The diaphragm is a dome-shaped, tendinous, muscular structure and the most important muscle involved in breathing. The diaphragm's action is the mechanical driving force for inspiration. Diseases of the diaphragm are few and rare, but diaphragmatic dysfunction can be fatal, as breathing is a vital and involuntary process.
Anatomy
The diaphragm is attached to the lower part of the sternum, the ribs and the first three lumbar vertebrae. It acts as a partition between the abdominal cavity and the thoracic cavity. The diaphragm has openings that allow for passage of the aorta and other smaller arteries, major veins, the esophagus and nerves.
Mechanics of Breathing
According to "Pulmonary Physiology" by Dr. Michael Levitsky, the mechanics of breathing are centered on a principle of physics called Boyle's Law, which states that pressure and volume of a gas are inversely proportional--that is, increasing one value will decrease the other, and vice versa. This concept will be used to explain the events that occur during the two phases of breathing, inspiration and expiration.
During inspiration, the diaphragm contracts, and the dome descends one to two centimeters into the abdominal cavity. This descent causes the overlying thoracic cavity to elongate and increase in volume, and according to Boyle's Law, decrease in pressure as well. At this time, the pressure in the lungs is below that of the atmosphere, and in order to maintain balance, air rushes into the lungs to increase the pressure within the thoracic cavity. During normal quiet breaths, expiration is a passive process. It is a result of relaxation of the diaphragm after inspiration. When the diaphragm relaxes, it elongates and ascends into the thoracic cavity. This decreases the volume of the thoracic cavity, thus increasing pressure, which forces air out of the lungs.
Diseases of the Diaphragm
According to Drs. Marco G. Patti and Pierro Fisichella, contributors to "Current Diagnosis and Treatment: Surgery," there are three primary diseases of the diaphragm: herniation, trauma and tumors.
Diaphragmatic hernias occur when abdominal organs protrude through the diaphragm and into the thoracic cavity. This can also occur in a more mild form, where abdominal contents bulge into the thorax via weaknesses in the diaphragm.
The diaphragm is vulnerable to rupture when subjected to penetrating trauma or severe blunt external trauma. This can make herniation more likely.
Tumors of the diaphragm are very rare, but when they occur they are generally benign and do not cause any symptoms.
Changes During Pregnancy
Drs. Daniel Kahn and Brian Koos, contributors to "Current Diagnosis and Treatment: Obstetrics and Gynecology," assert that diaphragmatic changes during pregnancy are primarily anatomic. The expanding uterus displaces the diaphragm upward by approximately four centimeters. However, this change alone does not alter the function of the diaphragm or cause changes in breathing.
Other Respiratory Muscles
Muscles that assist the diaphragm during quiet breathing include the external intercostal, parasternal intercostal and scalene muscles, which work together to enlarge the thoracic cavity to allow air to flow into the lungs. During heavy breathing, these muscles receive extra help from the sternocleidomastoid muscle.
Though expiration is generally due to passive relaxation of the diaphragm, it may be enhanced during heavy breathing by the abdominal muscles and the internal intercostal muscles.
References
- "Current Diagnosis and Treatment: Surgery 13e, Ch 20"; Marco G. Patti MD, Piero M. Fisichella MD; 2010
- "Pulmonary Physiology, Chapter 2"; Michael G. Levitzky, PhD; 2007
- "Current Diagnosis and Treatment: Obstetrics and Gynecology, Chapter 7"; Daniel A. Kahn MD, PhD, Brian J. Koos MD, PhD; 2007
