Good day everyone, you are welcome to my blog, today we are discussing about the lungs:
After studying this section, you should be able to:
- name the air passage of the bronchial tree in descending order of size
- describe the structure and changing functions of the different levels of airway
- describe the location and gross anatomy of the lungs
- identify the functions of the pleura
- describe the pulmonary blood supply.
Position and gross structure
There are two lungs, one lying on each side of the midline in the thoracic cavity. They are cone-shaped and have an apex, a base, a tip, costal surface and medial surface.
The apex This is rounded and rises into the root of the neck, about 25 mm above the level of the middle third of the clavicle. It lies close to the first rib and the blood vessels and nerves in the root of the neck.
The base This is concave and semilunar in shape, and lies on the upper (thoracic) surface of the diaphragm.
The costal surface This is the broad outer surface of the lung that lies directly against the costal cartilages, the ribs and the intercostal muscles.
The medial surface. The medial surface of each lung faces the other directly across the space between the lungs, the mediastinum. Each is concave and has a roughly triangular-shaped area, called the hilum, at the level of the 5th, 6th and 7th thoracic vertebrae. The primary bron- chus, the pulmonary artery supplying the lung and the two pulmonary veins draining it, the bronchial artery and veins, and the lymphatic and nerve supply enter and leave the lung at the hilum.
The mediastinum contains the heart, great vessels. trachea, right and left bronchi, oesophagus, lymph nodes, lymph vessels and nerves.
Pleura and pleural cavity
The pleura consists of a closed sac of serous membrane (one for each lung) which contains a small amount of serous fluid. The lung is pushed into this sac so that it forms two layers: one adheres to the lung and the other to the wall of the thoracic cavity.
The visceral pleura
This is adherent to the lung, covering each lobe and passing into the fissures that separate them.
The parietal pleura
This is adherent to the inside of the chest wall and the thoracic surface of the diaphragm. It is not attached to other structures in the mediastinum and is continuous with the visceral pleura round the edges of the hilum.
The pleural cavity
This is only a potential space and contains no al so the pressure within is negative relative to atmospher res sure. In health, the two layers of pleura are sepa by a thin film of serous fluid (pleural fluid), which WS them to glide over each other, preventing friction en them during breathing. The pleural fluid is secreby the epithelial cells of the membrane. The double brane arrangement of the pleura is similar to the sus pericardium of the heart.
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The two layers of pleura, with pleural fluid between them, behave in the same way as two pieces of glass sepa rated by a thin film of water. They glide over each other easily but can be pulled apart only with difficulty, because of the surface tension between the membranes and the fluid. This is essential for keeping the lung inflated against the inside of the chest wall.
Interior of the lungs
The lungs are composed of the bronchi and smaller air passages, alveoli, connective tissue, blood vessels, lymph vessels and nerves, all embedded in an elastic connective tissue matrix. Each lobe is made up of a large number of lobules.
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Pulmonary blood supply
The pulmonary trunk divides into the right and left pulmonary arteries, carrying deoxygenated blood to each lung. Within the lungs each pulmonary artery divides into many branches, which eventually end in a dense capillary network around the alveoli. The walls of the alveoli and the capillaries each consist of only one layer of flattened epithelial cells. The exchange of gases between air in the alveoli and blood in the capillaries takes place across these two very fine membranes (together called the respiratory membrane).