Lungs
The lungs are vital organs located in the chest (thoracic) cavity of the human body. They function in gas exchange by obtaining oxygen from the air to provide energy for cells throughout the body and by eliminating carbon dioxide. Air enters through the nasal cavities, travels into the main airway (called the trachea) and then into the lungs. There are two lungs. The left lung is smaller (to accommodate the heart), while the right lung is slightly larger. The trachea branches into the bronchi, which are tree-like tubes that connect to each lung. The bronchi branch further into smaller passageways called the bronchioles, which lead to the alveoli. The alveoli are sac-like structures (that look like clusters of grapes on a stem) where gas exchange occurs. Unlike the trachea, the bronchioles can be recognized by an absence of cartilage. Pulmonary arterioles are blood vessels that travel juxtaposition to the bronchi and branch more and more extensively, turning into pulmonary capillaries that cover the alveoli. Oxygen diffuses from the alveoli into the pulmonary capillaries that surround the alveolar sacs. This is how oxygen gets transported from the lungs to tissues throughout the body.
The airways are categorized into two major sections, the conducting portion and the respiratory portion. The conducting portion of the airways spans from the trachea to the terminal bronchioles, while the respiratory portion includes from the respiratory bronchioles to the alveoli. The conducting portion serves not only as a passageway for air to travel to the alveoli, but also to warm and humidify the air. It is also important in protecting the body from aerosolized pathogens (disease-causing microorganisms) and contaminants in the air. This is achieved this by tiny finger-like structures called cilia that beat synchronously to remove particulate material trapped in mucous secreted by airway cells, a process called mucociliary clearance. The surface area increases exponentially from the conducting to the respiratory portions of the airways. With over 300 million alveoli, there is a large enough surface area (enough to be the size of a tennis court) for massive amounts of gas exchange and coupled to a very short diffusion path, the body can obtain sufficient amounts of oxygen, especially helpful during strenuous cardiovascular exercise. Additionally, the lung volume has a large reserve capacity beyond normal breathing to account for increases in ventilation, or the movement of oxygen into the lungs by bulk flow.
The bronchus, bronchiolus, and alveolus can be distinguished morphologically in terms of differing cellular and structural organization. The airway epithelium (or the outermost cells in contact with the airway surface) is composed of a continuous network of cells that are anchored by a thin membrane called the basement membrane. The basement membrane is found throughout the airways. Below the basement membrane is a type of tissue called connective tissue that tapers off in the bronchiolar epithelia and is absent in the alveoli. The epithelium of the bronchi consists of Goblet cells, which are the primarily mucous secreting cells, as well as cells that have cilia. Morphologically, the cells in the bronchi are columnar meaning that they appear rectangular or column-shaped. There are also glands called submucosal glands in the epithelia that secrete various substances such as mucous. As the airways branch into the bronchioles, the cells become cuboidal, or cube-shaped, and there are no submucosal glands. The alveoli do not contain ciliated or secretory epithelial cells but are composed of Type I and Type II alveolar epithelial cells. These cells are squamous (or flattened) in shape and are widely spaced to allow for the diffusion of oxygen into the pulmonary capillaries.
The lungs can expand and contract without friction during breathing due to the pleura, a thin membranous structure. The visceral pleura surrounds the lungs, while the parietal pleura lines the wall of the thoracic cavity. The visceral and parietal pleura are separated from each other by a small fluid-filled space called the pleural cavity. Ventilation requires work and before the lungs can become inflated, a pressure change must take place. The elastic properties of the lung allow ventilation to take place more efficiently and the fluid in the pleural cavity serves as a lubricant that allows the lungs to slide against the chest wall.
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