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Lower Respiratory Tract


Dr. Flemings
The major structures of the lower respiratory tract include the trachea, bronchi and bronchial tree, and the lungs. Dr. Patel, can you tell us about this part of the respiratory system?

Dr. Patel
Sure thing, Dr. Flemings. Inhaled air that comes from the upper respiratory tract passes through the trachea, commonly called the windpipe. The walls of the trachea are supported by rings of C-shaped cartilage, called hyaline cartilage, that keep the trachea from collapsing.

The mucous membrane that lines the trachea contains goblet cells, which produce mucus to trap airborne particles and microorganisms. Hairlike structures called cilia move the mucus and particles upward, where they can be swallowed, coughed out, or sneezed out.

The trachea branches off to the two primary bronchi, which lead to more bronchi that continue branching until they reach the lungs. Like the trachea, the bronchi contain hyaline cartilage rings that provide support and a mucous membrane that helps protect against foreign particles, bacteria, and viruses.

In general, the right lung is shorter, broader, and has a greater volume than the left lung. It's divided into three lobes. The left lung, which is longer and narrower, has two lobes. The left lung also has an indentation, called the cardiac notch, to make room for the heart.

Each lung is surrounded by a double-layered membrane called the pleura. The space between the membrane layers is called the pleural cavity, which contains a thin fluid that acts as a lubricant as the lungs inflate and deflate.

Inside the lungs, the primary bronchi branch into smaller and smaller passageways, called lobar bronchi, segmental bronchi, interlobular bronchi, and bronchioles. These make up the bronchial tree. Unlike the trachea and bronchi, there is no cartilage in the bronchioles, which are the smallest airways.

The bronchioles end in air sacs called alveoli, which are bunched together in clusters, called alveolar sacs, that resemble bunches of grapes. Tiny blood vessels called capillaries cover the surface of the alveoli.

Here, oxygen from the alveoli and carbon dioxide from the blood are exchanged. This process is called external respiration. After the blood is oxygenated, it goes to the heart, where it is pumped to the rest of the body to provide tissue cells with oxygen.

The carbon dioxide exchanged from the blood is exhaled back through the lower and upper respiratory tracts and expelled from the body through the nose or mouth.