Cells are composed of organelles and other cell compartments that work together during normal cell function. In multicellular organisms, cells join together to form tissues. These web pages deal with how tissues are organized to form organs in animals. Animals have a variety of organs and each one develops particular functions. When comparing an animal with a cell, it can be observed the division of work in specialized compartments, but at different scale. In eukaryotic cell, these compartments are mostly organelles, whereas in animals the compartments are organs and systems.
The following pages deal with organs of vertebrates, mainly mammals. Mammals are probably the most studied and better known vertebrates because of the similarities with humans and the impact on the study of human organs, health and medicine. An organ is a functional unit of the body. A system, such as blood system, is a number of structures, some of them are organs, that carries out one or several functions. However, as it usually happens in biology, it is difficult to define the precise role of an organ or system in the many tasks that an animal must accomplish. So that, how the organs are distributed in tasks depends on the author or the focus of a particular study (physiology or anatomy, general or particular functions). Thus, an organ may take part in different systems at the same time, doing more than one function or sharing functions with other organs. Here, we will study the animal organs and systems in a way similar to that found in the syllabus of courses related to microscopic study of animal organs, taught at some Spanish universities, with some changes.