Mechanics and Muscles of Ventilation

There are two zones of the respiratory system that are involved in ventilation. The conducting zone is the portion of the respiratory system that carries air from the environment to the alveoli. This includes the nose, pharynx, larynx, trachea, bronchi, and bronchioles. The respiratory zone is the zone involved in gas exchange; it is made up of the terminal bronchioles, alveolar ducts, and alveoli. Ventilation is the process of moving air into and out of the lungs. Eupnea is normal, unlabored breathing, also called quiet breathing. Hyperventilation is very fast breathing that results in expiring more carbon dioxide than normal. A decreased amount of carbon dioxide in the blood is called hypocapnia and results in a rise in pH in the blood, or respiratory alkalosis. Hyperventilation has many causes, some of which include panic attacks, heart attack, severe pain, or different types of lung disease. Treatment for hyperventilation depends on the cause, but involves breathing less rapidly or inspiring carbon dioxide into the lungs (for example, breathing in a paper bag). Hypoventilation is very slow breathing that results in expiring less carbon dioxide than normal. Excess carbon dioxide in the bloodstream is called hypercapnia and results in a drop in blood pH, or respiratory acidosis. Common causes of hypoventilation and respiratory acidosis include asthma, severe obesity, multiple sclerosis, and sedative overdose of drugs. Treatment depends on the specific cause, but increasing oxygen supply to the lungs to meet the metabolic demands of the body is of primary concern. This may involve the use of bronchodilator medication, noninvasive positive-pressure ventilation (CPAP), supplemental oxygen, or if necessary, artificial ventilation. Involuntary breathing is controlled by the respiratory center in the brain stem. Chemoreceptors located in the medulla and in the aortic arch and carotid bodies detect changes in the pH of the blood caused by an increase or decrease of carbon dioxide. For example, increased levels of carbon dioxide lowers blood pH due to an increase in hydrogen ions from carbonic acid production. In response to this, the respiratory center in the medulla sends nerve impulses to the external intercostal muscles and diaphragm. This increases both breathing rate and lung volume during inspiration.

During the inspiration/expiration phases of eupnea, the diaphragm is the primary ventilatory muscle. The diaphragm is the skeletal muscle that separates the thoracic (chest) cavity from the abdominal (stomach) cavity. It is the muscle used for breathing. When it contracts and descends, pressure in the thoracic cavity drops in addition to the alveolar pressure. This pulls air into the lungs as it moves from a higher pressure gradient outside the body to a lower pressure gradient within the lungs. Expiration of eupnea is a passive event. As the diaphragm relaxes and rises, the elastic lungs and rib cage return to their original position and air is pushed out of the lungs.

On the contrary, active breathing, which occurs during exercise or when in distress, involves many more muscles in the act of respiration. During inspiration, the external intercostal muscles contract, elevating the ribs and pulling them out, thereby increasing the space in the thoracic cavity. This is both a lateral and anteroposterior expansion. The sternocleidomastoid and scalene muscles work to pull out the upper ribs and sternum, further opening up more thoracic space. Abdominal wall muscles are recruited during active expiration, particularly the rectus abdominis, the external and internal obliques, and the transversus abdominus. When these muscles contract, they force the intra-abdominal pressure to rise. The diaphragm is pushed up, the thoracic pressure rises, as does the pressure in the alveoli, and air is expelled from the lungs. Further, the internal intercostal muscles pull the ribs down and in when they contract. This also aids in active expiratory efforts.