The lungs are the two main organs of respiration. They are located in the chest cavity on the sides of the heart and have the ability to expand and relax following the movements of the rib cage and diaphragm.
The right lung - heavier (600 g) - is divided by deep fissures into three lobes (upper, middle and lower), while the left one - less voluminous (500 g) - has only two (an upper and a lower lobe). .
The lungs are made up of a spongy and elastic tissue, which adapts well to the volume variations induced by respiratory movements.
The two lungs are separated by the mediastinum and joined by the trachea.
The mediastinum is a region between the sternum and the thoracic vertebrae, inside which there are various organs (thymus, heart, trachea, extrapulmonary bronchi, esophagus), as well as vessels, lymphatic structures and nerve formations.
The trachea, 10-12 cm long and 16-18 mm in diameter, is a semi-flexible cylindrical tube supported by cartilaginous rings. Superiorly it flows into the larynx, while
Each primary bronchus penetrates inside the respective lung, giving rise to further, numerous, ramifications called bronchioles. In turn, the bronchioles undergo various divisions, until they reach, in the terminal tract, small vesicles called alveoli. To get an idea. of the complexity of these branches, just think that each lung contains approximately 150-200 million alveoli; taken together, the alveolar surfaces reach an impressive extension, similar to that of a tennis court (75 m2, that is about 40 times the outer surface of our body).
At the level of the alveoli there is the exchange of gas between the air and the blood, which releases water vapor and carbon dioxide, charging itself with oxygen. Each alveolus is surrounded by hundreds of very thin capillaries, whose diameter is so small (5-6 µm) to allow the passage of a single red blood cell, while the peculiar thinness of their walls facilitates the exchange and diffusion of respiratory gases.
The dense capillary network is fed by the branches of the pulmonary artery - in which venous blood circulates - and drained by those of the pulmonary vein (in which arterial blood flows which will distribute oxygen to the various tissues). The blood flow is linked to the action of the right heart, whose activity is entirely dedicated to supporting pulmonary circulation. For this reason, the blood flow to the lungs is in percentage equal to that which reaches the rest of the body in the same amount of time. engaged in strenuous physical exercise (25 L / min), the flow rate of blood to the lungs will always be equal to 100%. Unlike what happens in the large circle, however, blood pressure remains at significantly lower levels, from since the resistance offered by the flow during right ventricular systole is very low (thanks to the high sectional area of the pulmonary arterioles and the shorter length of the vessels).
The thin membrane that delimits the alveolar walls gives the lungs the characteristic spongy appearance. While the trachea and bronchi are supported by hyaline cartilage, smooth (involuntary) muscle tissue is present in the walls of the bronchioles; consequently, the bronchioles have the ability to increase or decrease their caliber in response to stimuli of various kinds. During physical exertion, for example, the bronchioles dilate to allow better oxygenation of the blood in response to the increase in CO2 in the exhaled air, while they tend to constrict themselves with the cold.
Excessive bronchoconstriction in response to various types of agents (environmental pollution, physical exercise, excessive mucus production, inflammation, emotional factors, allergies, etc.) underlies various lung diseases, such as asthma or COPD.
Second part "
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