2. The Heart Oxygen rich blood is pumped out of the heart to the muscles and organs via artery’s (red areas) De-oxygenated blood is returned to the heart via veins (blue areas) Artery’s have plenty of pressure on them from the heart contracting to force the blood away from the heart. Think of a bottle of tomato sauce being squeezed. Veins however are not under any pressure. To stop the blood going backwards veins have valves.
3. The Lungs Think of the lungs as being like a tree. You start with the trunk (Trachea), moving down the main branches (bronchus), through to the individual twigs (bronchiole) and finally at the end you have the leaves (alveoli).
