Okay so we know that air enters the body through the nose and mouth. As it enters, it passes down the trachea and divides through the bronchioles eventually ending at these small sacks called Alveoli.   

Its here where the majority of gasses exchange occurs.

Observe the diagram to the right. The alveoli look like bunches of grapes coming off a vine.

The capillaries wrap around the alveoli, allowing for simple gas diffusion.

The animation to the left shows how carbon dioxide diffuses off the red blood cells and oxygen in.

Scroll over the image to see the animation.

Although the atmosphere is only 21% oxygen, 78% nitrogen and 1% other gases, our red blood cells act as sponges only for oxygen. This is due to the haemoglobin protein found in red blood cells.

Specific cells throughout the respiratory system secrete mucus which trap foreign pathogens or dust. However if a pathogen gets through and reaches the alveoli it could cause serious damage.

This is why, within the alveoli are white blood cells called alveolar macrophages.

These Macrophages patrol very close to the surface of the alveolus and enter its thin walls when an invader is detected. The macrophage quickly engulfs and digests the intruder.

Click here for questions on the human respiratory system