Breathing is divided into two processes: inspiration and expiration.
During inspiration, the diaphragm contracts, decreases in size and moves downward to allow the lungs to expand and increase volume. The air from outside the body is inhaled through the nasal or oral cavity and is filtered by cilia, or small hairs that line the cavity, to prevent foreign particles from entering air pathway. The inhaled air enters and passes through the pharynx, or throat, towards the larynx. The air then passes through the larynx and enters the trachea, or windpipe, which then splits into the left and right bronchial tubes that lead into each lung. The bronchial tubes branch off into smaller tubes called bronchioles. Located at the end of each bronchiole are small sacs called alveoli which is the location where oxygen from the inhaled air diffuses into the bloodstream through the respiratory membrane.
During expiration, the diaphragm relaxes, increases in size and moves upward to compress the lungs and decrease in volume. Carbon dioxide from the bloodstream is diffused through the respiratory membrane into the alveoli and forced out the same way the air entered the body.The process of ventilation is driven by the concept of Boyle’s law which states that the pressure is inversely proportional to the volume of a given quantity of gas. Essentially, the respiratory system works by producing pressure gradients between the atmospheric pressure and the intra-alveolar pressure, the pressure inside the alveoli. During inspiration, the diaphragm contracts, increasing the volume of the lungs which decreases the intra-alveolar pressure below atmospheric pressure causing air to move into the body. Expiration is the opposite; the diaphragm relaxes and enlarges and decreases the volume of the lungs which increases the intra-alveolar pressure above atmospheric pressure causing air to move out of the body.