Cells are structural and functional units of life. Cells interact with each other and form tissues - a group of cells possessing similar structures and functions. In the human body, cells form four major types of tissues including muscle, epithelial, connective and nerve. All these issues have a specific function due to certain structural features observed in cells. Different tissue can form organs. Each organ has a unique structure and function. For example, the heart consists of four types of tissues, has a specific shape and structure, and performs a function of blood transportation through the body. Organ systems consist of several organs which have different structures. However, the organ system performs mostly a certain function. For instance, heart and vessels form the cardiovascular system which is specialized in the transportation of blood.