The number of organs in any organism depends on the definition used. In the thoracic cavity the heart is a hollow, muscular organ. The hollow organs of the abdomen are the stomach, intestines, gallbladder, bladder, and rectum. The solid organs are the liver, pancreas, spleen, kidneys, and adrenal glands. The abdominal organs may be classified as solid organs, or hollow organs.
In the study of anatomy, viscera (singular viscus) refers to the internal organs of the abdominal, thoracic, and pelvic cavities. In single-celled organisms such as bacteria, the functional analogue of an organ is known as an organelle. Organs exist in most multicellular organisms. The main tissues that make up an organ tend to have common embryologic origins, such as arising from the same germ layer. For example, the gland's 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. 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.Īn organ's tissues can be broadly categorized as parenchyma, the functional tissue, and stroma, the structural tissue with supportive, connective, or ancillary functions. The intestinal wall for example is formed by epithelial tissue and smooth muscle tissue. Tissues of different types combine to form an organ which has a specific function.
Tissues are formed from same type cells to act together in a function. In the hierarchy of life, an organ lies between tissue and an organ system. In biology, an organ is a collection of tissues joined in a structural unit to serve a common function.
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