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Lung is the main organ of respiratory system of human and many other animals (including some fish and some snails). In mammals and most other vertebrates, two lungs are located near the main trunk on both sides of the heart. Their function in the respiratory system is to extract oxygen from the atmosphere in the process of gas exchange, transfer it to the blood, and release carbon dioxide from the blood to the atmosphere. Breathing is driven by different muscle systems in different species. Mammals, reptiles and birds use their different muscles to support and promote respiration. In early tetrapods, the pharyngeal muscles pumped air through the cheeks into the lungs, which is still present in amphibians. In humans, the main respiratory muscle that drives breathing is the diaphragm. The lungs also provide air flow, making it possible for human voice, including human voice.

Humans have two lungs, the right and the left. They are located in the chest cavity of the chest. The right lung is larger than the left, sharing the chest space with the heart. The lung weighs about 1.3 kg (2.9 lb) in total, and is heavier on the right. The lung is part of the lower respiratory tract, which starts from the trachea and branches into bronchioles and bronchioles, and inhales air through the conduction area. The conduction region ends at the end of the bronchiole. They are divided into the respiratory bronchioles in the respiratory area, and the bronchioles are divided into alveoli. These alveoli form microscopic alveoli where gas exchange takes place. The lungs contain a total of about 2400 kilometers (1500 miles) of airway and 30 to 500 million alveoli. Each lung is enclosed in a pleura sac, so that the inner and outer walls slide against each other during respiration without too much friction. The capsule also divides each lung into parts called lobes. There are three lobes in the right lung and two lobes in the left lung. The lobes were further divided into bronchopulmonary segments and lobules. The lung has a unique blood supply that receives deoxygenated blood from the heart in the pulmonary circulation to receive oxygen and release carbon dioxide, while in the bronchial circulation, oxygen-containing blood is supplied to the lung tissue alone.

Lung tissue can be affected by a variety of diseases, including pneumonia and lung cancer. Chronic obstructive pulmonary diseases, including chronic bronchitis and previously known as emphysema, may be related to smoking or exposure to harmful substances such as coal dust, asbestos fibers and crystalline silica dust. Diseases such as bronchitis can also affect the respiratory tract. Medical terms relating to the lung usually begin with the Latin for lung (such as lung) or pneumonia (Greek for lung).

During embryonic development, the lung begins to develop as an outer pouch of the foregut, which continues to form the upper part of the digestive system. When the lungs form, the fetuses are held in fluid filled amniotic sacs, so they cannot breathe. Blood is also transferred from the lung through an artery catheter. However, at birth, air begins to pass through the lungs and the duct is closed so that the lungs can begin to breathe. Lungs develop completely only in early childhood.

The main or main bronchus enters the lung at the hilus of the lung, initially branching into the secondary bronchus, also known as lobar bronchus, which supplies air to each lobe of the lung. The lobar bronchi branch into three-stage bronchi, also known as segmental bronchi, which provide air for the further division of the lung lobes, known as the bronchopulmonary segments. Each bronchopulmonary segment has its own (segmental) bronchi and arteries. Parts of the left and right lungs are shown in the table. The segmental anatomy can be used to locate the disease process in the lung. [5] Segments are discrete units that can be removed surgically without seriously affecting surrounding tissue.

The chest on either side of the heart in a rib cage. They are conical, with a narrow circular apex at the top and a wide recess at the bottom on the convex face of the diaphragm. The apex of the lung extends to the base of the neck and reaches shortly above the level of the sternum end of the first rib. The lung extends from the skeleton near the ribs to the front of the chest, then from the lower part of the trachea down to the transverse diaphragm membrane. The left lung shares space with the heart and has a dent at its boundary, called the left lung's cardiac notch, to accommodate this situation. The front and outside of the lungs face the ribs, which form shallow dents on their surfaces. The inner surface of the lung faces the center of the chest and is adjacent to the heart, the large blood vessels and the trachea, which is divided into two main bronchi. A heart impression is an indentation on the surface of the lung against the heart.

Both lungs have a fovea at the base of the lung called the hilum, where blood vessels and airways enter. There are also bronchopulmonary lymph nodes on the hilum.

The lung is surrounded by the pleura. The pleura is two serosa. The parietal pleura is lined on the inner wall of the ribs, and the visceral pleura is directly lined on the surface of the lungs. Between the pleura is the potential space known as the pleural cavity, which contains a thin layer of lubricating pleural fluid. Each lung is divided into lobes by folds of the pleura. A tear is a double fold of the pleura that cuts the lung open and helps it expand.

At birth, the baby's lungs are filled with fluid secreted by the lungs and do not swell. After birth, the baby's central nervous system responds to sudden changes in temperature and environment. This triggers the first breath approximately 10 seconds after delivery. Before birth, the lungs are filled with fetal lung fluid. After the first breath, the liquid will be quickly absorbed into the body or exhaled. The resistance in the pulmonary vessels decreases, which increases the surface area of gas exchange, and the lungs begin to breathe spontaneously. This, along with other changes, results in an increase in the amount of blood entering the lung tissue.

At birth, the lungs are very underdeveloped, with only one sixth of the adult lung alveoli remaining. Alveoli continue to form into early adulthood, and their ability to form when necessary can be seen in lung regeneration. The alveolar septum has a double capillary network rather than a single network of developed lungs. Only when the capillary network matures, can the lungs enter the normal growth stage. With the early increase of the number of alveoli, the alveoli entered another stage.