 Dear students, in this topic we shall discuss about some general considerations and general principles of respiration. You know that animals use oxygen for cellular respiration. They generate carbon dioxide as toxic waste which may result in the fall of pH level of the body. To obtain oxygen and remove carbon dioxide, animals exchange these gases with the environment. The gas exchange between the animal and the environment takes place passively by diffusion. Oxygen of gases always occurs across a respiratory surface. The respiratory surface may be skin, general body surface or a specialized respiratory epithelium. Dear students, a specialized respiratory epithelium has three basic features. In this surface may be these three features which can be used to exchange gas. 1. Large surface area and low diffusion distance. 2. The epithelium should be thin and 3. It should be highly vascularized i.e. supplied with blood capillaries. Dear students, we shall discuss these three features in detail one by one. First we are going to discuss the surface area of respiratory epithelium. The animals oxygen requirements and carbon dioxide production is proportional to its mass, body mass. However, the rate of gas exchange is related to the surface area and diffusion distance. To facilitate the gas transfer, the surface area of respiratory epithelium should be large and diffusion distance should be small. Dear students, the gas exchange across the body surface can fulfill the needs of very small animals because they have small diffusion distances and they have large surface area to volume ratios. In the case of small animals, the gas exchange is sufficient for small animals such as rotifers and protozoan, which are unicellular organisms whose diameter is less than 0.5 mm. In such animals, only the gas exchange is enough for the body surface. Second, when animals' size increases, it results in increase in diffusion distance and reduced surface area to volume ratios. In that case, animals' gas exchange requirements cannot be fulfilled by its general body surface. These animals have to develop specialized argons which have specialized respiratory epithelium whose surface area to volume ratio increases and to fulfill the body's requirements, the gas exchange can be done through the diffusion process. Dear students, the respiratory argons which the animals have evolved have extensive respiratory epithelium for gas exchange. For example, humans have specialized argons which are called the lungs. The respiratory area of the lungs is up to 50 to 100 m2. Dear students, the second major feature of a respiratory epithelium is that it is a thin surface. It is made up of thin layer of cells. It has or it should have a thickness between 0.5 to 15 micrometers. The third feature of a respiratory epithelium is that it is highly vascularized. Highly vascularized means that the supply of blood capillaries is very high with such a surface. The effect of blood flow is that it reduces the diffusion distance for gas transfer. Dear students, a gas transfer system involves four basic steps through which the body is not only exchanged with the outside but also with the body. All the requirements are that the oxygen reaches the proper place. For this, the body has gas transfer systems. These four basic steps are number one, breathing movements, which assure continuous supply of air or water to the respiratory surfaces which are mostly lungs or gills in animals. Number two, the diffusion of gases across the respiratory epithelium. After air flow or water flow of the respiratory epithelium, the next step is the diffusion of the respiratory gases. Number three, the next step after diffusion is to transport the gases in the body. This is the gas or bulk transport of gases by blood and the fourth step of the gas transfer system is the diffusion of gases across the capillary walls between the blood and body cells. Blood or body cells may have diffusion of gases so that oxygen cells are found and the carbon dioxide is transported from the body to the body.