All animals produce heat internally when they metabolize, or produce energy from food. Mammals are endotherms, meaning they conserve and regulate this body heat. Most other animals, such as insects and lizards, are ectotherms, animals that are heated or cooled by their surroundings. Endothermy allows mammals to live in cold climates yet remain active. Moreover, the type of metabolism needed for endothermy also provides energy to perform strenuous activities for extended periods, such as migrating long distances.
Mammalian organ systems are uniquely adapted for endothermy. Figure 43-5 shows the internal anatomy of a mammal. Because of its faster metabolism, a mammal uses more oxygen and food than does a reptile of the same size. So, mammals have unique circulatory and digestive systems. Also, the body temperature of a mammal is often above that of its environment, so heat constantly escapes through the animal's skin and breath. The energy used to heat a mammal's body would be wasted without adaptations such as body insulation. Often, mammals that live in very cold climates have heavy coats of fur and, or thick layers of fat, called blubber.
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The organs and organ systems of modern mammals have a high demand for energy. The mammalian heart efficiently pumps oxygenated blood throughout the body, delivering nutrients that fuel the high-energy requirements of endothermy.
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The structure of the mammalian heart allows efficient pumping of blood throughout the body, as shown in Figure 43-6. The mammalian heart has two atria and two ventricles. A septum, or wall of tissue, completely separates the ventricles. The septum prevents oxygenated and deoxygenated blood from mixing. Recall that in the hearts of lizards and turtles, an incomplete septum allows the oxygen-rich blood and oxygen-poor blood to mix when the animal is inactive. The complete septum is an adaptation that allows mammals' bodies to transport oxygen more efficiently.
A mammal's respiratory system is adapted for efficient gas exchange. The lungs are large and contain millions of alveoli, the small sacs in which gas exchange occurs. As a result, mammalian lungs have a much larger surface area available for gas exchange than reptilian lungs do.
Mammals breathe using two mechanisms: one they share with some reptiles and one that is unique. Lizards and snakes inhale by using their rib-cage muscles to expand the thoracic cavity, the body cavity that holds the lungs. Mammals inherited this breathing mechanism but use it mostly under conditions of strenuous activity. The second mammalian breathing mechanism uses a sheet of muscle below the rib cage called the diaphragm. Contraction of the diaphragm enlarges the thorax and thus expands the thoracic cavity. At rest, mammals breathe primarily with the diaphragm.
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The red arrows show the path of oxygenated blood through the heart, and the blue arrows show the path of deoxygenated blood. Notice that the ventricles are completely separated by a septum. As a result, the blood pumped to the body contains a higher percentage of oxygen than a reptile's or fish's pumping heart can circulate.
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