Levels of organisation

In order of increasing complexity, multicellular organisms are made of:

cells → tissues → organs → organ systems

OrganelleA specialised unit within a cell which performs a specific function
CellThe basic building block of all living organisms
TissueA group of cells working together to perform a shared function, and often with similar structure
OrganA structure made up of groups of different tissues, working together to perform specific functions
Organ systemA group of organs with related functions, working together to perform certain functions within the body

A diagram of the the circulatory system.An example of the levels of organisation using the circulatory system