Animal physiology is the study of how animals work, or more specifically the physical and chemical processes that occur within animals. Examples of these processes include gas exchange, blood and circulation, osmoregulation, digestion, nervous and muscle systems and endocrinology. In the animal body, the milieu interieur is often considered to be the body fluids, especially the “blood”. Components that can vary include; its pressure, temperature, gas content, nutrient content, waste content, pH etc. Sensors include specialized receptor cells that are responsive to physical or chemical components and/or their associated nerve endings. The primary control center of the body is an expanded region of the nervous system known as the Brian. Secondary controls are in the spinal cord, the endocrine system and in “nerve nets” built directly into particular organs or organ systems. The Effectors are generally the “maintenance systems” of the body, such as the respiratory, circulatory, digestive and excretory systems. Additional effectors can be in the muscular and skeletal systems if the response requires animal movement. Central among the maintenance systems is the circulation. This system is charges with the responsibility of distributing the liquid (blood) to all the cells of the body. A main factor in the distribution is the provision of all cells with the means to maintain their energy levels. In this regard, the circulation must be coordinated with the (intermittent) digestion and constantly with the respiratory system to supply the fuel and oxygen needed to burn that fuel to the body cells. Thus, the circulatory system must be highly responsive to changing conditions. The text provides a comprehensive overview of physiological systems in a well-organized and concise manner by describing the importance of the interaction between an animal and its environment.