Refer to figure.
BREATHING

Air is breathed in through the nose or mouth. The air then passes through the trachea and reaches the two bronchi, where it is subdivided to reach the two lungs. The lungs consist of air vessels that become progressively smaller until they end up to tiny sacs called alveoli. Alveoli sacs are covered by tiny blood vessels (capillaries). Both the alveoli sacs and the capillaries have very thin walls to allow gas exchange to take place.

• At the alveoli sacs, the breathed in oxygen is transferred from the alveoli to the capillaries (to the blood), and the carbon dioxide is transferred from the capillaries to the alveoli to be exhaled through the lungs. This process is called external respiration.

Note: The term 'gas exchange' refers to the gas exchange between the lungs and the blood. Oxygen has to be transferred from the lungs to the blood and carbon dioxide has to be transferred from the blood to the lungs to be exhaled.