The lungs are the major organs of the respiratory system, which helps provide the body with a continuous supply of oxygen. The lungs take more than 6 million breaths per year and affect every aspect of our bodies and health. This article looks at the form and function of the lungs, diseases that affect them, and how to maintain healthy lungs. The lungs’ main role is to bring in air from the atmosphere and pass oxygen into the bloodstream. From there, it circulates to the rest of the body. The organs require help from surrounding structures in the body in order to breathe properly. To breath, we use the muscle of the diaphragm, the intercostal muscles between the ribs, the muscles of the abdomen, and sometimes even muscles in the neck. The diaphragm is a muscle that is domed at the top and sits below the lungs. It powers most of the work necessary in breathing. As it contracts, it moves down, allowing more space in the chest cavity and increasing the lungs’ capacity to expand. As the chest cavity volume increases, the pressure inside goes down, sucking in air through the nose or mouth and down into the lungs. As the diaphragm relaxes and returns to its resting position, the lung volume decreases because the pressure inside the chest cavity goes up, and the lungs expel the air. The lungs are like bellows — as they expand, they suck in air for oxygen. As they compress, the exchanged carbon dioxide waste is pushed back out during exhalation. When air enters the nose or mouth, it travels down the trachea also known as the windpipe. After this, it reaches a section called the Carina. At the Carina, the windpipe splits into two, creating two main stem bronchi. One leads to the left lung and the other to the right. From there, similar to branches on a tree, the pipe-like bronchi split again into smaller bronchi and then even smaller bronchioles. This ever-decreasing pipework eventually terminates in the alveoli, which are tiny air sacs. Here, gas exchange occurs.
Although they are a pair, the lungs are not equal in size and shape. While the left lung has an indentation bordering where the heart resides, called the cardiac notch, the right lung is shorter to allow space for the liver below. Overall, the left lung has a slightly smaller weight and capacity than the right. Two membranes, known as the pulmonary pleurae, surround the lungs. The inner layer directly lines the outer surface of the lungs, and the outer layer attaches to the inner wall of the rib cage. The space between the two membranes contains pleural fluid, which helps the pleura stay moist and reduces friction between the membranes when breathing. The alveoli are the endpoint of the oxygen’s journey from the outside atmosphere to the depths of the lungs. The total surface area of membrane that the alveoli provide is roughly 70 meters squared, or around the size of half a tennis court. After the lungs, the body takes oxygen out of the bloodstream to its other tissues as it travels around the circulatory system. The blood that has given up its oxygen in exchange for carbon dioxide from the tissues then passes through the heart and travels to the lungs to reach the capillaries surrounding the alveoli. The alveoli now contain a new supply of oxygen that a person has inhaled. This oxygen passes across a membrane, called the alveolar-capillary membrane, into the bloodstream. At the same time, the carbon dioxide that has collected in the bloodstream during its travels around the body enters the alveoli. From there, a person’s body breathes it back out into the atmosphere during exhalation. Special cells in the alveoli produce a compound known as pulmonary surfactant. It comprises lipids, proteins, and carbohydrates. Surfactant features both hydrophilic and hydrophobic regions — water attracts hydrophilic regions but repels hydrophobic regions. Pulmonary surfactant has several vital functions, including allowing for better breathing efficiency and preventing the alveoli from collapsing on themselves. Each alveolus is similar to a plastic bag that is wet inside. If there were no surfactants, the bag would collapse in on itself and the internal sides would stick together. Surfactant prevents this from happening to the alveoli. Pulmonary surfactant carries out its role by reducing the amount of surface tension. By doing this, it reduces the effort necessary to inflate the alveoli.
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