Breathing

Breathing (or ventilation) is the process of moving air into and out of the lungs to facilitate gas exchange with the internal environment, mostly by bringing in oxygen and flushing out carbon dioxide. Breathing (or ventilation) is the process of moving air into and out of the lungs to facilitate gas exchange with the internal environment, mostly by bringing in oxygen and flushing out carbon dioxide. All aerobic creatures need oxygen for cellular respiration, which uses the oxygen to break down foods for energy and produces carbon dioxide as a waste product. Breathing, or 'external respiration', brings air into the lungs where gas exchange takes place in the alveoli through diffusion. The body's circulatory system transports these gases to and from the cells, where 'cellular respiration' takes place. The breathing of all vertebrates with lungs consists of repetitive cycles of inhalation and exhalation through a highly branched system of tubes or airways which lead from the nose to the alveoli. The number of respiratory cycles per minute is the breathing or respiratory rate, and is one of the four primary vital signs of life. Under normal conditions the breathing depth and rate is automatically, and unconsciously, controlled by several homeostatic mechanisms which keep the partial pressures of carbon dioxide and oxygen in the arterial blood constant. Keeping the partial pressure of carbon dioxide in the arterial blood unchanged under a wide variety of physiological circumstances, contributes significantly to tight control of the pH of the extracellular fluids (ECF). Over-breathing (hyperventilation) and under-breathing (hypoventilation), which decrease and increase the arterial partial pressure of carbon dioxide respectively, cause a rise in the pH of ECF in the first case, and a lowering of the pH in the second. Both cause distressing symptoms. Breathing has other important functions. It provides a mechanism for speech, laughter and similar expressions of the emotions. It is also used for reflexes such as yawning, coughing and sneezing. Animals that cannot thermoregulate by perspiration, because they lack sufficient sweat glands, may lose heat by evaporation through panting. The lungs are not capable of inflating themselves, and will expand only when there is an increase in the volume of the thoracic cavity. In humans, as in the other mammals, this is achieved primarily through the contraction of the diaphragm, but also by the contraction of the intercostal muscles which pull the rib cage upwards and outwards as shown in the diagrams on the left. During forceful inhalation (Figure on the right) the accessory muscles of inhalation, which connect the ribs and sternum to the cervical vertebrae and base of the skull, in many cases through an intermediary attachment to the clavicles, exaggerate the pump handle and bucket handle movements (see illustrations on the left), bringing about a greater change in the volume of the chest cavity. During exhalation (breathing out), at rest, all the muscles of inhalation relax, returning the chest and abdomen to a position called the “resting position”, which is determined by their anatomical elasticity. At this point the lungs contain the functional residual capacity of air, which, in the adult human, has a volume of about 2.5–3.0 liters. During heavy breathing (hyperpnea) as, for instance, during exercise, exhalation is brought about by relaxation of all the muscles of inhalation, (in the same way as at rest), but, in addition, the abdominal muscles, instead of being passive, now contract strongly causing the rib cage to be pulled downwards (front and sides). This not only decreases the size of the rib cage but also pushes the abdominal organs upwards against the diaphragm which consequently bulges deeply into the thorax. The end-exhalatory lung volume is now less air than the resting 'functional residual capacity'. However, in a normal mammal, the lungs cannot be emptied completely. In an adult human, there is always still at least one liter of residual air left in the lungs after maximum exhalation. Diaphragmatic breathing causes the abdomen to rhythmically bulge out and fall back. It is, therefore, often referred to as 'abdominal breathing'. These terms are often used interchangeably because they describe the same action. When the accessory muscles of inhalation are activated, especially during labored breathing, the clavicles are pulled upwards, as explained above. This external manifestation of the use of the accessory muscles of inhalation is sometimes referred to as clavicular breathing, seen especially during asthma attacks and in people with chronic obstructive pulmonary disease. Usually, air is breathed in and out through the nose. The nasal cavities (between the nostrils and the pharynx) are quite narrow, firstly by being divided in two by the nasal septum, and secondly by lateral walls that have several longitudinal folds, or shelves, callednasal conchae, thus exposing a large area of nasal mucous membrane to the air as it is inhaled (and exhaled). This causes the inhaled air to take up moisture from the wet mucus, and warmth from the underlying blood vessels, so that the air is very nearly saturated with water vapor and is at almost body temperature by the time it reaches the larynx. Part of this moisture and heat is recaptured as the exhaled air moves out over the partially dried-out, cooled mucus in the nasal passages, during breathing out. The sticky mucus also traps much of the particulate matter that is breathed in, preventing it from reaching the lungs.