Exhalation (or expiration) is the flow of the respiratory current out of the organism. In humans it is the movement of air out of the bronchial tubes, through the airways, to the external environment during breathing Figure 1.
This happens due to elastic properties of the lungs, as well as the internal intercostal muscles that lower the rib cage and decrease thoracic volume. As the thoracic diaphragm relaxes during exhalation it causes the tissue it has depressed to rise superiorly and put pressure on the lungs to expel the air.
During forced exhalation, as when blowing out a candle, expiratory muscles including the abdominal muscles and internal intercostal muscles generate abdominal and thoracic pressure, which forces air out of the lungs.
Exhaled air is rich in carbon dioxide, a waste product of cellular respiration during the production of energy, which is stored as ATP. Exhalation has a complementary relationship to inhalation; the cycling between these two efforts define respiration.
The main reason for exhalation is to rid the body of carbon dioxide, which is the waste product of gas exchange in humans.
The air then flows through the trachea, the larynx, and pharynx to the nasal cavity and oral cavity where it is expelled out of the body. Exhalation takes longer then inhalation since it is believed to facilitate better exchange of gases. Parts of the nervous system help to regulate respiration in humans. The exhaled air is not just carbon dioxide; it contains a mixture of other gases. Human breath contains volatile organic compounds (VOCs).
Brain control of exhalation can be broken down into voluntary control and involuntary control. During voluntary exhalation, air is held in the lungs and released at a fixed rate. Examples of voluntary expiration include: singing, speaking, exercising, playing an instrument, and voluntary hyperpnea. Involuntary breathing includes metabolic and behavioral breathing.
The neurological pathway of voluntary exhalation is complex and not fully understood. However, a few basics are known. The motor cortex within the cerebral cortex of the brain is known to control voluntary respiration because the motor cortex controls voluntary muscle movement. This is referred to as the corticospinal pathway or ascending respiratory pathway.
The pathway of the electrical signal starts in the motor cortex, goes to the spinal cord, and then to the respiratory muscles. The spinal neurons connect directly to the respiratory muscles. Studies indicate that there are numerous other sites within the brain that may be associated with voluntary expiration.
The inferior portion of the primary motor cortex may be involved, specifically, in controlled exhalation. Activity has also been seen within the supplementary motor area and the premotor cortex during voluntary respiration. This is most likely due to the focus and mental preparation of the voluntary muscular movement.
Voluntary expiration is essential for many types of activities. Phonic respiration (speech generation) is a type of controlled expiration that is used every day. Speech generation is completely dependent on expiration, this can be seen by trying to talk while inhaling. Using airflow from the lungs, one can control the duration, amplitude, and pitch. While the air is expelled it flows through the glottis causing vibrations, which produces sound. Depending on the glottis movement the pitch of the voice changes and the intensity of the air through the glottis change the volume of the sound produced by the glottis.
Involuntary respiration is controlled by respiratory centers within the medulla oblongata and pons. The medullary respiratory center can be subdivided into anterior and posterior portions. These centers are located in the brainstem and work together to control involuntary respiration. In our case, the ventral respiratory group (VRG) controls involuntary exhalation.
Yawning is considered a nonrespiratory gas movement. A nonrespiratory gas movement is another process that moves air in and out of the lungs that does not include breathing. Yawning is a reflex that tends to disrupt the normal breathing rhythm.
Several receptor groups in the body regulate metabolic breathing. These receptors signal the respiratory centers to initiate inhalation or exhalation. Peripheral chemoreceptors are located in the aorta and carotid arteries. They respond to changing blood levels of oxygen, carbon dioxide, and H+ by signaling the pons and medulla. Irritant and stretch receptors in the lungs can directly cause exhalation. Both sense foreign particles and promote spontaneous coughing. They are also known as mechanoreceptors because they recognize physical changes not chemical changes. Central chemoreceptors in the medulla also recognize chemical variations in H+. Specifically, they monitor pH change within the medullary interstitual fluid and cerebral spinal fluid.