Along with the upper respiratory tract, the lower respiratory tract makes up the respiratory system. In descending order, it begins with the trachea, which is also known as the windpipe. The trachea divides into two main bronchi, and one bronchi each enters one of the pair of lungs. As the bronchi branch out and become smaller in the lungs, they eventually allow for gas exchange to take place. The site of the exchange of oxygen and carbon dioxide is known as the respiratory zone.
At the top of the lower respiratory tract is the trachea. It connects the larynx above it to the main bronchi. It is a flexible, hollow tube that can stretch with the movements of the neck. It also has a set of sturdy cartilage rings that keep it from collapsing during breathing. This could otherwise happen because of the changes in air pressure that move air in and out of the lungs.
The two main bronchi branch off from the trachea. Each one carries air into a separate lung. Together, they form the head of what is called the bronchial tree. The bronchial tree contains a network of increasingly smaller bronchial tubes that first divide from the main bronchi. Eventually they end in structures called bronchioles, which are extremely small.
The bronchioles are nearly the smallest part of the lower respiratory tract. At the furthest and smallest end of the bronchial tree are the alveoli. This is the site where oxygen from the air enters the blood and carbon dioxide passes into the air-filled alveoli. Here these gases pass in and out through extremely small blood vessels exposed to air.
The lungs are responsible for moving air throughout the upper and lower respiratory tract. They draw in air when contracting muscles cause them to expand. The difference in air pressure that results causes air to be drawn into the lungs. After expanding, they naturally recoil before the next inward breath.
The lower respiratory tract can be affected by a number of disease and disorders. Chronic bronchitis has a negative effect on normal gas exchange and is mostly seen in smokers. Pneumonia, caused by viruses or bacteria, can seriously impair breathing by causing fluid to build up in the longs and is a common cause of death in sick people. Tuberculosis, caused by bacterial infection in the lungs, has a weakening effect on the body through repeated breakouts. Emphysema is usually caused by smoking and is marked by permanent damage to the alveoli.