<P> In mammals, inhalation at rest is primarily due to the contraction of the diaphragm . This is an upwardly domed sheet of muscle that separates the thoracic cavity from the abdominal cavity . When it contracts the sheet flattens, (i.e. moves downwards as shown in Fig. 7) increasing the volume of the thoracic cavity . The contracting diaphragm pushes the abdominal organs downwards . But because the pelvic floor prevents the lowermost abdominal organs moving in that direction, the pliable abdominal contents cause the belly to bulge outwards to the front and sides, because the relaxed abdominal muscles do not resist this movement (Fig. 7). This entirely passive bulging (and shrinking during exhalation) of the abdomen during normal breathing is sometimes referred as "abdominal breathing", although it is, in fact, "diaphragmatic breathing", which is not visible on the outside of the body . Mammals only use their abdominal muscles only during forceful exhalation (see Fig. 8, and discussion below). Never during any form of inhalation . </P> <P> As the diaphragm contracts, the rib cage is simultaneously enlarged by the ribs being pulled upwards by the intercostal muscles as shown in Fig. 4 . All the ribs slant downwards from the rear to the front (as shown in Fig. 4); but the lowermost ribs also slant downwards from the midline outwards (Fig. 5). Thus the rib cage's transverse diameter can be increased in the same way as the antero - posterior diameter is increase by the so - called pump handle movement shown in Fig. 4 . </P> <P> The enlargement of the thoracic cavity's vertical dimension by the contraction of the diaphragm, and its two horizontal dimensions by the lifting of the front and sides of the ribs, causes the intrathoracic pressure to fall . The lungs' interiors are open to the outside air, and being elastic, therefore expand to fill the increased space . The inflow of air into the lungs occurs via the respiratory airways (Fig. 2). In health these airways (starting at the nose or mouth, and ending in the microscopic dead - end sacs called alveoli) are always open, though the diameters of the various sections can be changed by the sympathetic and parasympathetic nervous systems . The alveolar air pressure is therefore always close to atmospheric air pressure (about 100 kPa at sea level) at rest, with the pressure gradients that cause air to move in and out of the lungs during breathing rarely exceeding 2--3 kPa . </P> <P> During exhalation the diaphragm and intercostal muscles relax . This returns the chest and abdomen to a position determined by their anatomical elasticity . This is the "resting mid-position" of the thorax and abdomen (Fig. 7) when the lungs contain their functional residual capacity of air (the light blue area in the right hand illustration of Fig. 7), which in the adult human has a volume of about 2.5--3.0 liters (Fig. 3). Resting exhalation lasts about twice as long as inhalation because the diaphragm relaxes passively more gently than it contracts actively during inhalation . </P>

Why do we need to study respiratory system