface (gills, tracheae, or lungs). Increasing this flow ensures a fresh supply of O2and the removal of CO2. Swimming fish simply open their mouths and let water flow over the gills while pumping water across the gills by the coordinated opening and closing of the mouth and operculum, the stiff flap that covers and protects the gills. Countercurrent ExchangeThe arrangement of capillaries in a fish gill enhances gas exchange. Blood flows opposite the movement of water past the gills. This makes it possible to transfer oxygen to the blood by an efficient process called countercurrent. Countercurrent exchange is the transfer of a substance from a fluid moving in one direction to another fluid moving in the opposite direction. 5Air contains a much more higher concentration of O2, the air is much lighter and easier to move than water. Thus, a terrestrial animal expends much less energy than an aquatic 2move than water. Thus, a terrestrial animal expends much less energy than an aquatic animal ventilating its reparatory surface. The main problem facing any air breathing animal is the loss of water to the air by evaporation.Figure 5: Illustrates the tracheal system in a grasshopper. The largest tubes, called tracheae, open to the outside and are reinforced by rings of chitin, as shown in the blowup on the bottom right of the figureFigure 6: An insect in flight has a very high metabolic rate and consumes 10-200 times more O2than it does at rest. In many insects, alternating contraction and relaxation of the flight muscles rapidly pumps air through the tracheal system.6It now seems clear that tetrapods first evolved in shallow water from what some researchers jokingly call ”fishapods’. These ancient forms had both gills and lungs. The researchers jokingly call ”fishapods’. These ancient forms had both gills and lungs. The adaptations for air-breathing evident in their fossils include a stronger and elongated snout and a muscular neck that enabled the animal to life the head clear of water into the unsupportive air. Strengthening of the lower jaw may have facilitated the pumping motion presumed to be used by early air-breathing tetrapods and still employed by frogs to inflate their lungs. The recently discovered 375 million year old fossil of Tiktaalik illustrates some of these air-breathing adaptations.7Mammalian lungs are located in the chest or thoracic cavity and protected by the supportive rib cage. The thoracic cavity is separated from the abdominal cavity by a sheet supportive rib cage. The thoracic cavity is separated from the abdominal cavity by a sheet of muscle called the diaphragm.Diagram 4 shows the human respiratory. Air enters our respiratory system through the nostrils. It is filtered by hairs and warmed, humidified, and sampled for orders as it flows through a maze of space in the nasal cavity. We can also draw in air through the mouth, but mouth breathing does not allow the air to be processed by the nasal cavity. From the nasal