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An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. In the hierarchy of life, organs lie between tissue and organ systems. A tissue is an organizational level between cells and organs. A tissue is an ensemble of similar cells and their extracellular matrix from the same origin that together carry out a specific function. Organs are then formed by the functional grouping together of multiple tissues. Two or more organs working together in the execution of a specific body function form an organ system, also called a biological system or body system.
A given organ's tissues can be broadly categorized as parenchyma, the tissue peculiar to (or at least archetypal of) the organ and that does the organ's specialized job, and stroma, the tissues with supportive, structural, connective, or ancillary functions. For example, in a gland, the tissue that makes the hormones is the parenchyma, whereas the stroma includes the nerves that innervate the parenchyma, the blood vessels that oxygenate and nourish it and carry away its metabolic wastes, and the connective tissues that provide a suitable place for it to be situated and anchored. The main tissues that make up an organ tend to have common embryologic origins, such as arising from the same germ layer. Functionally related organs often cooperate to form whole organ systems. Organs exist in most multicellular organisms. In single-celled organisms such as bacteria, the functional analogue of an organ is known as an organelle. In plants, there are three main organs. A hollow organ is an internal organ that forms a hollow tube, or pouch such as the stomach, intestine, or bladder.