The lung is a vital organ that is a part of the respiratory system. Generally a person has two lungs; however, the body can function with only one. The lung’s main function is to move oxygen and carbon dioxide into and out of the bloodstream through gas exchange. The lungs are located lateral to the heart in the thoracic cavity. Each lung has a tip called the apex, which is superior in the body. The bases of the lungs rest on the diaphragm. They appear to be spongy due to their bundles of elastic fibers which allow them to expand. The left lung has two parts called the superior and inferior lobes, while the right lung has three parts called the superior, middle, and inferior lobes. Each part is divided by a fissure, and while the medial aspect of the right lung appears straight, the medial aspect has a dent in it, called the cardiac notch, which allows room for the heart in between the two lungs.
The trachea, or windpipe, branches off into bronchi, which are hollow, tube-like structures that enter the lungs. Within each lung are structures called bronchioles. These small, hollow, tube-like structures support air flow through the lungs. Instead of their walls being made of cartilage, they are of smooth muscle so they can either tighten (bronchoconstriction) or loosen (bronchodilation). The bronchioles branch off into terminal bronchioles to supply air to its particular lobule. A lobule is a piece of lung that is supplied by a single bronchiole and parts of pulmonary arteries and veins. The terminal bronchioles then split into respiratory bronchioles to move air into the alveoli. Aveoli are sac-like structures in the lungs that function in gas exchange.
Each lung is surrounded by a serous membrane called pleura, which secretes fluid to lubricate the lungs and reduce friction. The visceral pleura cover the outside of the lungs, while the parietal pleura lines the inside of that particular body cavity, in this case the thoracic cavity. The rib cage surrounds the lungs and serves mainly as protection.
For atmospheric air to reach the lungs, it must pass through the nasopharynx, oropharynx, larynx, trachea, and finally the bronchi, bronchioles, and alveoli, all of which are in the lungs. Air then follows the reverse pattern to leave the lungs and exit the body. The act of breathing, or ventilation, is controlled by the autonomic nervous system. The diaphragm is one of the largest and strongest muscles in the body; it is located below the lungs and directly controls how much air is moved into and out of the body. When the ANS tells it to contract, it pulls the bottom of the lung cavities downwards, which increases the volume within the lungs and thus decreases the pressure to allow air to move into the body. Tidal volume is the amount of air that is taken in with each breath. This part of breathing, called inhalation or inspiration, is considered to be an active process. However, when the diaphragm relaxes, it forces air back out of the lungs. This process, called exhalation or expiration, is considered to be passive.
Gas exchange, called respiration, occurs within the alveoli. Each one is individually wrapped in blood vessels; once deoxygenated blood is pumped by the heart through the pulmonary artery into the lungs, the blood will release carbon dioxide and the hemoglobin, or proteins of the red blood cells, will pick up oxygen. Through a process called diffusion, the gas molecules will travel through the alveolar and capillary walls to reach the blood.
Although the lungs play a very important role in respiration and breathing, they also have other functions. The lungs help maintain the pH of blood by facilitating changes to the partial pressure of carbon dioxide. If the blood pH becomes more acidic (roughly less than 7) or more basic (roughly greater than 8), it can be life-threatening. The lungs also function to filter out small blood clots that may be formed in veins. The trachea is lined with tiny hairs called cilia which protect the body by sweeping out any foreign particles. The lungs are essential to the cilia because they provide the airflow needed. The lungs also provide the airflow needed for the larynx to make vocal sounds. Lastly, the lungs store roughly ten percent of the body’s total blood volume.
Many illnesses involving the lungs are either bacterial or viral, due to the lung’s moist internal environment. Inflammation of the lungs is known as pneumonia, while inflammation of the pleura surrounding the lungs is known as pleurisy. Inflammation of the bronchi is known as bronchitis. In diseases such as Chronic Obstructive Pulmonary Disease, chronic bronchitis or emphysema, the alveolar sacs containing the alveoli are damaged. There are so many alveoli that the body can afford for many of them to burst without causing a problem. But if over time they continue to be damaged, the body will no longer be able to compensate. Smoking greatly increases the risk of COPD. One of the most common traumatic injuries to a lung is called a pulmonary contusion, which is a bruise to the lung caused likely by blunt force. Another common injury is called a pneumothorax, which is the abnormal build up of gases within the chest cavity outside of the lungs themselves.
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