Pharynx: It is a common channel to the digestive and respiratory systems and communicates with the mouth and nasal passages. The air inspired by the nostrils or mouth necessarily passes through the pharynx before reaching the larynx.
Larynx: It is a tube supported by articulated cartilage pieces, located in the upper neck, in continuation of the pharynx. The Adam's apple, a protrusion that appears on the neck, is part of one of the cartilaginous parts of the larynx. The entrance to the larynx is called a glottis. Above it is a kind of cartilage “tongue” called epiglottis, which works as a valve. When we feed, the larynx rises and its entrance is closed by the epiglottis. This prevents ingested food from entering the airways.
The epithelium lining the larynx has folds, the vocal chords, capable of producing sounds during airflow.
Trachea: is a tube approximately 1.5 cm in diameter by 10-12 cm in length, whose walls are reinforced by cartilaginous rings. It forks in its lower region, giving rise to the bronchi, which penetrate the lungs. Its mucociliary lining epithelium adheres to dust particles and bacteria present in the air, which are then swept out (thanks to the movement of the lashes) and swallowed or expelled.
Lungs: The human lungs are spongy organs, approximately 25 cm long, surrounded by a serous membrane called pleura. In the lungs the bronchi branch off profusely, giving rise to increasingly thin tubes, the bronchioles. The highly branched set of bronchioles is the bronchial tree or respiratory tree.
Each bronchiol ends in small pockets formed by flattened epithelial cells (paved epithelial tissue) covered by blood capillaries, called pulmonary alveoli.
Diaphragm: The base of each lung rests on the diaphragm, the musculoskeletal organ that separates the thorax from the abdomen, which is present only in mammals, promoting, along with the intercostal muscles, respiratory movements. Located just above the stomach, the phrenic nerve controls the movements of the diaphragm.
The work of the pulmonary alveoli
The alveoli are elastic structures, formed by a very thin membrane and surrounded by a network of blood capillaries.
There are millions of alveoli in each lung. It is in each of them that the gas exchange between the lung and the blood occurs. In the alveoli gas diffusion occurs by concentration difference and consequently the gas pressure. The blood that reaches the alveoli absorbs the inspired oxygen gas from the atmosphere. At the same time, the blood eliminates carbon dioxide inside the alveoli; This gas is then expelled from the body through exhalation.
On inspiration, the diaphragm and intercostal muscles contract. Upon contracting, the diaphragm descends and the thoracic cavity swells vertically. When the intercostal muscles contract, they take the ribs and the volume of the chest cavity increases horizontally. As the chest volume increases, air pressure inside the chest cavity and lungs decreases. Then the atmospheric air pressure becomes greater than the internal air pressure, and the atmospheric air enters the body going to the pulmonary alveoli: it is the inspiration.
In a second movement, the diaphragm and intercostal muscles relax, decreasing the volume of the chest cavity. Then the internal air pressure (inside the lungs) increases, becoming higher than the atmospheric pressure. Thus, the air leaves the body to the external environment: it is the expiration.
In the pulmonary alveoli, oxygen gas, present in the inspired air, passes into the blood, which is then distributed by red blood cells to all living cells in the body. At the same time, living cells release carbon dioxide into the blood. In the lungs, carbon dioxide passes from the blood into the alveoli and is excreted to the external environment through exhalation.