LUNGS :::
There are two lungs with a bronchus attached to each of them they are cone shaped spongy organs Their apices are situated slightly above the clavicle bones and the bases are near the diaphragm
The right lung is thicker and broader than left The right lung has three and the left lung has two lobes Each lobe is composed of lobules A small bronchial tube enters each lobule where it divides and subdivides into pulmonary bronchi and terminal bronchioles terminal bronchioles subdivide into microscopic branches called respiratory bronchioles which subdivide into several (2-11) alveolar ducts Alveolar ducts open into numerous alveoli Two or more alveoli that share a common opening are called alveolar sacs There are two types of alveolar eithelial cells or pneumocytes in alveolar wall type 1 alveolar (squamous pulmonary epithelial ) cells which is the main site for gas exchange and Type II alveolar (septal) cells which secrete alveolar fluid Around the alveoli the lobules arteriole and venule disperse into capillary network The exchange of respiratory gases between lungs and blood takes place by diffusion across alveolar and capillary walls The layers through which respiratory gases diffuse are known as alveolar capillary (respiratory) membrane it consists of a layer of type I and II Alveolar cells constituting alveolar (epithelial) wall epithelial basement membrane underlying alveolar wall capillary basement membrane fused to epithelial basement membrane and costly endothelial cells of capillary Inspite of several layers the respiratory membrane is very thin allowing rapid diffusion of respiratory gases There are 350 million of alvecli in the lungs of an adult This amounts to 100 square metres of area for aeration
Pleura :::
Lung is covered by a double serous membrane known as pleura the inner layer of the pleura is closely attached to the lungs and is known as visceral layer The free layer on the thoracic wall is known as parietal layer between two pleural layers there is a fluid which lubricates the surfaces and prevents friction between the lungs and the chest wall during respiration The intrapleural pressure is important for bringing out respiratory movements
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