BIOLOGY Volume 1 - A Guide to General Biology - 2004

9. UTILIZATION OF ENERGY

9.4. Gas Exchange

9.4.2. The Need for Specialized Respiratory Structures and Pigments

As body size increases, the surface-area-to-volume ratio decreases. Simple diffusion can no longer ensure a sufficient supply of oxygen to Cells that are not in direct contact with the external environment. Furthermore, due to high metabolic activity, larger animals often have a greater overall demand for oxygen.

In the course of evolution, this increased oxygen demand led certain Regions of the body to develop into specialized respiratory surfaces. The types of respiratory surfaces vary across different animals, yet in each case they are adapted to specific environmental conditions. Fig. 9.14 illustrates how they can be classified accordingly. Typically, the surface area of these structures is greatly enlarged, and they are often associated with a transport system, such as the Circulatory system. The transport system connects the respiratory surface with all other body Tissues, enabling a continuous exchange of oxygen and carbon dioxide between the respiratory surface and the cells. The presence of a respiratory pigment in the Blood further enhances its oxygen-carrying capacity (see below). Additionally, specialized ventilation movements accelerate gas exchange between the Organism and the environment by maintaining steep diffusion gradients.

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Fig. 9.14. Some types of respiratory surfaces in animals.

Respiratory pigments

Respiratory pigments are colored substances capable of reversible oxygen binding and transport. All known respiratory pigments consist of a colored non-protein moiety—such as the heme group in Hemoglobin—bound to a protein molecule. Hemoglobin is a red pigment. At high oxygen concentrations, the pigment readily binds oxygen, while at low concentrations, it rapidly releases it. Blood containing a respiratory pigment is a much more efficient oxygen carrier than blood without one. The pigment allows for the Uptake and Transport of significantly larger quantities of oxygen. In mammals and other vertebrates, hemoglobin contained within red Blood Cells serves as the respiratory pigment.

In Ch. 14, we will examine Oxygen transport by hemoglobin in greater detail.