The lungs are the organs contained in the abdomen that play a abecedarian part in breathing by allowing gas exchanges. The right and left lungs, of which only one is sufficient to insure an nearly normal life, force oxygen to the whole body and allow the elimination of carbon dioxide contained in the blood. They’re indeed and asymmetrical organs. The right lung has 3 lobes, the leftism has 2 lobes. The lungs are enveloped by defensive and covering membranes the pulmonary pleura framing the mediastinum, the total rests on the diaphragmatic pate.
In addition to gas exchange, the lungs share in other functions, similar as the regulation of acid- base metabolism as well as a part in blood filtration by barring small clots.
The invertebrate lung has its embryological origin in an outgrowth of the pharynx. It’s now accepted that this origin is different from that of the syncope bladder. One of the strong arguments is that the lungs are a divergence of the frontal and not the rearward wall of the pharynx.
Man has two lungs, left and right, two thoracic organs, separated from each other by the mediastinum, medially. They rest on the diaphragm and are defended by the thoracic pen in front, outdoors and before, except at the position of their peak, because they exceed the upper edge of the first caricature, and indeed go up to the top of the clavicle, at the base of the neck, in the supraclavicular concave.
The right lung is divided into three lobes (upper, middle and lower), the left divided into two lobes (upper and lower). On the left, the lingular part of the upper lobe corresponds to the right middle lobe, while the culminating part (culmen) corresponds to the right upper lobe. The lobes are separated by scissors, two on the right (the large or “oblique”, and the small or “horizontal”) and one on the left (the oblique).
Each lobe of the lungs is divided into lung segments.
The pulmonary arterial vascularization is double: the pulmonary and bronchial system. The pulmonary arteries bring venous blood from the right ventricle for oxygenation, their course following the bronchi. The bronchial arteries come from the aorta or intercostal arteries and bring oxygenated blood to the bronchial wall at the level of the terminal bronchioles.
The lungs are connected to the ribs of the rib cage by two membranes called pleura. Inspiration and expiration are controlled by the intercostal muscles and the diaphragm, which deform the rib cage and therefore the lungs through the play of the pleura.
Diagram of the human respiratory system.
Detail of the alveoli and the pulmonary circulation
The lungs are ventilated by thoracic movements during inspiration and expiration, which constitute a respiratory cycle. At the same time, the alveoli receive blood pumped by the right heart. At rest, 4 liters of air and 5 liters of blood flow through the lungs per minute. During exercise, these amounts can vary considerably (up to 160 liters of air and 30 liters of blood per minute). These inputs allow the alveoli to perform their gas exchange role, through thin membranes that separate the alveoli from the blood capillaries.
The lung is a gateway for certain microorganisms, viruses, gases and toxic micro- or nanoparticles. In case of chronic exposure or exceeding a threshold of acute toxicity, these organisms and contaminants can cause intoxication and/or inflammatory and allergic phenomena. Thus, exposure to particulate air pollution is a source of inflammatory phenomena (a factor favoring cancer).
The air passes through the nose (the usual route at rest) or through the mouth, to cross the pharynx and the larynx, which constitute the upper airways. It then reaches the level of the trachea, which divides into two stem bronchi (at the level of T5, of the carina), to subdivide many times, until forming the terminal bronchioles. Up to this level, there are no alveoli, hence its name of conducting part. Then the respiratory bronchioles branch off, the starting point of the respiratory part. This part contains the alveoli, where gas exchanges can take place.
In addition to their role of air conduction, the upper airways ensure air conditioning. They allow the air to be warmed up to 37°C (body temperature) and to be saturated with water. In addition, the air undergoes a filtering, indeed all along the airways are arranged cells secreting mucus, glands and hair cells. This creates a layer of mucus lining the airways, and thus fixes the particles (dust, bacteria, …) crossing the airways. The movement of the cilia (of the ciliated cells) moves this mucus in the direction of the pharynx allowing its elimination in the digestive tract (one speaks about mucociliary escalator). This is an important defense mechanism of the lungs against external aggressions. In addition, there are macrophages, which, through their phagocytosis action, complete this defense system.
Cross-sectional diagram of the cells of an alveolus.
It is in the alveoli, small sacs terminating the airways, called pulmonary sacs or pulmonary vesicles, that gas exchange occurs. They are lined with a very thin wall (up to 0.2 μm; for comparison, the diameter of red blood cells is 7 μm) containing capillaries. The total surface area for exchange is about 130 m², the size of a volleyball court. This allows the alveoli to perform their role of transmitting oxygen to the blood and extracting carbon dioxide from it.
At this level, we find the type 2 pneumocytes, which secrete surfactant. The presence of surfactant is essential, as it reduces surface tension and allows for easier pulmonary distension. In comparison, its role is the same as the soap that is added to water to form soap bubbles. It prevents the collapsing of the alveoli during the exhalation phase. It is washed away by water during drowning, which requires intensive monitoring of resuscitated drowning victims.
Air movements during pulmonary ventilation
Depend essentially on the contraction of the respiratory muscles which causes a pressure gradient drawing air into the lungs. Inspiration is therefore described as active, the contraction of the diaphragm, which increases the vertical diameter of the rib cage and the external intercostal muscles, which increases the anteroposterior diameter, leads to a decrease in the pressure inside the lungs and therefore an entry of air.
Natural exhalation is a passive phenomenon, resulting from elastic restoring forces when the muscles relax, which cause the rib cage to return to its volume at the beginning of inspiration and thus expel air from the lungs. Nevertheless, it is possible to perform a forced expiration, which is active. It involves the abdominal muscles and the internal intercostal muscles.
Exchanges and transport of gases
The external respiration, pulmonary, allows the transformation of the deoxygenated blood which comes from the heart into oxygenated blood, which will return there to be redistributed to the whole body. The exchanges between the alveoli and the blood are a function of the differences in partial pressures, a gas will diffuse from the high pressure to the low pressure according to Fick’s law. The partial pressure of the alveoli being 100 mmHg for dioxygen and 40 mmHg for carbon dioxide when respectively it is 40 mmHg and 46 mmHg in the capillary, O2 goes from the alveoli to the blood and carbon dioxide makes the opposite way.
The contact time between the blood and the alveoli is 0.75 seconds, but only one third of the time is needed to reach equilibrium. The heart-lung system is called the small circulation; the latter was first demonstrated by the Arab physician Ibn Nafis in 1242 in Cairo
The regulation of breathing
Breathing takes place unconsciously and rhythmically thanks to the activity of certain neurons in the brainstem. Its regulation depends essentially on the partial pressure of carbon dioxide in the blood, which is captured by two types of chemoreceptors located in the periphery and in the central nervous system. The first are located in the aortic arch and at the bifurcation of the carotid arteries, the second are located on the ventral side of the medulla oblongata. Any change in the carbon dioxide content of the blood causes a response in the rate and depth of ventilation. Modulations of respiratory activity can also be due to other stimuli, such as during emotions (fear, excitement…).
Maintaining sterility to ensure respiratory capacity
The lung, a complex organ, is maintained sterile by the secretions it generates, in particular by a number of antimicrobial constituents present in the mucus. In addition to glycoproteins, e.g. mucins, there are antimicrobial proteins such as lactoferrin, lysozyme, lactoperoxidase. There are also other duox-type proteins which allow the production of hydrogen peroxide, a peroxide necessary for the production of hypothiocyanite. It should be noted that this function is impaired in patients with cystic fibrosis.
Role of the autonomic nervous system
Sympathetic stimulation of the bronchial tree causes dilation of the bronchi and inhibition of mucus secretion. On the other hand, parasympathetic stimulation causes bronchial constriction and stimulation of mucus secretion.