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 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.

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.

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 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 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 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 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.

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.

Oxygen is taken up from the blood by all cells to be used in ATP production. The process of cellular respiration (energy production) creates carbon dioxide as a waste product, which, if accumulated, can cause the blood to become dangerously acidic. Gases in the lungs diffuse passively into or out of the air entirely based on where the concentration is lowest. Thus, oxygen levels in the blood must be lower in concentration than those in the lungs in order for oxygen to enter the blood, and carbon dioxide levels in the blood must be higher than those in the lungs order for carbon dioxide to exit.