<P> One of the reasons we can breathe is because of the elasticity of the lungs . The internal surface of the lungs on average in a non-emphysemic person is normally 63m2 and can hold about 5lts of air volume . Both lungs together have the same amount of surface area as half of a tennis court . Disease such as, emphysema, tuberculosis, can reduce the amount of surface area and elasticity of the lungs . Another big factor in the elasticity of the lungs is smoking because of the residue left behind in the lungs from the smoking . The elasticity of the lungs can be trained to expand further . </P> <P> 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 . </P> <P> 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 . Initiation of voluntary contraction and relaxation of the internal and external internal costals has been shown to take place in the superior portion of the primary motor cortex . Posterior to the location of thoracic control (within the superior portion of the primary motor cortex) is the center for diaphragm control . 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 . </P> <P> 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 . </P>

Describe the function of the diaphragm during inhalation and exhalation