Organs sit between tissues and systems—with fuzzy human counts
An organ is tissues united structurally to share a function—midway between tissue and organ system. Parenchyma does the specialty work; stroma supports with vessels, nerves, and connective scaffolding. Humans are said to have about seventy-nine organs, though definitions keep the tally contested.
Biology stacks its levels. Cells of one type act together as a tissue; different tissues join into an organ with a specific job; and two or more organs cooperating make an organ system. The gut wall shows the idea, combining epithelial tissue with smooth muscle. Within any organ, the working tissue is the parenchyma, while the stroma supplies nerves, blood vessels and connective scaffolding. In a hormone gland, for example, the secreting cells are parenchyma and everything holding and feeding them is stroma. An organ's main tissues usually share an embryonic origin, often the same germ layer.
How many organs does a person have? It depends on the definition. The usual estimate is about 79, but the figure remains debated. Organ systems overlap too. The nervous and endocrine systems share the hypothalamus, so they are often studied together as the neuroendocrine system, and muscles and bones are paired as the musculoskeletal system.
Human anatomy lists a familiar set of systems: cardiovascular, digestive, endocrine, excretory, lymphatic and immune, integumentary, nervous, reproductive, respiratory and skeletal. Each has its own roster, from the heart and vessels that move blood to the kidneys, ureters, bladder and urethra that manage fluid and electrolyte balance. Internal organs of the chest, abdomen and pelvis are called viscera. In the abdomen they divide into solid organs, such as the liver, pancreas, spleen and kidneys, and hollow ones, such as the stomach, intestines and bladder.
Organs are not universal. Single-celled eukaryotes use organelles as the functional equivalent, and plants have three main organs. Among animals, tissues first unite into true organs in flatworms and the other bilaterians; placozoans, sponges, comb jellies and cnidarians never make that step, and placozoans are the one multicellular animal group without organ systems.
Source: Organ (biology)
