What is the mechanism of breathing?

Mechanisms of Human Breathing 
To understand ventilation, the manner in which air is drawn into and expelled out of the lungs, it is necessary to remember first that when a person breathes, there is a con­tinuous column of air from the pharynx to the alveoli of the lungs; that is, the air passages are open.
Second, it should be noted that the lungs lie within the sealed-off thoracic cavity. The ribs, which are hinged to the vertebral column at the back and to the sternum at the front, along with the muscles that lie between them, com­pose the top and sides of the thoracic cavity. The di­aphragm, a dome-shaped, horizontal muscle, forms the thoracic cavity floor. The lungs themselves are enclosed by the pleural membranes. One of these, the parietal mem­brane, adheres closely to the walls of the thoracic cavity and diaphragm, while the other, the visceral membrane, is fused to the lungs. The two pleural layers lie close to one another, being separated only by a thin film of fluid. Normally, the intrapleural pressure is less than the atmospheric pressure. This reduced pressure is important because when, by de­sign or accident, air enters the intrapleural space, the lungs collapse, making inspiration impossible. (Presence of air in the intrapleural space is called a pneumothorax.)
The lungs are completely enclosed and, by vvay of the pleural membranes. adhere to the vvalls of the thoracic cavity.
Inspiration 
The respiratory center, located in the medulla oblongata, consists of a group of neurons that exhibit an automatic rhythmic discharge that triggers inspiration. Carbon diox­ide (C02) and hydrogen ions (H+) are the primary stimuli that cause changes in the activity of this center. The respira­tory center is not affected by low oxygen (02) levels. Chemoreceptors in the carotid bodies (located in the . carotid arteries) and in the aortic bodies (located in the aorta) respond primarily to hydrogen ion concentration but also to the level of carbon dioxide and oxygen in blood. These bodies communicate with the respiratory center. When levels of carbon dioxide and hydrogen rise, the rate and depth of breathing increase. (Do not confuse the carotid and aortic bodies with the carotid and aortic si­nuses, which monitor blood pressure.) The Medical Focus reading on this page describes the modified breathing rates that result when disease affects the respiratory center.
The respiratory center sends out nerve impulses by way of nerves to the diaphragm and the rib cage. In its relaxed state, the diaphragm is dome-shaped, but upon stimulation, it contracts and lowers. Also, the exter­nal intercostal muscles contract, and the rib cage moves upward and outward. Both of these contractions increase the size of the thoracic cavity, which expands the lungs. When the lungs expand, air pressure within the enlarged alveoli lowers and is immediately rebalanced by air rush­ing in through the nose or the mouth.
Inspiration is the active phase of breathing. During this time, the diaphragm and the intercostal (rib) muscles contract, intrapleural pressure decreases even more, the lungs expand, and air rushes in. Note that the air comes in because the lungs already have opened up; ai: does not force the lungs open. This is why humans are sometimes said to breathe by negative pressure. The creation of a partial vacuum causes air to enter the lungs.
Stimulated by nerve impulses. the rib cage lifts up and out. and the diaphragm lovvers to expand the thoracic cavity and lungs. allovving inspiration to occur. 
Expiration 
When the respiratory center stops sending signals to the di­aphragm and the rib cage, the diaphragm relaxes and re­sumes its dome shape. The abdominal organs press up against the diaphragm, and the rib cage moves down and inward. Now the elastic lungs recoil, and air is pushed out.
The respiratory center acts rhythmically to bring about breathing at a normal rate and volume. If by chance we in­hale more deeply, the lungs are expanded, and the alveoli stretch. This stimulates stretch receptors in the alveolar walls, and they initiate inhibitory nerve impulses that travel from the inflated lungs to the respiratory center. This causes the respiratory center to stop sending out nerve impulses.
While inspiration is the active phase of breathing, expiration is normally passive-that is, the diaphragm and external intercostal muscles are relaxed during expira­tion. In deeper and more rapid breathing, expiration can also be active. Contraction of internal intercostal muscles
can force the rib cage to move downward and inward. Also, when the abdominal wall muscles are contracted, in­creased pressure helps to expel air. Air can also be moved into and out of the lungs for reasons not associated with respiration (for example, coughing, sneezing, yawning).
When nervous stimulation ceases, the rib cage lovvers and the diaphragm rises, allovving the lungs to recoil and expiration to occur. 
Lung Capacities 
The amount of air moved in and out with each breath is called the tidal volume (lV). Normally, the tidal volume is about 500 milliliters (ml), but a person can increase the amount inhaled and exhaled by deep breathing.
The total volume of air that can be moved in and out of the lungs during a single breath is called the vital capacity (Ve). A person can increase inspiration to as much as 3,100 m!. The increase beyond tidal volume is called the inspiratory reserve volume (IRV). Similarly, a person can increase expiration beyond tidal volume by contracting the thoracic muscles. This volume of air is called the expiratory reserve volume (ERV) and measures approximately 1,400 ml of air. Vital capacity is the sum of the tidal volume plus the inspiratory reserve and expiratory reserve volumes.
Even after very deep breathing, some air (about 1,000 ml) remains in the lungs. This vol­ume of air is called the residual volume (RV) and is no longer useful for gas exchange purposes. In some lung dis­eases, such as emphysema and asthma, the residual vol­ume builds up because the individual has difficulty emptying the lungs. Thus, the lungs tend to be filled with useless air and vital capacity is reduced.
Dead Space 
Some of the inspired air never reaches the lungs; instead, it fills the conducting airways. These passages are not used for gas exchange and, therefore, are said to contain dead space. Breathing more slowly and deeply ensures that a greater percentage of the tidal volume actually reaches the lungs.
If a person breathes through a tube, there is an increase in the amount of dead space and in the amount of air that never reaches the lungs. Any device that increases the amount of dead space beyond maximal inhalation capac­ity means death to the individual because the air inhaled never reaches the alveoli.
Any blockage of air passages may require extreme
The air used for gas exchange excludes both the residual volume in the lungs and that in the dead space of the respiratory tract.