QUESTION = Why don't we inhale close to (or all of) our lung's capacity worth of air when we breathe naturally? GENERATION = It's called the Mammalian Diving Reflex. It's an automatic response that's used when diving for a long time to get enough oxygen in, which is why diving underwater can make you feel so "full" when you're just as close as possible to the surface. It doesn't necessarily work well for humans, because we're upright, but it does give you a great feeling of being underwater. If you "close your mouth" and hold your breath, you should feel very buoyant. This is because your lungs expand when you're breathing, which pulls in air. When you do this, you're also using a lot of oxygen. The extra oxygen that your lungs absorb is actually pretty toxic, so you need to slowly adjust how much you hold your breath in order to get the same amount of oxygen in. The best way to do this is with something called a shallow breath tank. When you inhale, your diaphragm pulls down and your rib cage expands. As your lungs inflate, the air inside them pushes out against the diaphragm. This causes the diaphragm to contract, but a big enough contraction can make your lung deflate. RETRIEVAL #0 = Exhalation---but ventilated alveoli, such as a pulmonary embolism or smoking, excessive ventilation of the alveoli, brought on in relation to perfusion, in people with chronic obstructive lung disease, and “shunt dead space,” which is a mistake between the left to right lung that moves the higher CO2 concentrations in the venous blood into the arterial side. The anatomical factors are the size of the airway, the valves, and tubing of the respiratory system. Physiological dead space of the lungs can affect the amount of dead space as well with factors including smoking, and diseases. Dead space is a key factor for the lungs to work because of the differences in pressures, but it can also hinder the person. One of the reasons we can breathe is because of the elasticity of the lungs. The internal surface of the lungs on average in a non-emphysemic person is normally 63m2 and can hold about 5lts of air volume. Both lungs together have the same amount of surface area as half of a tennis court. Disease such as, emphysema, tuberculosis, can reduce the amount of surface area and elasticity of the lungs. Another big factor in the elasticity of the lungs is smoking because of the residue left behind in the lungs from the smoking. The elasticity of the lungs can be trained to expand further. Section::::Brain involvement. Brain control of exhalation can RETRIEVAL #1 = Lung volumes---Lung volumes Lung volumes and lung capacities refer to the volume of air in the lungs at different phases of the respiratory cycle. The average total lung capacity of an adult human male is about 6 litres of air. Tidal breathing is normal, resting breathing; the tidal volume is the volume of air that is inhaled or exhaled in only a single such breath. The average human respiratory rate is 30-60 breaths per minute at birth, decreasing to 12-20 breaths per minute in adults. Section::::Factors affecting volumes. Several factors affect lung volumes; some can be controlled and some cannot be controlled. Lung volumes vary with different people as follows: A person who is born and lives at sea level will develop a slightly smaller lung capacity than a person who spends their life at a high altitude. This is because the partial pressure of oxygen is lower at higher altitude which, as a result means that oxygen less readily diffuses into the bloodstream. In response to higher altitude, the body's diffusing capacity increases in order to process more air. Also, due to the lower environmental air pressure at higher altitudes, the air pressure within the breathing system must be lower in order to inhale; in order to meet this requirement, the thoracic diaphragm has a tendency to lower to a greater extent during inhalation, which in turn causes an increase RETRIEVAL #2 = Breathing---purpose of breathing is to bring atmospheric air (in small doses) into the alveoli where gas exchange with the gases in the blood takes place. The equilibration of the partial pressures of the gases in the alveolar blood and the alveolar air occurs by diffusion. At the end of each exhalation, the adult human lungs still contain 2,500–3,000 mL of air, their functional residual capacity or FRC. With each breath (inhalation) only as little as about 350 mL of warm, moistened atmospherically is added, and well mixed, with the FRC. Consequently, the gas composition of the FRC changes very little during the breathing cycle. Since the pulmonary capillary blood equilibrates with this virtually unchanging mixture of air in the lungs (which has a substantially different composition from that of the ambient air), the partial pressures of the arterial blood gases also do not change with each breath. The tissues are therefore not exposed to swings in oxygen and carbon dioxide tensions in the blood during the breathing cycle, and the peripheral and central chemoreceptors do not need to "choose" the point in the breathing cycle at which the blood gases need to be measured, and responded to. Thus the homeostatic control of the breathing rate simply depends on the partial pressures of oxygen and carbon dioxide in the arterial blood. This then also maintains the constancy of the pH of RETRIEVAL #3 = Respiratory system---composition of the dry outside air at sea level, where the partial pressure of oxygen is 21 kPa (or 160 mm Hg) and that of carbon dioxide 0.04 kPa (or 0.3 mmHg). During heavy breathing (hyperpnea), as, for instance, during exercise, inhalation is brought about by a more powerful and greater excursion of the contracting diaphragm than at rest (Fig. 8). In addition the "accessory muscles of inhalation" exaggerate the actions of the intercostal muscles (Fig. 8). These accessory muscles of inhalation are muscles that extend from the cervical vertebrae and base of the skull to the upper ribs and sternum, sometimes through an intermediary attachment to the clavicles. When they contract the rib cage's internal volume is increased to a far greater extent than can be achieved by contraction of the intercostal muscles alone. Seen from outside the body the lifting of the clavicles during strenuous or labored inhalation is sometimes called clavicular breathing, seen especially during asthma attacks and in people with chronic obstructive pulmonary disease. During heavy breathing, exhalation is caused by relaxation of all the muscles of inhalation. But now, the abdominal muscles, instead of remaining relaxed (as they do at rest), contract forcibly pulling the lower edges of the rib cage downwards (front and sides) (Fig. 8). This not only drastically decreases the size of the rib cage, but RETRIEVAL #4 = Breathing---atmospheric pressure is 50 kPa, a doubling of the volume of the lungs results in a pressure gradient of the only 25 kPa. In practice, because we breathe in a gentle, cyclical manner that generates pressure gradients of only 2–3 kPa, this has little effect on the actual rate of inflow into the lungs and is easily compensated for by breathing slightly deeper. The lower viscosity of air at altitude allows air to flow more easily and this also helps compensate for any loss of pressure gradient. All of the above effects of low atmospheric pressure on breathing are normally accommodated by increasing the respiratory minute volume (the volume of air breathed in – "or" out – per minute), and the mechanism for doing this is automatic. The exact increase required is determined by the respiratory gases homeostatic mechanism, which regulates the arterial and . This homeostatic mechanism prioritizes the regulation of the arterial over that of oxygen at sea level. That is to say, at sea level the arterial is maintained at very close to 5.3 kPa (or 40 mmHg) under a wide range of circumstances, at the expense of the arterial , which is allowed to vary within a very wide range of values, before eliciting a corrective ventilatory response. However, when the atmospheric pressure (and therefore the atmospheric ) falls to below 75% of its value at RETRIEVAL #5 = Lung---Lung The lungs are the primary organs of the respiratory system in humans and many other animals including a few fish and some snails. In mammals and most other vertebrates, two lungs are located near the backbone on either side of the heart. Their function in the respiratory system is to extract oxygen from the atmosphere and transfer it into the bloodstream, and to release carbon dioxide from the bloodstream into the atmosphere, in a process of gas exchange. Respiration is driven by different muscular systems in different species. Mammals, reptiles and birds use their different muscles to support and foster breathing. In early tetrapods, air was driven into the lungs by the pharyngeal muscles via buccal pumping, a mechanism still seen in amphibians. In humans, the main muscle of respiration that drives breathing is the diaphragm. The lungs also provide airflow that makes vocal sounds including human speech possible. Humans have two lungs, a right lung and a left lung. They are situated within the thoracic cavity of the chest. The right lung is bigger than the left, which shares space in the chest with the heart. The lungs together weigh approximately , and the right is heavier. The lungs are part of the lower respiratory tract that begins at the trachea and branches into the bronchi and bronchioles, and which receive air breathed in via the conducting zone. RETRIEVAL #6 = Hypoxemia---due to the effect of gravity. The highest concentration of blood in the pulmonary circulation occurs in the bases of the pulmonary tree compared to the highest pressure of gas in the apexes of the lungs. Alveoli may not be ventilated in shallow breathing. Shunting may also occur in disease states: BULLET::::- Acute lung injury and adult respiratory distress syndrome, which may cause alveolar collapse. This will increase the amount of physiological shunting, and unlike many forms of shunting, can be managed by administering 100% Oxygen. BULLET::::- Pathological shunts such as patent ductus arteriosus, patent foramen ovale, and atrial septal defects or ventricular septal defects. These states are when blood from the right side of the heart moves straight to the left side, without first passing through the lungs. This is known as a right-to-left shunt, which is often congenital in origin. Section::::Causes.:Exercise. Exercise-induced arterial hypoxemia occurs during exercise when a trained individual exhibits an arterial oxygen saturation below 93%. It occurs in fit, healthy individuals of varying ages and genders. Adaptations due to training include an increased cardiac output from cardiac hypertrophy, improved venous return, and metabolic vasodilation of muscles, and an increased VO Max. There must be a corresponding increase in V