Cells, tissues, organs, system, organism

Our body is organized as a country. It's organized in levels with an increasing complexity.
Our country has cities, these cities join together to form provincies, provincies join together to form a larger level called region. Several regions together will make up a nation.
Same way bodies of multicelled organisms are made up by the joining of basic units called cells, cells join together to perform specific works forming a tissue. Different tissues can join to form an organ and different organs will make up an organ system or apparatus. Once more different apparatus come together to bulid an organism.
Let's give an example to make it clear.
In our body muscle cells join together to form mucle tissue. Muscle tissue along with others (like epithelial, or nerve, or adipose tissues) will form an organ like the heart.
Heart and other organs will make up the cardiovascular system. This system together with the rest of the body systems will build the organism.
Now we can start learning about each level!
Check this site for learning about these issues
http://www.usoe.k12.ut.us/CURR/SCIENCE/sciber00/7th/cells/sciber/levelorg.htm
http://www.usoe.k12.ut.us/curr/science/sciber00/7th/cells/sciber/levelcom.htm
http://www.coldwater.k12.mi.us/nicholsk/courses/chs/Ana/levelorg.htm
http://peer.tamu.edu/curriculum_modules/Cell_Biology/module_1/whyitmatters.htm

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