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2.2 – Respiratory Structures and Their Function
The content of this chapter was adapted from the Concepts of Biology-1st Canadian Edition open textbook byCharles Molnar and Jane Gair (Chapter 20 -The respiratory system).
2.2. Describe select respiratory structures in aquatic and terrestrial animals.
2.3. Explain relationships between respiratory structure and function in generalized aquatic and terrestrial animal systems.
Ventilation and Perfusion
Two important aspects of gas exchange in the lung are ventilation and perfusion. Ventilation is the movement of air into and out of the lungs, and perfusion is the flow of blood in the pulmonary capillaries. For gas exchange to be efficient, the volumes involved in ventilation and perfusion should be compatible. Therefore, ventilatory structures and perfusion surfaces have evolved to maximize this compatibility in both aquatic and terrestrial environment and examples of such structures and surfaces will be provided next.
Skin and gills
Fish and many other aquatic organisms have evolved gills to take up the dissolved oxygen from water (Figure 2.4). Gills are thin tissue filaments that are highly branched and folded. When water passes over the gills, the dissolved oxygen in water rapidly diffuses across the gills into the bloodstream. The circulatory system can then carry the oxygenated blood to the other parts of the body. In animals that contain coelomic fluid instead of blood, oxygen diffuses across the gill surfaces into the coelomic fluid. Gills are found in mollusks, annelids, and crustaceans.
The folded surfaces of the gills provide a large surface area to ensure that the fish gets sufficient oxygen. Diffusion is a process in which material travels from regions of high concentration to low concentration until equilibrium is reached. In this case, blood with a low concentration of oxygen molecules circulates through the gills. The concentration of oxygen molecules in water is higher than the concentration of oxygen molecules in gills. As a result, oxygen molecules diffuse from water (high concentration) to blood (low concentration), as shown in Figure 2.5. Similarly, carbon dioxide molecules in the blood diffuse from the blood (high concentration) to water (low concentration).
In terrestrial environments, animals showcase a diversity of respiratory system adaptations to obtaining oxygen (Figure 2.6), such as treachea in insects, alveoli in mammals and parabronchi in birds. You can also use the following gif animation as an overview of how air moves through respiratory system structured of a few terrestrial animals.
Question 2.3
How would you describe gas exchange and breathing cycle in each of the examples you see in the gif animation? What are their similarities? What are their differences?
Tracheal systems
Insect respiration is independent of its circulatory system; therefore, the blood doe