The oropharynx is capable of accepting both air and food from the mouth and nose. In the laryngopharynx, however, a crucial division occurs. Food, solids, and liquids are passed down the esophagus, while air travels through the trachea to the lungs. During swallowing, the entrance to the trachea, the larynx, must be covered so that food does not go through the trachea and enter the lungs. This is accomplished by the epiglottis, an elastic cartilage structure that covers the larynx during swallowing.

The alveoli are lined with a single layer of squamous epithelial cells, which allow for easy diffusion of vital gases. Basal and apical cells refer to cells located at the bottom and top of structures, respectively. Endothelial cells line the circulatory system and blood vessels. There is no formal class of cells known as "respiratory cells."

The spherical or grape-like shape of the alveoli allows for maximum contact between the alveoli and the capillaries that surround them. The alveoli are filled with air that has been taken into the lungs from the environment, so a high surface area allows for maximum contact between air from the environment and capillaries. Oxygen rapidly diffuses through the exceptionally thin alveolar walls to the capillaries, which carry hemoglobin-containing blood cells that bind to the oxygen and shuttle it around the body. Blood cells also release carbon dioxide into the alveoli and lungs, which is why this process is called gas exchange. 

Capillaries are considerably smaller than alveoli; they surround the alveoli like a mesh, and are certainly not the cause of the alveoli's shape. Furthermore, though the cells of the alveoli do secrete extracellular matrix material, the cells affect the structure of the extracellular matrix, rather than the other way around. The shape of the alveoli is crucial to their function in gas exchange and cannot be considered an "accident," or the unexpected result of the shapes of other biological structures. 

Alveoli are the site of gas exchange in the lungs. Because rapid diffusion of gases is necessary between the capillaries and the alveoli, a very thin epithelial layer is needed. As a result, alveoli use simple squamous epithelium so that gases can easily diffuses to and from the bloodstream.

Alveoli are at the end of the respiratory pathway in humans, and act as a site of gas exchange (carbon dioxide and oxygen).

The path of air through the respiratory tract is: trachea, bronchi, bronchioles, alveoli. It is important to note that no gas exchange takes place in the bronchi, but does in the bronchioles, which are passageways that branch off from the main bronchi and eventually lead to alveolar ducts.

Micorvilli are found int he small intestine and act to increase the surface area in order to increase nutrient absorption. Secretory vesicles are used to transport proteins, hormones, and other molecules from a cell into the extracellular space.

The vast majority of the surface area of an alveolus is made up of type 1 alveolar cells, which are squamous (flat), thin epithelial cells that allow rapid gas exchange between the air inside the alveoli and blood in the surrounding capillaries. The healthy adult human has millions of alveoli in his/her lungs, providing a huge total surface area across which gas can diffuse, letting oxygen into the bloodstream and carbon dioxide out.

The right lung contains three lobes: upper, middle, and lower. The left lungs contains two lobes: upper and lower. The left lung is designed to be smaller than the right in order to accommodate the heart, which is situated slightly to the left.

The respiratory system allows air to enter the lungs from the outside environment and facilitates gas exchange with the blood. Air initially enters through the mouth or nose, passes through the pharynx and larynx, and enters the trachea. From the trachea, air travels through branching structures from bronchi, to bronchiole, to alveoli. Gas exchange occurs between the air in the alveoli and the capillaries surrounding the alveoli.

The pharynx, located posteriorly (behind) the nasal passages and the mouth, is responsible for collecting the air that is taken in via the nose and mouth. The pharynx then passes the air to the larynx before it flows into the trachea. The trachea carries the air to the bronchioles, which end in terminal alveoli in the lungs.

Air enters the body through the nose or mouth, and is transferred to the pharynx (the upper portion of the throat located at the back of the mouth). The larynx is commonly called the "voice box," and is the lower portion of the throat connected to the pharynx. From there, air enters the trachea and flows into the chest. The trachea branches into two bronchi, which continue to branch and divide as the air is carried into the lungs.