 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. Even in this surface, these three features can be used to exchange gas. 1. Large surface area and low diffusion distance. 2. The epithelium should be thin. 3. It should be highly vascularized, that is 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 animal's 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. However, when the animal's 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 types of animals have to develop specialized argons which have specialized respiratory epithelium which increases the surface area to volume ratio and can fulfill the body's requirements 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 surface 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 layers 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 a lot of blood capillaries are supplied to such surfaces. 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 for gas from outside but also for 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, the next step is diffusion of the respiratory gases. Number three, the next step is to transport the gases inside the body. The fourth step of the gas transfer system is the diffusion of gases across the capillary walls between the blood and body cells. The diffusion of gases in the blood and body cells is so that oxygen cells are found and the carbon dioxide is transported from the body to the outside.