An organ is a structure in the body that is made of different tissue types – so these are things like the heart, the liver, the brain, kidneys, stomach etc. Now you might also be wondering what I mean by tissue? Tissue is a collection of cells, all doing the same job. You actually only have 4 different tissue types in the body (muscle, epithelial, connective and nervous) and so an organ contains most, or all of these tissue types together.
To give you an idea os structures, anatomists like to look at things like this
cells<tissues<organs<organ systems
so a good example is cardio cell (heart cells)< cardiac muscle < heart < cardiovascular system
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