How structure is related to function
Air passing through the nose is warmed, moistened and filtered, it also stimulates the sense of smell.
Warmed air prevents the lower respiratory tract being chilled. Moistening prevents the lower respiratory tract from drying out and smell detects the possible presence of poisonous or dangerous gases. Smell is also vital to our sense of taste. Filtering removes such foreign bodies such as small insects and other possible larger contaminants in the air, the hairs in the nose are necessary for this process. The epiglottis flaps down over the top of the trachea to allow swallowing and therefore prevent food going down `the wrong way`.
Rings of cartilage, around the trachea and larger bronchial passages, maintain the patency of the upper airways by holding them open. Clearly if the trachea is closed the individual will asphyxiate. Rings of cartilage around the trachea are incomplete so there is a gap at the back to allow expansion of the oesophagus during swallowing of a food bolus. The oesophagus is immediately posterior to the trachea. The alveoli and the very small respiratory bronchioles are elastic so they expand when air enters them during inspiration and then passively recoil to aid the process of expiration.
The lumen of the respiratory passages is lined with a mucous membrane which produces mucus. Inhaled particles stick to the mucus and are then wafted up towards the trachea by cilia. The cilia are microscopic hair like projections from cell surfaces which move the mucus in one direction to help to clear foreign material and bacteria from the lungs. Once the mucus is in the trachea it may be coughed up buy a blast of air. This propels mucus through the vocal cords into the back of the mouth, from here it may be spat out or swallowed.Function of the pleural membranes
There are two pleural membranes surrounding the lungs. Firstly the visceral is the inner membrane, one visceral membrane surrounding the surface of each lung. Secondly the parietal is the outer membrane lining the inside of the thoracic (chest) cavity, this pleural membrane also lines the surface of the diaphragm. Between these two membranes there is a negative pressure of about 4mm of mercury, this has the effect of sucking the two membranes together. When the ribs move up and out and the diaphragm moves down the parietal pleural membrane will move with these structures as it is adherent to them and invaginates them. Because there is a negative pressure between the parietal and visceral pleural membrane the visceral pleural membrane will be drawn up and out with the chest wall and down with the diaphragm. Because the visceral pleural membrane is adherent to the surface of the lungs and invaginates the surface of the lung tissue this means the lungs expand with their overlying visceral pleural membrane. The visceral pleural membrane expands because of the suction of the visceral pleural membrane onto the parietal pleural membrane. Therefore if any air gets into the potential pleural space, (that is between the visceral and parietal pleural membranes) for example, through a stab wound, the negative pressure will be lost. Movement of the parietal pleural membrane would then no longer result in movement of the visceral pleural membrane and the lung would collapse. This condition is known as pneumothorax and requires immediate emergency medial treatment.