All organisms are characterized by a high degree of complexity and organization. Single-celled organisms have multiple organelles—the small structures that maintain the cell—and complex structures, such as the cell membrane, that enable them to meet their energy and nutrient requirements.

Multicellular organisms have even greater complexity. In single cells, nutrients can move into the cells and waste can move out of the cells by diffusion. In large multicellular animals, specialized cells, tissues, organs, and organ systems are required to meet these needs. Specialization of cells into nerve cells and muscle cells, for example, has been possible because other cells and systems of cells have evolved to serve other vital functions. Whereas single-celled organisms each have to take care of all of their basic needs—finding food, reproducing, sensing danger—multicellular organisms have a variety of different types of cells, each of which specializes in meeting just one or two basic needs.

Cells specialized to perform certain functions have become grouped among similarly specialized cells to form tissues, including epithelial tissue, connective tissue, muscle tissue, and nerve tissue. A structure made up of two or more tissue types working together in a common function is called an organ. The heart, for example, is made up of muscle, nerve, and connective tissues that work together to provide the rhythmic motion that pumps blood throughout the body.

Together, organs that share a common function or functions make up one of ten major systems in the body. For example, the heart, blood, and blood vessels make up the circulatory system, which is responsible for supplying oxygen and nutrients to tissues throughout the body and removing waste products. However, despite its functional specialization, each system is integrally connected to every other system. Oxygen enters the circulatory system as it passes through the respiratory system. Nutrients are absorbed in the small intestine of the digestive system. The oxygen and nutrients are then carried by the circulatory system to provide energy to all other organ systems, including the nervous system, which maintains the nerve signals required to keep the heart beating. The limbic system helps clean the blood of pathogens, while the reproductive system makes it possible to pass the individual's genes on to the next generation.

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