Lungs Health Article

Definition

The two lungs are spongy and highly elastic organs of respiration in the pulmonary cavities of the thorax, where the aeration of blood occurs.

Description

Each lung has an irregular conical shape with a blunt top, called the apex, extending into the root of the neck. They have concave bottoms resting on the arc of the diaphragm, a mostly concave inner mediastinal surface that follows the lines of the pericardium, and a convex outer (costal) surface. The right lung is larger than the left, and consists of three lobes (upper, middle, and basal or lower). The left lung consists of two lobes, an upper and a basal, or lower, lobe.

Each lung consists of an exterior plasma coat comprised of an organ coat which folds back to make an interior lining for the chest cavity. The inner lung contains sub-serous areolar tissue with elastic fibers interspersed over the surface of the organ. The parenchyma, or functional part of the organ, is composed of secondary lobules (alveolar ducts) that differentiate into primary lobules (alveoli) consisting of blood vessels, lymphatics, nerves, and an alveolar duct that connects with air space.

The lung, as it relates to inspiration and expiration, has two distinct zones in which the lung passages convey air to the alveolar sacs. The zones relate to the two functions of these passages. One is for conducting air, and the other is for respiration. The parts of the conducting zone do not participate in gas transfer, rather they convey air to and from the respiratory zone. All of the parts of the respiratory zone can take part in gas transfer. However, the uppermost branches, such as the respiratory bronchioles, participate in respiration only in times of exertion.

The conducting zone starts at the trachea and branches out to the bronchi. The bronchi differentiate into bronchioles and then into terminal bronchioles. The respiratory zone starts after the terminal bronchioles at the respiratory bronchioles. These differentiate into the alveolar ducts, which terminate at the alveolar sacs. The lungs consist mainly of the tiny air containing alveolar sacs.

Function

The lung is the sole means of gas exchange in respiration. Air is brought into the body through the mouth or nose and trachea to the lung. There oxygen diffuses from the airspace of the alveoli into the blood stream and carbon dioxide diffuses from the blood into the alveoli's airspace.

The alveoli are small hollow sacs. Their ends connect to the lumens of the airways. The air adjacent to surfaces of the alveolar wall are lined by a single cell layer of flat epithelial cells called type I alveolar cells. In between type I cells are type II cells. They are thicker, and secrete a fluid called surfactant. In the alveolar walls this fluid and connective tissue fills the interstitial space and is interspersed with capillaries. In some places the interstitial space is nonexistent and the epithelial cell membranes are in direct contact with the capillaries. The blood in the capillaries is separated from the air by a single layer of flat epithelial cells. The surface area in a single alveoli is roughly the size of a small basketball court due to the undulating terrain of the type I and II epithelial cells. There are around 300 million alveoli in the adult male. Thus, there is a large surface area where the air and the blood stream are in close proximity. This large surface area is necessary for gas exchange to easily occur. The respiratory system also needs a continual supply of fresh air, which is supplied by the process of breathing.

The process of breathing is aided by the position of the lungs in the thorax (chest). The thorax is a closed chamber that extends from the neck muscles to the diaphragm. The diaphragm is a dome shaped sheet of skeletal muscle that separates the thorax from the abdomen. The sides of the thorax are bounded by connective tissue around the spine, ribs, intercostal muscles, and sternum.

A completely enclosed sac consisting of a thin sheet of cells, called the pleura, surround each lung. Between the pleura and the lung is interstitial fluid. As the diaphragm expands and contracts the intra-pleural pressure placed on the lungs causes the lung to inflate and deflate. Breathing allows a fresh supply of air and oxygen to enter the lung upon inflation and carbon dioxide to exit the lung upon deflation. It also causes a change in the pressure of the lung.

The epithelial surface from the conducting zone to the respiratory bronchioles is lined with cilia that continually beat in the direction of the pharynx. There are epithelial cells and glands on this surface that secrete mucus. This mucus catches particulate and bacterial matter, and the material (and mucus) is slowly moved by the cilia toward the pharynx. There it is either swallowed or coughed up as sputum. The epithelial layer also secretes another viscous fluid that allows the cilia to move mucus easily out of the lung.

Toxic substances can inhibit ciliary action. Agents like cigarette smoke can paralyze the cilia for extended periods of time. This inhibits the movement of mucus and particles out of the lungs. The suspension of this process can inhibit gas exchange and eventually cause prolonged oxygen deficiency.