The Respiratory System and Gas Exchange

Anatomical Organization of the Respiratory Tract and Lungs

The primary purpose of the respiratory system is to deliver oxygen to the bloodstream and expel carbon dioxide from the body. To accomplish this, air travels through a continuous series of structures divided into two major anatomical divisions: the upper respiratory tract and the lower respiratory tract. The upper respiratory tract consists of the nose, nasal cavity, sinuses, and pharynx. The lower respiratory tract begins below the pharynx and includes the larynx, trachea, bronchi, bronchioles, and microscopic alveoli.

Air enters through the nasal cavity, which prepares incoming air for the delicate tissues downstream by warming, humidifying, and filtering it. The internal lining of the airway contains specialized goblet cells that produce protective mucus, alongside ciliated epithelial cells. Working together, mucus and cilia trap inhaled debris and move it toward the pharynx, backward through the nasal passages and upward through the lower airways, for removal or swallowing.

From the nasal cavity, air passes into the pharynx, a shared muscular passageway that conducts both air for the respiratory system and food or liquid for the digestive system. Directly beneath the pharynx sits the larynx, which houses the vocal cords and connects the pharynx to the trachea. To protect the lower airway during swallowing, a flexible flap of cartilage called the epiglottis folds down over the opening of the larynx, guiding swallowed material safely past the respiratory tract and helping prevent food and liquid from entering the airway.

Air continues from the larynx into the trachea, a hollow tube reinforced with rigid C-shaped rings of hyaline cartilage that prevent airway collapse during pressure shifts. At its base, the trachea branches into the right and left primary bronchi, which channel air into the lungs. Anatomically, the right primary bronchus is wider, shorter, and more vertically oriented than the left primary bronchus. The lungs themselves are asymmetric: the right lung is divided into three lobes (superior, middle, and inferior), whereas the left lung is divided into only two lobes (superior and inferior). The left lung also features an indentation called the cardiac notch, which accommodates part of the heart. Surrounding each lung are protective serous membranes: the visceral pleura adheres directly to the outer surface of each lung, while the parietal pleura lines the inner surface of the thoracic cavity. Between these layers is pleural fluid, which reduces friction as the lungs move, while its surface tension keeps the lungs mechanically coupled to the chest wall.