________________
costal muscles are stimulated to contract by the intercostal nerves.
Abdominal muscles: The abdominal muscles are the most powerful and important muscles for forced exhalation, but are not used in quiet breathing. Contraction puts inward pressure on the abdominal viscera, which then push the diaphragm up and reduce lung volume. In addition, these muscles may assist expiration by pulling down and deflating the lower rib cage. The important abdominal muscles for respiration are the rectus abdominous, the transverse abdominous, and the external and internal obloquies.
Process of respiration There are about 15 complete respiratory cycles per minute in normal, quiet breathing. The breath passes from the nasal cavities through the pharynx at the top of the throat and into the larynx, which commences at the point where the oesophagus and the trachea meet. The movement of food, liquid and air is guided by the epiglottis, a cartilaginous flap which closes off the larynx during swallowing. The larynx is made of cartilage and contains the vocal cords, which are responsible for the production of sound. From the larynx, the breath moves down the trachea, through the bronchial system and into the alveoli, where it flows into the capillaries surrounding the alveoli.
Approximately 79 percent of the inhaled breath is nitrogen (which plays no part in the respiratory process), 20 percent is oxygen, and 0.04 percent is carbon dioxide. The exhaled breath contains the same proportion of nitrogen but the oxygen composition is reduced to about 16 percent, and the carbon dioxide composition increases to about 4.04 percent. Exhaled air is also warmer and more humid, as water vapour is exhaled from the lungs, which can be seen by exhaling over a mirror. As much as 20 percent of body heat can be lost in exhalation.
The oxygen content in air is one of the most important factors for the maintenance of life. It is required for many
87
