Before any air can enter the lungs, it must pass through the mouth or nose into the trachea. Here are some of the strucures along the way:
Mouth/Nose: These openings serve as entrances and exits for the air we breath (6).
Epiglottis: This soft flap prevents foreign objects (mainly food) from entering the Trachea (6).
Trachea: The opening into the lungsit houses the vocal chords and helps provide structure, protection, and support for the delicate parts of the throat. It extends down until it splits into the right and left bronchi (6).

The diaphragm is a muscle located between the lungs and the ribcage, and it stretches under the lungs. This muscle is key to breathing as it controls inspiration. The diaphram can contract (seen below) to expand the lungs, lowering the air pressureinside of them, therefore causing air to flow into them (inspiration)
(6)
Exhalation occurs when the diaphragm relaxes-returning to its original shape and compressing the lungs. The intercoastal muscles help power this process. (6)

1.) The mouth and the nose take in oxygen from the air around the body, initiating the respiratory process. (2)
2.) The oxygen inhaled from the body travels through the trachea. (2)
-The trachea is lined with 15 to 20 C-shaped lines of cartilage down its length to prevent it from collapsing. (4)
3.) The trachea splits in to two bronchus tubes
(plural: bronchi). Each respective bronchus connects to one lung. (2)
4.)The bronchi split into many bronchial tubes.
(2)
Picture (1)

5.) The bronchial tubes split into several lesser tubes.
6.) Each of the lesser tubes ends in an alveoli sac (1).
-Each alveoli is surrounded by tiny blood vessels called capillaries (1). It is here that the respiratory and circulatory system connect.
-Alveoli are "spongy, air-filled sacs". This sponge-like structure makes it easier for the oxygen in air to diffuse out of the alveoli and into the capillaries (1).
-The human body contains approximately 600 million of these alveoli sacs (1).
Picture (1)

-Oxygen diffuses between the alveoli and capillaries (5).
-Oxygen attaches to hemoglobin in red blood cells and his carried throughout the body in the bloodstream (5).
-Carbon dioxide diffuses from the bloodstream into the alveoli and is exhaled (5).

(1) Ambulance Technician Study. “Respiratory System.” Ambulance Technician Study . Nixon Mcinnes, 2006. Web. 23 Oct. 2011. http://www.ambulancetechnicianstudy.co.uk/respsystem.html
.
(2) The Franklin Institute. “Respiratory System: Oxygen Delivery System.” The Franklin Institute Resources for Science Learning :
The Human Heart . The Franklin Institute, 2011. Web. 23 Oct. 2011. http://www.fi.edu/learn/heart/systems/respiration.html
.
(3) National Center for Biotechnology Information , Pneumonia.
A.D.A.M. Inc,
2011.
< http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001200/2011 >
(4) O’Rahilly, Ronan, et al. “Chapter 21: The esophagus, trachea and main bronchi.” Basic Human Anatomy: A Regional Study of
Human Structur . Dartmouth Medical School, 2008. Web. 23 Oct. 2011. http://www.dartmouth.edu/~humananatomy/part_4/chapter_21.html
.
(5) Starr, Cecie, Lisa Starr, and Ralph Taggart.
Biology the Unity and Diversity of Life . Belmont, Calif. [u.a.: Brooks Cole Thomson
Learning, 2002. Print.
6)Tamarkin, Dawn A. PhD. Inspiration and Expiration.
STCC Foundation Press, 2011.
< http://faculty.stcc.edu/AandP/AP/AP2pages/Units21to23/respiration/inspirat.htm
>