Atoms and molecules associate to form the organelles and cells(building block of the body)of the cellular level.The human body is composed of about 100 trillion cells of many types,such as bone cells,muscle cells and blood cells.Cells vary in size and shape according to their function,most of them are so small that they can be seen only with a microscope.Each cell consists of specialized cell parts called organelles.One organelle,nucleus,serves as the information and control centre of the cell.Several other kinds of organelles scattered throughout the cell perform specific functions like breaking down fuel molecules to provide energy and manufacturing needed substances.
Cells associate to form tissues such as muscle or bone tissue.A tissue is a group of closely associated cells specialized to perform particular functions.The four main types of tissues are muscle tissue,nervous tissue,epithelial tissue and connective tissue.
Tissues may be organized to form organs such as the brain or heart.The heart consists mainly of muscle tissue but it is covered by epithelial tissue and also contains connective and nervous tissue.
Certain tissues and organs function together to make up an organ system or a body system.For example,the circulatory system consists of the heart,blood vessels,blood,lymph structures and several other organs.
The body systems work together to make up the human organism.