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 nasal passage's main function is to heat or cool air before it enters the lungs. The cilia, mucous and hair also help filter air since the respiratory system is very sensitive to allergens and infection. Also, the lungs need to be kept moist and lubricated, so dry air is damaging. No gas exchange occurs in the nasal passage, it is merely a conduction zone through which air must travel before it gets to the alveoli where gas exchange occurs with the pulmonary capillaries.

The nasal passage's main function is to heat or cool air before it enters the lungs. The cilia, mucous and hair also help filter air since the respiratory system is very sensitive to allergens and infection. Also, the lungs need to be kept moist and lubricated, so dry air is damaging. No gas exchange occurs in the nasal passage, it is merely a conduction zone through which air must travel before it gets to the alveoli where gas exchange occurs with the pulmonary capillaries.

The nose is the primary passageway of air into the lungs. Before air can safely enter the body, it must be brought to body temperature, moisturized and cleansed of any particles that could damage the respiratory system or cause infection. Mucous is produced by goblet cells. Olfaction is carried out by specialized neurons in the nose that bind to certain molecules and send information to the brain.

The nose is the primary passageway of air into the lungs. Before air can safely enter the body, it must be brought to body temperature, moisturized and cleansed of any particles that could damage the respiratory system or cause infection. Mucous is produced by goblet cells. Olfaction is carried out by specialized neurons in the nose that bind to certain molecules and send information to the brain.

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 alveoli's structure maximizes the efficient transfer of gas from air to the capillaries and vice versa. Therefore the contact point between air and the capillaries needs to be as thin as possible so gas has only a short distance to diffuse. Alveoli are therefore made up of a thin layer of epithelial cells that are in direct contact with endothelial cells in the capillaries.