The role of the lungs in pulmonary circulation is as important as that of the heart. The lungs are a pair of organs that occupy the human chest cavity, and are positioned on either side of the heart. The lungs play two critical roles in pulmonary circulation. First, they provide the source of oxygen that is transported by the pulmonary circulation back to the heart, which is then distributed by the systemic circulation to all of the cells. Second, they eliminate carbon dioxide produced by the tissue in our bodies as the waste product of cellular metabolism.
The lungs function by a process known as external respiration, or simply known as breathing, which consists of inhaling and exhaling. When a person inhales, the large, dome-shaped muscle that lies below the lungs contracts, then descends. This muscle is called the diaphragm. As it descends, it creates a negative pressure around the lungs. This causes air to be drawn in through the nose, down the trachea or windpipe, which then branches into tubes called bronchi-one going to each lung. In the lungs, these bronchi continue to branch out into progressively smaller tubes called bronchioles. The smallest tubes, called the terminal bronchioles, then inflate the tiny air sacs inside each lung, called alveoli.
Air is primarily made up of 79% nitrogen and 21% oxygen. It is the oxygen, however, that our bodies need for cellular metabolism, the process of converting the food we eat into energy, to take place. As oxygen enters the alveoli, it crosses a thin barrier called the alveolar-capillary membrane, then enters tiny blood vessels called capillaries. These capillaries come together to form progressively larger blood vessels called veins, which transport this oxygen-enriched blood into the left atrium, when it then flows into the left ventricle. The left ventricle pumps the oxygenated blood to the tissue by the process known as systemic circulation. This oxygen is then transported to all cells of each organ in our bodies.
As oxygen is transported to the cells, the cells release carbon dioxide, the waste product of cellular metabolism. This gas is transported by the veins of systemic circulation to the right atrium of the heart. It then enters the right ventricle, which pumps this blood that is rich in carbon dioxide and low in oxygen to the lungs by way of the pulmonary artery. This is called pulmonary circulation. The carbon dioxide then crosses from the capillaries to the alveoli, where it is eliminated during exhalation. Exhalation begins when the diaphragm ascends, forcing air out of the lungs, then up through the trachea.
It is clear that both the heart and lungs need to function properly in order for the pulmonary circulation to be effective. If the lungs are impaired by diseases as emphysema and pneumonia, less oxygen is transmitted to the blood. If the heart is impaired by disease, it will transport oxygen to the cells at a diminished rate. Cells that are deprived of oxygen will die. The role of the lungs in pulmonary circulation, therefore, is critical to sustaining life.