Anatomy and Physiology of the Lungs

Before air enters the lungs, it travels through the upper airway: the nose, pharynx, larynx, and trachea. Lining the nose is a soft, moist layer of cells called mucosa that warms and humidifies the air. The pharynx and larynx form the throat and lead to the trachea or windpipe.

The trachea connects the upper airway to the lungs. It is a tube-shaped structure made up of rings of cartilage surrounded by thin, smooth muscle. Usually, the trachea is midline, but it may be shifted slightly to the right and sit near the aortic arch. It is about four to five inches long and between one-half and three-quarters of an inch wide.

After the trachea, the airway splits into the left and right main bronchi. The right bronchus is about an inch long and is wider and more vertical than the left. It is nearly in a straight line with the trachea so, when someone chokes on a foreign object, it is likely to be lodged in the right bronchus. The left bronchus is about two inches long and crosses in front of the esophagus.

The main bronchi divide into smaller branches called bronchioles. There are three types of bronchoiles: conducting, terminal, and respiratory. Each lung has about 20 to 25 conducting bronchioles. As they continue to spread out and get narrower, they become terminal bronchioles, marking the end of a path. These then divide further into the smallest, narrowest branches called respiratory bronchioles.

The respiratory bronchioles give rise to the alveoli. These small sacs are responsible for gas exchange and make up about 90 percent of lung volume. They are arranged in units called acini, of which there are about 30,000. Each alveolus has a septum that allows for gas exchange, providing structure to prevent collapse and overdistention. The lungs also release surfactant, which protects the alveoli from collapse when air volume is low.

Each alveolus has a lining of tiny blood vessels called capillaries. Oxygen and carbon dioxide move freely between the lungs and the blood at the membrane of each capillary. Oxygen molecules attach to the hemoglobin and travel back to the heart, then throughout the body. Carbon dioxide crosses into the lungs, and the body expels it with each exhalation.

The right and left lungs are very similar in size and shape but are not symmetrical. The right lung has three lobes, while the left is slightly smaller and divided into two. This is because the left lung has a cardiac impression to accommodate the heart, which is located slightly left of center.

The pleural cavity surrounds and protects the lungs. It is made of membranes that fold back onto themselves to form two layers. The outer layer or parietal pleura attaches to the chest wall and is highly sensitive to pain. The inner layer is the visceral pleura. It covers the lungs and does not have any sensory innervation. The thin space between them is the pleural cavity. It contains a small amount of fluid that acts as a lubricant.

While the ribs are not directly part of the respiratory system, they are necessary to keep the lungs functioning properly and are one of the strongest structures in the body. They completely surround and protect the lungs and heart. The ribs connect to the sternum with costal cartilage, which gives them the flexibility to expand when the lungs fill with air.

With each inhalation, the lungs pull air into the mouth and nose and through the upper airway, where it is heated and humidified. From there, it travels through the trachea and into the main bronchi, where it splits and enters the right and left lungs. Inside each lung, the air travels down the bronchioles and into the alveoli where oxygen and carbon dioxide cross through the capillary membranes. Oxygen enters the blood and goes into the heart, where it travels out into the body. When the lungs exhale, the carbon dioxide that is now in the alveoli travels up through the bronchioles, bronchi, trachea, mouth, and nose, out of the body.