Lungs

 Lungs

The lungs are anatomical structures belonging to the respiratory system, they are located in the thoracic cage, on both sides of the mediastinum. Due to the space occupied by the heart, the right lung is larger than its left counterpart. They have three faces; mediastinal, costal and diaphragmatic, it is irrigated by the bronchial arteries and the pulmonary arteries carry blood for oxygenation. Embryologically it is derived from the endoderm.

The lungs are the organs in which the blood receives oxygen from the inspired air and releases carbon dioxide, which passes into the expired air. This exchange occurs through simple diffusion of gases thanks to the difference in partial pressures of oxygen and carbon dioxide between the blood and the alveoli.

Anatomy 

The lungs are located inside the thorax and on both sides of the heart. They are protected by the ribs and separated from each other by the mediastinum. They are covered by a double membrane called the pleura. A cavity (pleural cavity) is formed between both pleurae, which is occupied by a thin sheet of serous fluid.

 The surface of the lungs is pink in children and somewhat more grayish in adults. The weight depends on the sex and the hemithorax they occupy: the right lung weighs an average of 600 grams and the left 500 g. These figures are a little lower in the case of women, due to the smaller size of the thoracic cage, and somewhat higher in men.[1] A vertex or apex corresponding to its upper portion and a base are described in both lungs. or lower portion that rests on the diaphragm muscle. The right lung is divided into 3 lobes by two fissures, while the left lung has only 2 lobes, upper and lower, separated by a fissure.

The right lung is divided by two fissures (major and minor) into 3 parts or lobes (upper, middle and lower). The left lung has two lobes (upper and lower) separated by an oblique fissure. The major fissure of both lungs runs from the fourth posterior intercostal space to the anterior third of the corresponding hemidiaphragm. In the right lung it separates the upper and middle lobes from the lower lobe, while in the left lung it separates the only two lobes: upper and lower. The minor fissure separates the upper and middle lobes of the right lung and runs from the anterior wall of the chest to the major fissure. It may be absent or incomplete in up to 25% of people.

 In each lobe, different, well-differentiated segments are distinguished, each corresponding to a segmental bronchus (3rd bronchial generation). There are several classifications to name the different segments, one of the most accepted being Boyden's.[2]​The segmental bronchi are subdivided into bronchi proper and bronchioles (generations 12-16). The latter lack cartilage and branch into terminal bronchioles and respiratory bronchioles (generations 17 to 19) that lead to the alveoli: the functional gas exchange units of the lung.

 There is also the lobe of the azygos vein as a variation of normal anatomy. 

Pleura

 The pleura is an elastic connective tissue membrane that prevents the lungs from rubbing directly against the inner wall of the rib cage. It has 2 layers, the parietal or external pleura that covers and adheres to the diaphragm and the inside of the thoracic cage, and the visceral pleura that covers the outside of the lungs, entering their lobes through the fissures. Between both layers there is a small amount (about 15 cm³) of lubricating fluid called pleural fluid.

Functions

The main function of the lungs is to carry out gas exchange with the blood. In the alveoli, the passage of oxygen from the air to the blood and the passage of carbon dioxide from the blood to the air occurs. This exchange occurs by simple diffusion due to the difference in partial pressures of oxygen and carbon dioxide between the blood and the alveoli. In the pulmonary capillaries that surround the alveoli, gas exchange takes place, in such a way that hemoglobin is released from a molecule of carbon dioxide and takes on that of oxygen. All cells in the body use this oxygen to oxidize glucose, thus generating the energy necessary for each of them to continue functioning. Oxidation occurs in a cell organelle called the mitochondria, where carbon dioxide is generated as a byproduct.

Respiratory membrane

 For gas exchange to occur, it is necessary for them to cross the respiratory membrane, which is made up of four layers:

 The wall of the alveolus, formed by type I and II pneumocytes together with macrophages.

 Epithelial basement membrane located below the alveolar wall.

 Basement membrane of the capillary.

 Capillary endothelium, that is, the layer of cells that forms the wall of the capillary vessel.

 The set of these four layers is very thin and only has a thickness of 0.5 microns, that is, half a thousandth of a millimeter. This arrangement allows gas exchange to be very efficient.

Pulmonary ventilation

Ventilation is the process by which oxygen-rich atmospheric air enters the lungs, which then expel oxygen-poor, carbon dioxide-rich air. It consists of 2 phases, inspiration and expiration:

 Inspiration occurs when contraction of the diaphragm and external intercostal muscles increase the volume of the thoracic cavity and cause the lung to expand. The intraalveolar pressure becomes negative with respect to atmospheric pressure and air enters the lung until the pressure gradient disappears.[7]​

 Expiration occurs when the diaphragm relaxes and the diaphragmatic dome rises passively due to its elasticity, the intrapulmonary pressure becomes positive with respect to atmospheric pressure, the lung decreases its volume and the air escapes to the outside until the pressure gradient disappears and the intraalveolar pressure is equal to atmospheric pressure.[7]​

 At rest, the volume of air that enters the lung during an inspiration is called tidal volume and is 500 cm³, the respiratory rate is 12 cycles per minute. Total lung capacity ranges between 4000 and 6000 cm³, depending on age, weight and sex; being higher in men than in women.