Reptiles have a well-developed blood circulation that plays a central role in transport of respiratory gases. In most species oxygen is acquired from the environment at the internal lung surfaces, which are ventilated by movements that alternately expand and contract the body compartment surrounding the lungs. Lung ventilation is intermittent, and the depth and frequency of breathing vary greatly among species and even among individual animals. Oxygen is transported in blood largely in chemical combination with hemoglobin, which is contained in red blood cells.
The lungs are paired structures, except in various limbless species in which one of the lungs has become reduced or absent. The functional lung surfaces are complexly honeycombed to increase surface area, and they receive a profuse blood supply. In various reptiles, especially snakes and lizards, the posterior part of the lung is a simple membranous sac that functions to conduct or store air but does not participate directly in gas exchange. The saccular part of the lung may also be used to produce defensive inflation of the body. Among highly aquatic reptiles, the skin and linings of the throat and cloaca serve as accessory respiratory surfaces. However, most oxygen and carbon dioxide are exchanged across the lungs of most species, which require periodic access to air.
The blood is circulated by a central heart connected to distributing arteries, which service the tissue capillaries, where exchange of respiratory gases, nutrients, and other materials take place. Blood leaving the capillaries is returned to the heart in a system of veins, and excess fluid filtered from the blood at the capillaries is returned to the circulation via the lymphatic system. Except for crocodilians, all reptiles have a heart with a single ventricle filled by two atria. Thus in the ventricle there is an admixture of oxygenated blood returning from the lungs with oxygen-depleted blood returning from the tissue capillaries elsewhere in the body. In many species, these two bloodstreams are kept relatively well separated despite the potential for mixture in the single ventricle. On the other hand, the capacity for shunting blood between the lung and body circuits depends on physiological demands. For example, in aquatic reptiles blood flow to the lung is highest during air breathing, when lung and blood oxygen stores are renewed, and declines considerably during diving or submergence as the lung oxygen is used.
Both heart rate and blood pressure are generally lower than those in mammals, but they vary considerably with temperature and activity. The volume of circulating blood in reptiles varies from a few percent to approximately 14% of body mass. The common value is approximately 5-7% of body mass.
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