The lungs are the two main organs of respiration. They are found in the thoracic cavity on the sides of the heart and have the ability to expand and relax following the movements of the rib cage and the diaphragm.
The right lung - heavier (600 g) - is divided by deep fissures in three lobes (upper, middle and lower), while the left - less voluminous (500 g) - has only two (one upper and one lower lobe) .
The lungs are made up of a spongy and elastic tissue, which is well suited to the volume changes induced by respiratory movements.
The two lungs are separated from 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 branches called bronchioles. In turn, the bronchioles undergo various divisions, until they reach, in the terminal tract, small blisters 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, or about 40 times the external surface of our body).
It is precisely at the level of the alveoli that gas is exchanged 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) as to allow the passage of only one 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 from those of the pulmonary vein (in which the 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 the pulmonary circulation. For this reason the blood flow to the lungs is equal in percentage to that which reaches the rest of the body in the same period of time. Whether you are in a state of rest (cardiac output 5 L / min) or engaged in strenuous exercise (25 L / min), the flow rate of blood to the lungs will always be 100% . Unlike what happens in the large circle, however, the arterial pressure remains at much lower levels, since the resistance offered by the flow during the right ventricular systole is very low (thanks to the high area of section of the pulmonary arterioles and to the lower vessel length).
The thin membrane that surrounds the alveolar walls gives the lungs the characteristic spongy appearance. While trachea and bronchi are supported by hyaline cartilage, smooth (involuntary) muscle tissue is present in the bronchiole walls; consequently, the bronchioles have the ability to increase or decrease their caliber in response to various stimuli. During physical exertion, for example, the bronchioles dilate to allow better oxygenation of the blood in response to the increase in CO 2 in the expired air, while they tend to be constricted with cold.
Excessive bronchoconstriction in response to agents of various kinds (environmental pollution, exercise, excessive mucus production, inflammation, emotional factors, allergies, etc.) is the basis of various lung diseases, such as asthma or COPD.
Second part "
