Chapter 3: Respiratory System

Respiratory Physiology

The heart's function of delivering oxygenated blood to the brain would be futile without a system of oxygenating the blood. This is the function of the respiratory system. It consists of as we all know, two lungs joined together and connected to the throat and mouth. The two lungs can each be likened to a collapsed balloon, which contains air, and in their isolated state they will collapse down into a small volume. It is the function of the chest wall cavity to stretch out the lungs so that they can function to oxygenate the blood.

If the integrity of the chest cavity is compromised e.g. by a barotrauma or a penetrating chest wound, the space between the lung and the chest wall, which in reality is a vacuum and this vacuum is responsible for stretching the lungs up to their normal size, is lost and for this reason the lungs collapse. If air gathers in the space under pressure as it can do, in a tension pneumothorax (pneumothorax is the situation where air exists within the chest cavity collapsing the lung). A tension pneumothorax results in gas collecting under increasing pressure within the chest wall. This collapses the lung, compresses the heart, compromises the heart's action and results in cardiac tamponad shock and ultimately death. The situation can be remedied by an emergency drainage procedure to allow the gas to escape from the chest wall.

The upper airway that includes the nasopharynx, mouth and throat functions to warm and humidify the air as it is drawn into the respiratory system. It then passes down through the throat, past the vocal cords. This is an important area as it is the narrowest part of the airway and in serious injury the first sign of this narrow gateway being compromised is hoarseness. This can occur when for example a patient is subjected to a blast injury where they have experienced a burn to the upper airway and hot air is drawn in. The airway can be burned and the first sign of this will of course be soot to the face, throat with blistering and the most ominous sign is increasing hoarseness followed by difficulty breathing. This difficulty results in the subject making a whistling sound when breathing in and out (stridor). This is a life-threatening crisis and can be controlled by having a tube passed down the narrow space between the two vocal cords, known as intubation. (The area is known as the trachea and the technical term for this is endotracheal intubation.) If this fails a surgical airway can be made by cutting directly into the windpipe (trachea) below the area of the cords (crycothyroidotomy) or below this. In the normal situation once the air has been warmed and moistened by the upper airway, it then passes down into the lungs. The lungs as said before amount to a large spongy air space and the blood is oxygenated by being passed around this air space through very thin blood vessels. This allows gas to pass between the two systems. Breathing is controlled automatically by the brain (i.e. is not normally controlled by volition and responds to chemical changes in the blood). The most sensitive chemical change is increasing carbon dioxide. This has important consequences in that oxygen which is required to keep the brain and the body alive is not responsible for control of breathing and it is possible that the oxygen level could drop to dangerously low levels without breathing being stimulated and the subject would have no awareness of this situation e.g. where air is rebreathed oxygen is not replenished and C02 is extracted.

The fact that the pulmonary (lung) blood vessels are particularly fine and thin walled, is very useful for their function but makes them vulnerable. If the lung is damaged for example by a near drowning or inhalation of poisonous gases or by shock and by over infusion (giving too much saline to a shocked subject), this can result in fluid leaking from the small vessels into the air spaces in the lungs. This is known as pulmonary oedema and its treatment has been previously discussed.

The brain primarily controls the action of breathing although one can choose to increase the rate and depth of breathing. The mechanism of breathing is that the ribs are lifted up like bucket handles from a near vertical to horizontal position. A small thin muscle, which forms the base of the chest cavity known as the diaphragm pushes the abdominal contents down and this also, increases the space available in the chest. The net result is to temporarily increase the vacuum between the lungs and the inside of the chest wall and this sucks the lungs up and outwards, reducing the pressure within them and this forces air into the lungs. Breathing out is done by relaxation of the above mechanism and can also be done by using extra muscles to force air out. This does not normally happen unless there is some kind of airway obstruction such as one up in the vocal cords as described before or if the subject has a medical condition such as asthma. This result in the small passages in the lungs becoming smaller, increasing the resistance to flow and thus makes breathing outwards more difficult and noisy (wheeze).

The situation in asthma is remedied by drug therapy and is usually given with oxygen which allows the air passage in the lungs to return to normal size and this is evident by the subjects responses returning to normal i.e. breathing becomes easier and the wheeze is reduced. An asthma attack results in difficulty breathing, a wheeze during the expiration (breathing out) phase, increasing anxiety and in severe cases the subject may become confused. An asthmatic who has trouble talking in full sentences is regarded as having a moderately severe illness.