Speech, a form of human communication, is produced by three groups of organs working together: respiratory, phonatory and articulatory. The dominant elements of the respiratory system  are the lungs, the chest wall and the diaphragm. Working together, they provide the mechanical energy in form of air pressure, the aerodynamic energy of the speech (Kent & Read 2) needed to produce sound in the larynx. The tongue, the lips, the jaw and the velum, the articulatory elements of the speech organs, modify the properties of created sounds. The extent of modification depends on several factors, including the position of articulatory organs, the intensity of sound (pressure), physical properties of the tissues, etc. The larynx is the place of phonation.
The respiratory system  is located in the chest (thorax) – a cavity, created by rib cage and the muscles. The ribs are posteriorly connected to the vertebral column, and anteriorly to the sternum (breast-bone). This thoracic cavity is on its top limited by the shoulder blades (scapuae), and on the bottom by the diaphragm. The lungs are located within the thoracic cavity: they are a cone-shaped organ, made of sponge-like matter, consisting of many bronchioles that branch into numerous alveoli. The lungs and the inward surface of the cage are connected with pleural linkage, a fluid-like matter that makes possible for the lungs to expand or shrink simultaneously with the cavity. The lungs act as bellows (Crystal 20): after the chest muscles flex, the pressure inside the lungs increases, which forces air to exit; in reverse, by lowering the diaphragm or flexing the rib muscles, the pressure inside the lungs decreases, which forces the air to enter the respiratory system.
There are two important phases in the respiratory system that are related to speech: inspiration and exhalation. They make the respiratory cycle, which is relevant not only in providing the energy, but also “in the sequential organization of speech” (Clark, Introduction 21). Inspiration, or the process of inhaling, occurs when the thoracic volume increases, which causes the lowering of the pressure in lungs. This pressure difference causes air to enter the system. The increase of space within the thorax is achieved by the rib cage moving upwards (caused by shortening of intercostal muscles) or by lowering of the diaphragm. Expiration, or exhalation, is achieved by the “elastic recoil forces” or relaxation pressure (24).
However, in situations where more energy is needed (shouting, prolonged speaking), the muscles activate to help the air stream mechanism and increase the flow of air. The subcostal muscles and the traverse thoracic muscles shrink the rib cage, while abdominal muscles (the traverse, internal oblique, external oblique and the rectus abdominis) “compress the abdomen” (25). In an experiment  described by Clark (27), Ladefoged, among others, showed that significant energy must be used to maintain the air energy once the exhalation phase reaches zero capacity. This is why “speech does not exploit this part of the expiratory phase except under extreme conditions” (28).
This post is based on a draft for one of the introductory chapters in my paper.
Next text: The Speech Organs and Airstream
 Another system directly involved in speech is the nervous system.
The discussion in was adopted from Clark’s chapter about the speech organs.
 Although, the experiment was done on one subject only – warned Ladefoged.