The lungs are the main organ of the lower respiratory tract and responsible for the exchange of oxygen, which is necessary for cellular processes, for carbon dioxide, a cellular waste product. This gas exchange is possible because of the thin membranes and air sacs that make up the lungs, allowing easy access to the capillaries.
Structure of the Lungs
The lungs are divided into lobes. The right lung has three lobes, but the left lung has two lobes - it is smaller to accommodate the heart, which is positioned below the left lung and takes up the space of the third lobe in the chest cavity. Air enters the lungs via the bronchial tree, a series of increasingly smaller branches off of the windpipe (or trachea). Off the windpipe, the left bronchus branches into the left lung, and the right bronchus branches into the right lung. These bronchi then branch into bronchioles, which terminate at the alveolar sacs. The alveolar sacs contain the small, thin-walled air pouches known as alveoli.
The alveoli are the smallest units of the lung tissue and divided into type I and type II pneumocytes. The alveoli are essentially air sacs with thin membranes, allowing the passage of air into the full depth of the lung tissue. Because of these air sacs, the lungs inflate and deflate while breathing. The lungs do not collapse because of an alveolar cell (type II) secretion called pulmonary surfactant, a lipoprotein that reduces surface tension. Surfactant also keeps the airways dry by reducing the capillary fluid drawn in by surface tension.
Function of Breathing
Atmospheric air, which contains mostly nitrogen and oxygen, is inhaled through the upper respiratory system into the lungs. The oxygen passes into the circulation and carbon dioxide is exhaled out from the lungs through the mouth and nose along with the nitrogen and other gases the body doesn't use. This is the process known as breathing.
Oxygen is necessary for cellular processes, but the gas exchange that occurs in the lungs is also important for maintaining blood pH. The exchange occurs due to differences in the pressure in the lungs, atmosphere, and blood. Exhalation occurs passively, whereas inhalation requires energy (active process).
The lungs also contain macrophages, mature white blood cells, or immune cells, that scavenge foreign particles such as bacteria and dust. Particles may be inhaled into the lungs depending on their size, large particles do not make it past the upper respiratory system, but smaller particles will filter into the bronchial tree. The alveolar macrophages are also known as dust cells, and they prevent the infiltration of the body by inhaled pathogens.
There are a number of measurements and terms used to determine a person's breathing capacity and rate, which affects the oxygen saturation of the blood. Thinner air, meaning a lower atmospheric oxygen concentration, occurs at higher altitudes, resulting in a quickened rate of breathing to bring in enough oxygen to the lungs. Exertion also increases the breathing rate because of an increased demand for oxygen by the tissues. Also a determinant of breathing rate is lung capacity. The lungs have reserve air that can be forced out beyond exhalation to increase capacity, but they are never empty of all air.
Pulmonary Blood Supply
Alveolar ducts contain blood vessels fed by the pulmonary artery, which transports deoxygenated blood (i.e. containing carbon dioxide) from the heart to the lungs. Capillaries, the smallest blood vessels that act as bridges between arteries and veins, in close proximity to the alveoli have thin walls and allow the blood-gas barrier to function in gas exchange. The pulmonary vein transports oxygenated blood from the lungs to the heart for circulation throughout the body.