Mechanics of breathing

Thoracic volume. The volume of the thoracic cavity increases during inspiration and decreases during expiration. 

Assisting the diaphragm with inspiration are the external intercostal muscles, which connect adjacent ribs. When the external intercostal muscles contract, the ribs are lifted upward and outward (much like a handle on a bucket). Therefore, contraction of these muscles causes an increase in the horizontal dimension of the thoracic cavity and a further increase in thoracic volume. The external intercostal muscles are supplied by the intercostal nerves. 

Deeper inspirations are achieved by more forceful contraction of the diaphragm and external intercostal muscles. Furthermore, accessory inspiratory muscles, including the scalenus and sternocleidomastoid muscles, contribute to this process. Located mainly in the neck, these muscles raise the sternum and elevate the first two ribs. As a result, the upper portion of the thoracic cavity is enlarged. 

During exercise or voluntary hyperventilation, expiration becomes an active process. Under these conditions, a larger volume of air must be exhaled more rapidly. Therefore, two muscle groups are recruited to facilitate this process. The most important muscles of expiration are the muscles of the abdominal wall. Contraction of these muscles pushes inward on the abdominal contents and increases abdominal pressure. As a result, the diaphragm is pushed upward more rapidly and more forcefully toward its preinspiration 

As it does with all objects on the surface of the Earth, gravity exerts its effects on the molecules of the atmosphere. The weight generated by these molecules is referred to as atmospheric, or barometric, pressure (Patm). At sea level, atmospheric pressure is 760 mmHg. In order to simplify this discussion, atmospheric pressure will be normalized to 0 mmHg (or 0 cmH20) and all other pressures are referenced to this. 

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Horsfield, K. (1986). Morphometry of airways. In The respiratory system Section 3. Mechanics of Breathing, Part 1vol. Ill (P.T. Macklem and J. Mead, Eds.), pp. 75-88. American Physio logical Society, Bethesda, Maryland. 

In order to understand diseases of the respiratory system and how they can be treated, it is necessary to have an overview of the normal function of the respiratory system that is the structures involved, the mechanism of breathing and gas exchange and the control and variation of breathing rate. Air must be able to get in and out of the lungs efficiently and gas exchange must be adequate. Any condition that interferes with these processes will cause disease. 

The importance of Boyle s law becomes more apparent when you consider the mechanics of breathing. Our lungs are elastic, balloon-like structures contained within an airtight chamber called the thoracic cavity. The diaphragm, a muscle, forms the flexible floor of the cavity. 

These mechanisms can be observed in many common situations. For example, fog via mixing can be seen in the discharge of breath on a cold day. Fog via adiabatic expansion can be seen in the low-pressure area over the wing of an airplane landing on a humid summer day and fog via condensation can be seen in the exhaust from an automobile air conditioner (if you follow closely enough behind another car to pick up the ions or NO molecules needed for nucleation). All of these occur at a veiy low supersaturation and appear to be keyed to an abundance of foreign nuclei. All of these fogs also quickly dissipate as heat or unsaturated gas is added. 

Inhaled gaseous compounds are absorbed in all parts of the respiratory system whereas particle size determines how deep into the airways the parti cles will he transported in the airstrearn. Shortness of breath is a typical sign of a chemical exposure that has affected the lungs, and it may be evoked through iminunological mechanisms (e.g., formaldehyde, ethyleneoxide), or through toxic irritation (formaldehyde, isocyanates, sulfur dioxide, nitrogen dioxide, Frequently the mechanism depends on the concentration ol the com... 

A one-year-old male develops decreased breath sounds, and wheezing during a febrile episode, which is relieved by albuterol. What is the mechanism of action of albuterol ... 

Greer JJ, Carter JE. 1995. Effects of cyanide on the neural mechanisms controlling breathing in the neonatal rat in vitro. Neuro Toxicology 16(2) 211-216. 

Despite these findings, mediation by enz3rmes present in dust remains an attractive hypothesis. Enzymes may exert their action in one of three ways 1) an allergic mechanism like that shown in workers exposed to subtills during detergent manufacture (22,23,24) where wheezing, shortness of breath, and/or rhinitis symptoms are shown to be mediated by specific IgE antibodies. 

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