The Human Body
Levels of Organization in the Body
All vertebrates share the same basic body plan, with tissues and organs functioning in a similar manner. Here, we will focus on the human body, studying form (anatomy) and function (physiology). The two go hand-in-hand and are an extension of one of biology's central themes, evolution.
Animals are made of complex systems of cells, which must be able to perform all of life's processes and work in a coordinated fashion to maintain a stable internal environment. Early in a human's development, groups of cells specialize into three fundamental embryonic or germ layers: endoderm, mesoderm, and ectoderm. These embryonic layers differentiate into a number of specialized cells and tissues. Tissues are groups of cells similar in structure and function and may be held together by some sort of matrix. There are four primary groups of tissues: epithelial, connective, muscular, and nervous.
Different tissues functioning together for a common purpose are called organs (e.g., stomach, kidney, lung, heart).
Organ systems are composed of individual organs working together to accomplish a coordinated activity. For example, the stomach, small intestine and large intestine all play a role in digestion.
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