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.

After entering the nose and mouth, air is passed into the pharynx. It then travels to the larynx, which houses the vocal cords, before entering the trachea. The trachea transports the air into the thoracic cavity before branching into the primary bronchi. The right bronchus carries air to the right lung; the left bronchus carries air to the left lung. Of the given answer options, air only passes through the pharynx before entering the lungs.

Alveoli are found in the lungs and the primary site of gas exchange; however, the question specifies a structure before the air enters the lungs. The esophagus transports food, and is not involved in respiration. Nephrons are the function units of the excretory system, and are located in the kidneys.

The left atrium receives blood from the pulmonary veins, which carry blood that was freshly oxygenated from the lungs to the heart. The partial pressure of oxygen is always highest soon after oxygenation, thus blood returning from the lungs would have a high partial pressure.

The superior and inferior vena cavae return deoxygenated blood from the body to the heart, and would have very low oxygen partial pressures. The right atrium receives this deoxygenated blood from the vena cavae and transfers it to the right ventricle. From the ventricle, the deoxygenated blood is transported to the lungs via the pulmonary arteries. It only becomes oxygenated again after reaching the lungs, and then returns to the heart through the pulmonary veins.

This question requires a basic understanding of general chemistry and/or general physics. Remember that gas will only move from an area of high pressure to low pressure; thus, if air is moving out of the lungs, the pressure inside of the lungs must be greater than the pressure outside of the lungs. The point at which air does not move in or out of the lungs is a signal that the pressure of the gas inside of the lungs is equal to that of atmospheric pressure.

In a biological sense, remember that the diaphragm contracts to cause inhalation, which results from negative or decreased pressure in the lungs. When the diaphragm relaxes, the pressure in the lungs must increase again. The increase in pressure forces the air out of the lungs and back into the atmospheric environment.

We know that systemic arteries carry oxygenated blood away from the heart and that these vessels pass through tissues, allowing oxygen to diffuse into the tissues and carbon dioxide to diffuse out into the bloodstream. We also know that veins carry blood back to the heart, which by this time is oxygen-poor due to its gas exchange with the tissues of the body. The partial pressure of oxygen in veins must be lower than it is in arteries, since the veins carry deoxygenated blood.

The remaining statements regarding gas exchange are true.

Hemoglobin is the main oxygen carrier in the human body. Each hemoglobin protein is able to carry four oxygen molecules. As the hemoglobin travels through the blood vessels of the body, the oxygen is released to tissues and used in the electron transport chain.

If oxygen is unable to bind to hemoglobin, our bodies cannot carry the needed oxygen to the tissues of the body. This can occur when carbon monoxide displaces oxygen from hemoglobin.