Breathing and Gas Exchange
The air enters through the lungs, and passes down the trachea. The air passes into the left or right bronchus, then passing down into bronchioles, and then into microscopic air sacs, named alveoli. This is where the diffusion of gases in and out of the lungs takes place. There is a lower concentration of oxygen in the blood of the capillaries, because it is deoxygenated, so oxygen diffuses out of the alveoli. There is a higher concentration of carbon dioxide in the blood, so carbon dioxide diffuses out of the blood and into the alveoli. The blood is now oxygenated, and is taken to the heart via the pulmonary vein to be pumped all around the body. The carbon dioxide is removed from the body when we exhale.
We inhale air into the lungs because there is a slight pressure difference when we try to breathe in. Our ribs push up and out, due to our external intercostal muscles contracting (pulls ribs out) and because our diaphragm muscles pull down- the volume of our lungs increases, so that the pressure in the lungs falls. This means that air can enter the lungs. When we try to exhale air, our ribs pull down and in, due to the external intercostal muscles relaxing, and our internal intercostal muscles contracting. The diaphragm muscles relax, so diaphragm returns to normal dome shape. Volume of thorax decreases, so pressure in the thorax is raised slightly above atmospheric pressure, forcing air out of the lungs.
The trachea is kept clear because of cells secreting sticky liquid, mucus, trapping particles of dirt or bacteria that are breathed in. Cilia are tiny hair like structures, which beat back and forwards, sweeping mucus and trapped particles towards the mouth. This prevents infection, as dirt and bacteria don't enter the lungs.
There are hundreds of millions of alveoli in the two lungs. Between them they have a massive surface area. Their surface area is increased due to their grape like appearance, along with their large SA:V ratio. They also work very closely to the blood capillaries, so that the time it takes to diffuse is very small, less than a thousandth of a millimetre between the two cell layers, the alveoli wall and the capillary wall. It also keeps a high diffusion gradient, so that it is quicker to diffuse gases in and out of the alveoli.