Lungs are the main spongy organs of respiratory system. They are two in number-left and right which are mostly in the chest cavity. Lungs are situated at the both sides of the middle line of the body and remain separate to each other by the mediastynum. Mediastynum is the space between the two lungs in which big blood vessels, trachea, esophagus, thoracic duct and thymus gland are present. Lungs are extended from the lower portion of the neck to the diaphragm and they are conical in shape of which apex is upwards a little above from the clavicle gland and their base is downwards on the diaphragm at the surface of chest cavity. External or costal bulged surfaces of lungs touch to the ribs and intercostal muscles which are at the middle portion of ribs. Their medieval surface is inserted (concave) in which there is a triangle area hilum on the level of fifty to seventh thoracic vertebrae. Bronchus, pulmonary artery, pulmonary veins, pulmonary nerves and lymphatic vessels reach into the lungs by coming through hilum. The surface situated on the diaphragm of lungs’ base is called diaphragmatic surface. The frontal or anterior portion of lungs covers to the frontal portion of the heart and posterior edge remain in the contact of vertebral column.      

     Each lung is divided into lobes by the deep fissures. There are two lobes in the left lung which remain separate by the oblique fissure. The upper lobe is over and before the lower lobe. The lower lobe is conical in shape. There are three lobes in the right lobe. Its lower lobe is divided by oblique fissure and the rest of portion is divided by the transverse fissure into upper and middle lobes which are called bronco-pulmonary lobe. These lobes are separate from each other by the wall of connective tissues and each has artery and vein. Each small lobe is also divided into several units which are called lobules.

          There are bronchioles air sacs in these lobules. Numerous air sacs are the respiratory parts of the lungs. The wind inhaled in breathing reaches into the air sacs by going through the respiratory tracts where gases are exchanged between the walls of respiratory tracts and cells.


     Pleura are a double serous membrane which covers to each lung. Its one layer or visceral pleura is closely inserted with the lungs and separates the lobes by entering into the fissures of lungs and remains sticking with them.  By reaching close to the base of lungs, this visceral pleura make other layer parietal pleura after reversing which makes the lining of internal surface of chest wall and covers the upper surface of diaphragm. The portion which unstratified its ribs is called costal pleura; the portion close by the neck is called cervical pleura; the portion which covers to the diaphragm is called diaphragmatic pleura and the portion which covers to the mediastinnum is called mediastinal pleura.

     The middle portion between the both layers of pleura which is very short is called pleural cavity. There is thin serous fluid in it which lubricates to the surfaces of the pleura viz it keeps them wet and smooth. Generally, both layers of pleura remain in close contact to each other but the presence of fluid at their middle portion glide above one to another without any abrading and remain far from any kind of hurt. There is swelling of pleura in the disease named pleurisy because of the quantity of pleural fluid gets increased very much and the pleural cavity becomes enlarged. 

Blood supply of the lungs:

     Pulmonary artery supply impure blood viz blood without oxygen into the lungs from the right ventricle. Here it marches forwardly along with the bronchus by getting divided into two branches. Thereafter, it keeps on dividing into several minute and very minute arteries by marching along with bronchiole and finally a thick net of cells comes into existence around the walls of alveoli by these arteries. The walls of air sacs and cells are made of single layer of the cells of flat epithelium cells which are very thin. Air sacs and cells are attached to each other closely. So, oxygen and carbon dioxide gases are able in transferring from one side to another side. The process of exchanging gases is based on the physical rules of diffusion. Here oxygen absorbs hemoglobin from the red blood cells and the carbon dioxide of the blood is left outside. After the purification of blood, all the pulmonary cells or venules start to connect to each other and make some big veins. Many veins make very big veins by meeting to each other and finally two pulmonary veins take birth. These pulmonary veins come out by going through the hilum of lungs and supply the pure blood into the left atrium of hear from where the blood reaches in the left ventricle and from here it spreads all over the body by the aorta and its branches.