Respiration
Respiration
Energy bound up in food is released by oxidative processes, usually with molecular oxygen as the terminal electron acceptor. Oxygen for this purpose is taken into the body across some respiratory surface. Physiologists find it is convenient to distinguish two separate but interrelated respiratory processes: cellular respiration, the oxidative processes that occur within cells, and external respiration, the exchange of oxygen and carbon dioxide between the organism and its environment. In this section we describe external respiration and transport of gases from respiratory surfaces to body tissues.
In single-celled organisms, oxygen is acquired and carbon dioxide liberated by direct diffusion across surface membranes. Gas exchange by diffusion alone is possible only for very small organisms less than 1 mm in diameter, where diffusion paths are short and the surface area of the organism is large relative to volume. As animals became larger and evolved a waterproof covering, specialized devices such as lungs and gills evolved to increase the effective surface for gas exchange. But, because gases diffuse so slowly through living tissue, a circulatory system was necessary to distribute gases to and from the deep tissues of the body. Even these adaptations were inadequate for complex animals with high rates of cellular respiration. The solubility of oxygen in the blood plasma is so low that plasma alone cannot carry enough oxygen to support metabolic demands. With evolution of special oxygen-transporting blood proteins such as hemoglobin, the oxygen-carrying capacity of blood increased greatly. Thus what began as a simple and easily satisfied requirement resulted in evolution of several complex and essential respiratory and circulatory adaptations.
Energy bound up in food is released by oxidative processes, usually with molecular oxygen as the terminal electron acceptor. Oxygen for this purpose is taken into the body across some respiratory surface. Physiologists find it is convenient to distinguish two separate but interrelated respiratory processes: cellular respiration, the oxidative processes that occur within cells, and external respiration, the exchange of oxygen and carbon dioxide between the organism and its environment. In this section we describe external respiration and transport of gases from respiratory surfaces to body tissues.
In single-celled organisms, oxygen is acquired and carbon dioxide liberated by direct diffusion across surface membranes. Gas exchange by diffusion alone is possible only for very small organisms less than 1 mm in diameter, where diffusion paths are short and the surface area of the organism is large relative to volume. As animals became larger and evolved a waterproof covering, specialized devices such as lungs and gills evolved to increase the effective surface for gas exchange. But, because gases diffuse so slowly through living tissue, a circulatory system was necessary to distribute gases to and from the deep tissues of the body. Even these adaptations were inadequate for complex animals with high rates of cellular respiration. The solubility of oxygen in the blood plasma is so low that plasma alone cannot carry enough oxygen to support metabolic demands. With evolution of special oxygen-transporting blood proteins such as hemoglobin, the oxygen-carrying capacity of blood increased greatly. Thus what began as a simple and easily satisfied requirement resulted in evolution of several complex and essential respiratory and circulatory adaptations.
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