The lungs are the major organ of the respiratory system. They consist of small air sacs with thin walls that allow for efficient gas exchange between the blood and atmosphere.
Lung Function
The anatomy of the lungs allows for efficient gas exchange. Oxygen from the atmospheric air is transferred to the blood for circulation to the heart and tissues, while carbon dioxide, a waste product from cellular function, is released from the blood into the atmosphere. Oxygen is brought into the lungs during inhalation, and carbon dioxide is released through exhalation.
One inhalation and exhalation is a breathing cycle. They are also known as inspiration and expiration. Breathing is controlled by the nervous system and is involuntary under normal conditions. The pressures in and around the chest cavity play a major role in a person’s ability to breath and the passive exhalation response.
The Role of Pressure in Breathing
Three kinds of pressure are important for lung function:
- Atmospheric pressure is the pressure of the external environment, the pressure of the air around the mouth and/or nose. This pressure cannot be controlled by the body.
- Intrapulmonic pressure is the pressure within the lungs, which fluctuates below and above atmospheric pressure. As intrapulmonic pressure adjusts due to the contraction and relaxation of the respiratory muscles, which results in the expansion and relaxation of the chest cavity, air flows into and out of the respiratory system. Intrapulmonic pressure is also a determinant of lung capacity, the amount of air that can be taken into the lungs.
- Intrapleural pressure is the pressure within the potential pleural space and is always slightly below atmospheric pressure (negative pressure). The pleural space is actually not a space, it is the barrier between the membranes surrounding the lungs (parietal and visceral pleura) and only contains air when problems occur, such as traumatic injury or hemorrhage. If the intrapleural pressure increases, it compresses the lungs, restricts breathing, and can possibly cause asphyxiation (referred to as a collapsed lung).
Lung Volume
There are several lung volumes to consider:
- Tidal volume (TV) - amount of air involved in one normal breathing cycle. Average = 500 mL for adult, less for shallow breathing.
- Minute respiratory volume (MRV) - amount of air inhaled and exhaled in one minute. MRV = TV x number of breathing cycles in one minute. Average = 6 liters/minute.
- Inspiratory reserve - amount of air that can be taken in with the deepest inhalation, beyond the tidal volume. Average = 2 to 3 liters.
- Expiratory reserve - amount of air, beyond the tidal volume, that can be exhaled forcefully beyond the normal passive exhalation. Average = 1 to 1.5 liters.
- Vital capacity (VC) – amount of air that can be taken in and exhaled with the most forceful inhalation and exhalation. VC = TV + reserves. Average = 3.5 to 5 liters.
- Residual air – amount of air that remains in the lungs even after the most forceful exhalation. Average = 1 to 1.5 liters. The residual air in the lungs ensures efficient and consistent gas exchange.
Factors Determining Lung Capacity
The elasticity of the lung decreases as a person ages and can be affected by illness or obstruction, such as infection or emphysema. The loss of elasticity results in a lower lung capacity, and the respiration rate increases to compensate for the lower oxygen intake, and thus saturation of the blood, per inhalation.
The respiration rate may also increase under conditions of lower atmospheric oxygen concentration, referred to as thinner air, such as that found at higher altitudes. Also, any time oxygen demand is increased, such as with exercise, the breathing rate will increase as well.
Additional Information:
Scanlon and Sanders. Essentials of Anatomy and Physiology, 4th edition. Saunders.
Alicia Mae Prater - Alicia received her doctorate in Experimental Pathology in 2007. She has been a freelance writer and scientific editor since 2008.
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