Lung Respiration Alveolu Surfactant Inspiration Expiration Ventilation Trachea Bronchus

The lungs are part of the respiratory system of the body. The main function of the respiratory system is for gaseous exchange. Together with the exchange of gases, it also has immune functions to clense the air of pathogenic particles and prevent its entrance into the body. The upper respiratory tracts helps to warm and moisten the air so as to prevent the damage to the lining of the respiratory tract . The goblet cells of the respiratory epithelium has a role to play in the trapping of foreign matter and the cilliary action will move it upwards and be swallowed into the oesophagus and stomach where the stomach acids will destroy the pathogenic substances.

The bronchial tree leads onto the lungs. The respiratory tract is lined by the respiratory epithelium which is the pseudostratified columnar ciliated epithelium.  The trachea is the largest of the repiratory tubes and it branches into two primary bronchi. The carina lies at the bifurcation, which is a piece of cartilage. There are pulmonary lymph nodes at this junction. The right main bronchus is wider, shorter and more vertical than the left main bronchus and thus foreign objects do tend to lodge into the right lungs, specifically at the base of the left middle lobe.

The primary bronchi branches into the secondary bronchi; three in the right lungs and two in the left lungs. The right lung has a horizontal fissure and also the oblique fissure giving rise to three lobes. But the left lung only has the oblique fissure and thus only two lobes. each secondary bronchi supplies one lobe.  The secondary bronchi will branch further into bronchioles, terminal bronchioles, respiratory bronchioles and finally the alveoli. The alveoli are the sites of gas exchange.

The alveoli has a barrier that is suited for gas exchange. It has a squamous layer of type 1 pneumocytes and one large type 2 pneumocyte which releases surfactant. This substance reduces the surface tension within the alveoli, prevents the collapse of the alveoli and also reduces the work done by the lungs in order to expand. As the akveolus is the onlky region in the bronchial tree where respirationj can take place, collapse of the alveolus will reduce the surface area for gas exchange. The lungs also has an intrinsic capability to direct the pulmonary blood supply to the well ventilated alveolus instead leading to pulmonary hypertension and possibly pulmonary edema and right heart failure as it will have to pump against an incrased resistance.  

The pulmonary capillary membrane is close to the alveolar membrane with only the basement membranes in between. This reduces the distance the gas needs to travel to be dissolved into the blood and this makes gas exchange efficient. The presence of fibrous tissue or edemous fluid in between the basement membrane and the blood cappillary (ie: the alveolar interstitium) may reduce the efficiency of gas exchange my increasing the barrier distance.

Air is brought into the lungs during inspiration by increasing the thoracic volume and decreasing the pressure within the lungs below that of atmospheric pressure and thus sucking air into the alveolus. The ribs undergo two types of movements; pump handle and bucket handle movements that helps to increase the rib cage’s anteroposterior and transverse diamete respectively. the diaphragm contracts and increases the superior inferior diameter.  The accessory muscles of inspiration (sternocleidomastoid and the scalene muscles) only come into play during forceful inspiration and not during resting ventilation.  The thin layer of pleural fluid in the pleural space plays a role in maintaining a negative pressure surrounding the lungs so that it can be expanded easily.

During expiration, air is pushed out of the lungs as the thoracic volume decreases. accoridng to boyle’s law, pressure is invcersely related to volume. The elastic nature of the lungs allows the lungs to recoil back ton original size passively and the diaphragm also relaxes passively. During forced expiration, the abdominal muscles are used.