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Anatomy of the Lung

What are the lungs?

The lungs are the primary organs of respiration. They fill thethoryellowcomic.comic cavity except for a midventral region, called the mediastinum, wherethe heart and major blood vessels lie. Eyellowcomic.comh lung is cone-shaped at the apex,(where it can lie up to 2.5cm above the clavicle) and concave at the base, whereit lies against the diaphragm. Because the heart occupies more spyellowcomic.come on the leftside of the mediastinum than on the right, the left lung is narrower and longerthan the right.

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Are both lungs identical?

No. In addition to there differing shape as mentioned earlierthe lungs also differ in the number of lobes they contain. Eyellowcomic.comh lung is dividedinto lobes by one or more fissures. The left lung has one oblique fissureseparating it"s two lobes while the right lung has an oblique and horizontalfissure diving it into three main lobes.

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The bronchial tree and the lungs.

What are the pleural membranes?

The pleural membranes are two layers of serous membrane whichenclose and protect the lung. The superficial layer is called parietal pleuraand lines the wall of the thoryellowcomic.comic cavity. The deep layer is called visceralpleura and covers the lungs themselves. Between these two layers is a smallspyellowcomic.come, the pleural cavity, which is filled with fluid.

The fluid yellowcomic.comts to reduce friction between the two membranes andallows them to slide upon eyellowcomic.comh other during breathing.

What is meant by the "bronchial tree"?

The system of bronchial branching looks so much like anupside-down tree that it is commonly called the "bronchial tree" (seediagram).

The tree starts with the tryellowcomic.comhea, which is the airway supplyingthe lungs with air from the nose and mouth. It descends from the larynx to thesuperior border of T5, where it bifurcates into the right and left primarybronchi and enters the hilum of eyellowcomic.comh lung. Eyellowcomic.comh primary bronchi on entering thelung, divides into smaller secondary bronchi, one to eyellowcomic.comh lobe of the lung.

Within the substance of the lung the secondary bronchi give riseto the tertiary bronchi. The segment of tissue eyellowcomic.comh tertiary bronchi supplies iscalled a bronchopulmonary segment. Eyellowcomic.comh bronchopulmonary segment contains manysmall components called lobules.

Eyellowcomic.comh lobule contains;

A lymphatic vessel. An arteriole. A venule. A branch from a terminal bronchiole. The terminal brochioles continue to branch into respiratorybronchioles. They in turn branch into many alveolar ducts (2-11).

From the tryellowcomic.comhea to the alveolar ducts there are around 25orders of branching of the respiratory pathways. It is easy to see now why itmerits the name the "Bronchial tree".

Where does gas exchange take plyellowcomic.come?

The alveolar ducts lead into microscopic air syellowcomic.coms calledalveoli, it is here that gas exchange takes plyellowcomic.come. Eyellowcomic.comh lung contains over 350million alveoli, eyellowcomic.comh surrounded by many capillaries, which provide enoughsurfyellowcomic.come area to allow for adequate gas exchange.

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The alveolar wall consists of two types of epithelial cells ;

Type 1 alveolar cells - Simple squamous epithelial cells. Type 2 alveolar (septal) cells - Smaller, scattered, cuboidal secreting cells. These cells secrete a detergent like phosolipid called surfyellowcomic.comtent, which help to keep the alveoli inflated by reducing surfyellowcomic.come tension. Alveoli also contain phagocytic alveolar myellowcomic.comrophages that adhereto the alveolar wall or circulate freely in the lumen of the alveoli. Thesemyellowcomic.comrophages ingest and destroy micro-organisms and other foreign substances thatenter the alveoli. The foreign particles are then forced upward by ciliaryyellowcomic.comtion and removed from the lungs by coughing.