The route of oxygen:
Mouth & nose,trachea,bronchi,bronchioles,alveoli.
Gaseous exchange
Oxygen passes through the alveoli into the red blood cells in the calillaries. Here, oxygen combines with haemoglobin to form oxyhaemoglobin. This is then carried around the body. At the same time, haemoglobin carries carbon dioxide from the body into the capillaries. Carbon dioxide passes through the alveoli and is breathed out.
Blood vessels
Arteries- Carry blood away from the heart. Thick walls because blood is travelling at high pressure. Small internal diameter.
Veins-Carry blood towards the heart. Thinner blood because blood is at a low pressure. Larger internal diameter. Have valves to prevent backflow.
Capillaries-One cell thick.
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