Let us start with atoms that was define as the smallest piece of an element that can retain all the properties of that element. When atoms combines, it will form molecules. The body contains many different types of molecules that form cells, which provide substances like hormones and enzymes that are tiny motors in the body that keep us running. Everything has cells, and this is the reason why cells are very important for it is the fundamental unit of life.
Then, when cells of the same type, hang together and perform same function, a tissue is created. The four classes of tissue are connective tissue, epithelial tissue, muscle tissue and nerve tissue. A combination of two or more kinds of tissues working together makes an organ. Just like the stomach which has the four classes of tissues and has the specific physiologic function of breaking down food.
Finally, an organ system is made possible when a group of specialized organs are working together to achieve a major physiological need. One example is the digestive system that is responsible for breaking down food to become nutrients that can be transported in the entire circulatory system. The organs involve are the mouth, esophagus, stomach, small intestine and large intestine.