Unlike the blood pump, the air pump contains no valves and uses the same passage for air going in and out of the system. It uses negative pressure, a subatmospheric pressure pump. Respiration in man is associated with the movement of oxygen into and carbon dioxide out of the air sacs in the lungs, diffusion of these gases across the membranes that separate the alveolar lumen from that of the capillary, transport oxygen and carbon dioxide by the blood, the diffusion of these gases between blood and the cytoplasm of cells, and the utilization and storage of oxygen and the elimination of the products of oxidation by the cell.

Respiration, which lasts for about 4.0 sec. consist of:

•  Inspiration, which last only for 1.5 sec. and,
•  Expiration, which lasts for 2.5 sec.

During inspiration, the air is moistened, temperature raised, suspended particles removed, and the thorax enlarged by movements of the ribs outwards and upwards and by descent of the diaphragm.

In inspiration, the diaphragm is the major muscle of inspiration, and consists of a central tendon and an outer skeletal muscle. The phrenic nerve (C3, 4, 5) innervates it. Its contraction converts its dome shape to almost a flat surface. This lengthens the chest cavity.

The external intercostals muscles are responsible for rib movement in inspiration. They pass obliquely forwards and downwards from their origin at the lower border of the rib near the tubercle to an insertion into the upper border of the rib. Their movement results in an increase in the anterioposterior diameter of the chest, and by virtue of their bowed midpart, an increase in transverse diameter of the thorax (the so called “Bucket handle movement”). The thoracic segmental nerves innervate the external intercostals.

Muscles other than the diaphragm and the external intercostals are called accessory muscles of inspiration since they do not normally participate in quiet breathing. Most of these muscles lower the resistance to air flow and therefore aid inspiration.

Expiration at rest is purely a passive process –recoil of the lungs. However, active expiration begins in the average adult when the minute volume of air breathed begins to exceed 40 liters/min. or during yelling, coughing and sneezing.