Respiratory System 5, Podcast on breathing

Inspiration
To facilitate inspiration the diaphragm contracts, causing it to move down and flatten. At the same time the external intercostal muscles pull the rib cage up and out. Both of these movements result in an increase in the volume of the thoracic cavity. Because the volume of the thorax is increased the pressure of air left inside the thorax, in the lungs, is reduced. Gases will always move from areas of high pressure to areas of lower pressure. As there is a pressure reduction in the thorax, air moves in from the outside to equalise the pressures between the atmosphere and the reduced pressure inside the lungs. So in inspiration when air moves into the lungs the process is one of negative pressure ventilation. This is in contrast to artificial ventilation when air is actively blown into the lungs in the process of positive pressure ventilation. The increase in the volume of air in the lungs stretches the elastic tissues and smooth muscle associated with the walls of the bronchioles and alveoli. This is analogous to blowing up a balloon.
Expiration
During expiration, the smooth muscle and elastic tissue in the walls of the bronchioles and alveoli recoil. This reduces the volume of these structures and so increases the pressure of the air they contain. This is analogous to releasing the pressure on the neck of a balloon; as the elastic walls of the balloon recoil the pressure is increased so air is blown out. As a result of these elastic properties the lungs will passively recoil facilitating expiration. This increases the air pressure in the lungs as a whole, and air is blown out. To assist this process the diaphragm relaxes and so moves up. The external intercostal muscles also relax which allows the ribs to fall down and in. The movement of the diaphragm up, and the ribs down and in, reduces the volume of the thoracic cavity. Because the volume is reduced the pressure will be increased. This means the pressure inside the lungs is now greater than in the external atmosphere so air will be blown out of the lungs. From this it can be seen that, while inspiration is an active muscular process, expiration is a passive process caused by elastic recoil and muscle relaxation. During vigorous respiration the process of expiration needs to be faster, so the process is aided by contraction of the internal intercostal muscles. These pull the ribs down and in rapidly and actively. These internal intercostal muscles are also essential during periods of increased airway resistance. For example, during an asthmatic attack when the bronchial lumens are narrowed meaning air needs be forced out against the increased resistance.
Clinical applications
Monitoring the rate, depth and rhythm of breathing is a fundamental aspect of clinical observations. One respiratory cycle consists of one inspiration followed by one expiration. These may be counted by observation of chest movements and should be recorded over a period of one minute. The respiratory rhythm should be regular and a rate of about 12 breaths per minute is normal during periods of rest. In emergency situations, assessment of the airway is always the first priority. If we put our ear near the patients mouth and look down at their chest we are able to ‘look, listen and feel’ the breathing.

Source: https://www.youtube.com/watch?v=rndIw6rEjgI