The levels in one system

The five levels are clearer through one example — the blood circulatory system.

Level	Here	What it does
Cell	Cardiac muscle cell	Contracts
Tissue	Cardiac muscle tissue	Contracts as one
Organ	The heart	Pumps blood
System	Blood circulatory system	Carries blood round the body
Organism	A human being	All the systems working together

Figure to be added

Figure 8.6 — the blood circulatory system: how it grows more complex from the cardiac muscle cell through cardiac tissue and the heart (the organ) to the system.

What each level adds

A single cardiac muscle cell contracts — but that moves no blood. When millions of such cells contract together, the heart is a pump. The heart alone is not enough either; without blood vessels the blood goes nowhere. Each level adds something the level below cannot do on its own.

An organism is a collection of systems

The cell is the basic level of organization. The complex organization of tissues, organs and systems is found only within a multicellular organism.