Cells

A cell is the smallest building unit of the body. There are different cells in the body, each with its own specialised function; namely, nerve cells, muscle cells, sperm cells, blood cells, etc. Although cells vary in shape their structure remains the same with a nucleus surrounded by cytoplasm. Cells are continuously being replaced from a few days (skin cells) to a few years (bone cells). Each cell is a separate living organism and must be supplied with oxygen and nutrients by the bloodstream. If a cell does not receive oxygen it will die quickly. Irreparable damage can occur in three to four minutes in brain cells if they do not receive oxygen.

Tissues

A group of cells of the same type is called a tissue. Tissue fluids fill the spaces between the cells. The main tissues found in the body are: muscle 43%, fat 14%, organs 12%, skin 9%, blood 8%, etc.

Organs

An organ consists of the same type of tissues. An organ has a particular job to do in the body: the heart (pumps blood), stomach (digests food), eye (enables one to see), etc.

Systems

A system is a group of organs whose job is closely related. e.g. The circulatory system consists of the heart, blood vessels and to a lesser degree the bones of the skeleton which manufacture blood cells.