What is the difference between cells, tissues, organs and organ systems?

You can think of these 4 things as a hierarchy, where each 'element' is a building block for the next level. The smallest 'unit' are cells. There are trillions of cells in your body and these cells together make up what we call tissues. These tissues make up organs (so for example the heart is made up of many tissues). These organs then work together forming organ systems (such as the digestive system, which is made from organs such as the stomach, intestines and rectum) to perform a particular function. So as demonstrated below, it is not possible to move up the hierarchy without the 'stage' before being there. So if there are no cells, no tissues, organs or any organ systems can form. cells---->tissues---->organs---->organ systems