The lungs

The Lungs
You have two lungs, one on each side of the chest. The top, cone shaped part of the lung that fits under the collarbone is called the apex. The broad, bottom part of the lung that rests on the diaphragm is called the base. Unlike some other paired organs in the body, the two lungs are not identical (see Figure 1). The right lung is normally larger than the left. It is divided into three sections or   lobes, the upper (also called superior), middle, and lower (also called inferior). The left lung has two lobes, the upper and lower. The left lung has an indentation called the cardiac notch to make room for the heart. Each lung lobe is divided into segments. Health care providers often use the names of lung segments to describe specific locations in the lungs.
The lungs contain elastic fibers that allow the lungs to expand and contract. Healthy lungs have a smooth, shiny surface because they are encased in a thin, moist covering called the visceral pleura. There is also a pleural covering called the parietal pleura that lines the inner surface of the chest cavity. The visceral and parietal pleura are normally slippery and glide easily against each other as you breathe.

Figure 1: Organs of the Chest

The right and left main bronchi carry air in and out of the lungs. They are the beginning of a system of airways called the bronchial tree. The point at which the main bronchus enters the lung is called the hilum. The bronchial airway system is called a ‘tree’ for a very practical reason. The airways branch approximately 20 times in the lungs. At each branch point, the airways become smaller and more numerous much like the branches of trees (see Figure 2 ). The main bronchi branch into lobar bronchi, one for each lobe of the lung. From here, the airways divide into segmental bronchi, one for each lung segment. Segmental bronchi branch several times into smaller airways called bronchioles. The final branches of the bronchial tree are called atria. The atria end in tiny, microscopic air sacs called alveoli. Alveoli resemble clusters of grapes under the microscope. Each lung contains about 300 million alveoli. The walls of the alveoli are extremely thin, which makes them fragile and susceptible to damage. Damage to the alveoli is irreversible.

Figure 3: The Bronchial Tree

Alveoli are surrounded by tiny blood vessels called capillaries. There are about one billion capillaries in the lungs, more than three for each air sac. The blood in the capillaries is separated from the air in the alveoli only by the extremely thin alveolar and capillary walls. This close proximity allows gases to be exchanged between the blood and the lungs in a process called respiration. Inhaled oxygen enters the blood from the alveoli. Carbon dioxide leaves the blood and enters the alveoli to be exhaled. The close relationship between the circulatory and respiratory systems will be discussed in the following section, The Link Between the Respiratory and Circulatory Systems.

The lungs are traversed by another important system of vessels called the lymphatics. Each lung contains a network of lymphatic vessels that carry a mixture of fluid and proteins called lymph. Lymph is carried from the lung tissues through a series of lymph nodes to filter the fluid before it is returned to the bloodstream. This network of lymph vessels and lymph nodes are an important part of the body’s immune system. The work of the immune system and its role in lung cancer will be discussed in other posts