Respiratory system: summary anatomy, structure and functions

The respiratory system is made up of a group of highly specialized organs which has the main – but not exclusive – task of supplying the cells with oxygen and eliminating the carbon dioxide produced by the oxidation of organic substances. In humans it is made up of the airways, the two lungs and the internal and external pleura. The airways are formed by the external nose (nasal pits and nasal cavities), the pharynx, larynx, trachea, bronchi and bronchioles. The right lung, more voluminous, is divided into three lobes, while the left one is divided only into two lobes to make room for the apex (tip) of the heart. The inhalation, the exhalation and the short pause that occurs between the two movements constitute a respiratory act. Each respiratory act involves a series of changes in the rib cage where the lungs are  housed and in the position of the diaphragm, a flat muscle that separates the chest and abdomen

Function of the respiratory system
The main function of the respiratory system is to guarantee the correct oxygenation of our blood and the elimination of the CO2 that we possess in excess. Since these gases must enter or circulate from the bloodstream, the respiratory tract must necessarily combine ventilation and perfusion. The mechanical process commonly called breathing takes the name of lung ventilation, while the term breathing indicates the whole process that goes from the introduction of oxygen to its use for the oxidation of the substrates and the consequent production of energy. The airways are intended to distribute the air flow over an area of ​​approximately 70 m2 for a 70 kg man.housed and in the position of the diaphragm, a flat muscle that separates the chest and abdomen.

Anatomy and functions of individual parts of the respiratory system
The airways are divided into upper and lower. The upper airways include:

  • the nasal cavities, which have the task of humidifying the air and bringing it around to a temperature equal to that of the body. They provide about 50% of the flow resistance, also act like a filter and block particles larger than 10 micrometers in diameter.
  • The pharynx. It is not only a passageway but thanks to numerous lymphocyte aggregates it provides active protection against infectious agents.

The larynx that contains the organ of phonation and the glottis (which can be voluntarily occluded). the lower airways originate under the ring of the cricoid cartilage and are divided into:

  • trachea, a cylindrical duct that is formed by a membranous structure stretched between cartilaginous rings of structural support, not collapsible.
  • bronchi, which originate in two, one on each side, and constitute the ducts in which the trachea ends. penetrated into the lung they become arboreal, becoming lobar, lobular bronchi and giving ever smaller ramifications; the section of the 2 offspring branches is however larger than the section of the bronchus from which they originate. For ten orders of branching (which occur at an acute angle), up to a diameter of 1 micrometer, the bronchi retain cartilage plaques in their walls which confer a certain structural rigidity. Below this diameter, the bronchi are called bronchioles and become collapsible by pressure. After six orders of branches (at T) in the bronchioles, sporadic alveoli begin to appear along the wall. The subsequent branches take the name of terminal (or respiratory) bronchioles. The latter continue to divide until they generate alveolar ducts, or bronchioles whose surface is sprinkled with alveoli. In the lung, morphofunctional units are identified, defined as respiratory units formed by dozens of respiratory bronchioles and consequently a few thousand alveoli. These are identifiable in the number of about 60,000 per lung. The bronchial tree has its own innervation attributable to the sympathetic system. It is rich in sensitive endings that respond to stimuli such as irritants (hypersensitization causes asthma). Furthermore, the bronchial nutritional circulation and the muscular system are innervated. The bronchial muscular system responds to stimuli by forcing and increasing resistance to flow (it is for example the asthma effector), to opposite stimuli in opposite way. Bronchial circulation derives from the bronchial arteries of the thoracic aorta and constitutes the nutritional supply of the bronchial system. There are some anastomotic shunts between the pulmonary and bronchial circulation but in the normal subject they are not significant.
  • Alveoli. They are responsible for the exchange of oxygen between air and blood; this exchange takes place by passive diffusion, which is greatly facilitated since the air-blood barrier in the lungs is approximately 0.15 micrometers thick. (remember that according to the law of fick the diffusion occurs in an inversely proportional way to the distance).
  • The blood-air barrier is made up of the collation of laminar cells of the capillary endothelium and pneumocytes (also with the same laminar structure and protruding nucleus in the lumen) and the basement membrane between the two cell layers. Oxygen and carbon dioxide concentrations are at equilibrium after 0.2 seconds. The blood transit time is around 0.8 seconds, more than enough. During this time the systolic blood flow passes, about 70 ml (up to 200 ml under stress: the limited increase in range means that the metabolic demands for O2, which increase by up to six times under stress, are satisfied by increasing the withdrawal from hemoglobin, which also acts as a reserve), distributed over a capillary surface approximately equal to the alveolar one.
  • It is believed that the large extension of the surface is not functional to the oxygenation of the blood but to the distribution of the cardiac output in small vessels, those of an erythrocyte, perfectly oxygenatable.Each pulmonary alveolus has the wall consisting of a simple paving epithelium and a connective layer rich in capillaries distributed on a capillary surface approximately equal to the alveolar one. It is believed that the large extension of the surface is not functional to the oxygenation of the blood but to the distribution of the cardiac output in small vessels, those of an erythrocyte, perfectly oxygenatable.Each pulmonary alveolus has the wall consisting of a simple paving epithelium and a connective layer rich in capillaries distributed on a capillary surface approximately equal to the alveolar one.
  •  It is believed that the large extension of the surface is not functional to the oxygenation of the blood but to the distribution of the cardiac output in small vessels, those of an erythrocyte, perfectly oxygenatable.Each pulmonary alveolus has the wall consisting of a simple paving epithelium and a connective layer rich in capillaries. In addition to macrophages, the alveolar epithelium consists of two particular types of cells: pneumocytes of type Iand type II:
  1. Type I pneumocytesare structural flattened cells. They are also defined as small alveolar cells , covering about 90% of the total alveolar surface. They are small, thin cells, which develop like a thin film that covers the surface of the alveolus. Type I pneumocytes adhere to the surface of the capillaries via the basement membrane, allowing the diffusion and exchange of gases.
  2. Type II pneumocytesare also called Clara cells. They are cells present at the level of the lobular bronchi and intralobular bronchioles in the lung parenchyma. Their role is to secrete serous material that keeps the mucous material produced by the mucipar caliceous cells with a surfactant action that covers the surface of the epithelium of the bronchi and pulmonary alveoli .

The path of air inside our body

  • The air enters the nose from the nostrils and runs through the nasal cavities lined internally with mucus which has the function of moistening the air and retaining microbes and dust. It is possible to breathe in with the mouth, an action that normally takes place when you are very cold, but in this case the function of protection from microbes and dust is lacking.
  • The walls of the nasal cavities are thickly imbued with blood capillaries and covered with small hairs: the air is heated in contact with the capillaries and filtered by the hairs. The air then enters the pharynx, an organ that in the lower part communicates, posteriorly with the esophagus and anteriorly with the larynx. The pharynx is also an organ of the digestive system and in it air and food can be present simultaneously.
  • After passing through the larynx, the air passes through the trachea a flexible tube sprinkled internally with eyelashes which, moving from the bottom upwards, help to expel any impurities and foreign bodies.
  • Air passes into the bronchi, bronchioles and alveoli.
  • Air exchange occurs between the air and blood in the lung alveolus. The set of 300 million pulmonary alveoli constitutes the lungs, spongy and elastic organs.
  • The air follows the path backwards and is then expelled outside the body.

 

 

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