Inhalation and exhalation comprise what we normally call ‘breathing’ – taking air into the lungs (inhalation) and expelling the waste gases (exhalation). This occurs continuously throughout life.
Movements of breathing
The lungs are contained within the chest, and it is the rhythmic chest movements that bring about inspiration and expiration – the lungs follow passively. The chest is cone-shaped, narrower at the top and wider at the bottom. It is encased on each side by twelve ribs, each rib being joined to others by muscles called intercostals. When these muscles contract the ribs, which are hinged on the spine at the back, are pulled up and out at the front to increase the volume of the chest. The membranes of the surface of the lungs and on the inside of the chest are normally kept in close proximity, so that the volume of both chest and lungs increases during inspiration.
At the base of the chest, dividing it from the abdomen below, is a large sheet of muscle called the diaphragm. When it is relaxed it extends up into the chest in a dome shape. When it contracts the dome is flattened, pulling down the bases of the lungs which increases lung volume. Apart from the intercostals and the diaphragm, other muscles of the chest may also be brought into play in extreme circumstances. Inspir- ation is therefore an active, muscle-powered process. After inspiration comes expiration. Normally this is a passive process that occurs simply by the elastic recoil of the chest and lungs returning to their resting state. This can be speeded by contracting the abdominal muscles to push the diaphragm into the chest. During quiet breathing, at an average of 12-16 breaths per minute, the diaphragm is the main muscle used. As the respiratory needs increase the rib muscles are brought into play. Women tend to use their rib muscles more than men during quiet respiration. When respiratory needs are high the other chest muscles, such as pectorals, are used. In order for this to occur the arms have to be stabilized, either by holding on to a solid object or – as athletes do at the end of a race – placing the hands on the knees.
Regulation of respiration
It is important that the levels of the various gases dissolved in theare kept constant. The level of oxygen in the tends not to vary with small variations of respiration or atmospheric oxygen content. Therefore, the carbon dioxide content of the blood is mainly involved in regulating respiration. In the brain is an area that regulates respiration, called the respiratory centre. It is situated towards the lower part of the brain and is particularly sensitive to changes in the carbon dioxide content of the blood. During exercise there is an increase of carbon dioxide in the blood, and the respiratory centre detects this and stimulates, via nerves to the chest muscles, both the rate and depth of respiration. This causes the excess carbon dioxide to be ‘blown off and the level returns to normal. Conversely, if the level of carbon dioxide falls below normal the respiratory centre slows respiration until the level rises again. The oxygen content of the blood does not pose problems under normal circumstances, but there are instances where it may fall even when carbon dioxide levels are normal. In such instances there are areas in the carotid arteries (in the neck) which are sensitive to falls in the oxygen content of the blood. As the oxygen level falls these areas send messages to stimulate the respiratory centre. These messages over-ride the normal carbon dioxide control system and cause an increase in respiration, so returning the oxygen level to normal.
If you want to hold your breath for a long time, for example when swimming under water, you should first inhale and exhale rapidly for a few breaths to ‘clear out’ carbon dioxide from your blood. The removal of carbon dioxide, which normally acts to increase respiration, extends the length of time for which you can hold your breath until the level builds up again and you finally have to breath again. In certain circumstances, the centre in the brain that controls respiration can cease to function, for example as a result of a heart attack, electric shock, or a severe head injury. When someone has stopped breathing, there is no rise-and-fall chest movement, and the face becomes a bluish-grey colour. In most of these cases recovery may occur if ventilation can be quickly re-established by the administration of mouth-to-mouth resusitation. After the rapid clearance of any obstructions in the affected person’s airway, the nose is held, and the lungs are inflated by expiration of the person giving the treatment. Approximately 10 inflations per minute are made, and the lungs are allowed to empty spontaneously. This is continued until the patient’s breathing assumes a normal rhythm. It is possible to obtain an adequate exchange of gases this way, because the oxygen content in the air that is expired by someone who renders assistance is not very much lower than in normal air (just under 16 per cent compared to 21 per cent).
When fully inflated, the lungs contain six litres of air. But this does not mean we inhale six litres and then exhale it all with each breath. It is obvious that the chest does not collapse completely on expiration, because a certain amount of air remains in the lungs which cannot be exhaled. The maximum amount of air we can move in one breath, from full inspiration to full expiration, is called the vital capacity and in most people is about four and a half litres. At rest, the body needs only a fraction of its vital capacity: a normal breath (called tidal volume) uses less than half a litre. This means that there is a reserve capacity both in inspiration and expiration. Gas exchange takes place in the alveoli, the tiny air sacs in the lungs. Any gas passing through the larger airways, the trachea or bronchi, does not take part in the exchange, so this region is called the ‘dead space’ and is about 150ml in volume. During quiet breathing, eight litres of air move in and out of the lungs every minute.
During strenuous exercise the body can increase this amount to almost 100 litres per minute, but only for short periods.
The limiting factor on the amount of exercise the body can do is the rate at which breathing occurs. As breathing increases in rate and depth it takes more energy to drive the muscles involved with respiration. The limit is reached when the increased oxygen required to keep the respiratory muscles supplied is greater than the amount taken in by respiration.
A When a person breathes in (B) the diaphram moves downwards and the chest cavity is expanded as a result of the contraction of the muscles between the ribs (C2). The lungs then fill automatically with air. When exhaling (A) the muscles relax (CI), the diaphragm expands and the chest sinks downwards. This causes the lungs to shrink and air ex-capes from them.