Anatomical Dead Space and Functional Residual Capacity
Anatomical dead space and functional residual capacity (FRC) play a very important role to ensure the constancy of alveolar gas tensions. Firstly it is important to understand what the anatomical dead space and FRC actually are. The anatomical dead space refers to the gas in the conducting areas of the respiratory system where air does not come into contact with alveoli. Examples of places in the respiratory system where anatomical dead space is present are the mouth and trachea. The functional residual capacity is the amount of air that remains in the lungs after a normal and passive expiration. It is usually measured at approximately 2.5 litres of air.
Both the FRC and the anatomical dead space are essential in maintaining the constancy of alveolar gas tensions. To highlight there importance, it would be very useful to imagine the situation within the respiratory tract if they were not present. The consequence of these two factors not being present means that the inspired air would be equal to the normal tidal volume and the lungs would completely empty on expiration. This would mean that the air breathed in would equilibrate itself with the blood flowing in the lungs. Hence the pCO2 and pO2 levels in the blood flowing out of the lungs would have equilibrated itself with the gas tensions in the inspired air. Then when expiration takes place, no gas is present in the lungs as it has completely emptied itself. The effects of this are that on inspiration, diffusion would take place in between the inspired air and the blood. Due to this the blood pO2 levels would become atmospheric. This means the blood would have 20 Kpa of O2 and 0 Kpa of Co2 as the atmospheric air has a negligible concentration of CO2. As we have such a low concentration of CO2 in the lungs, there is a shift in the carbonic acid equilibrium. Due to this there is less carbonic acid produced as there is no CO2 and the blood becomes more alkaline. This is known as alkalosis. On expiration, as there
Both the FRC and the anatomical dead space are essential in maintaining the constancy of alveolar gas tensions. To highlight there importance, it would be very useful to imagine the situation within the respiratory tract if they were not present. The consequence of these two factors not being present means that the inspired air would be equal to the normal tidal volume and the lungs would completely empty on expiration. This would mean that the air breathed in would equilibrate itself with the blood flowing in the lungs. Hence the pCO2 and pO2 levels in the blood flowing out of the lungs would have equilibrated itself with the gas tensions in the inspired air. Then when expiration takes place, no gas is present in the lungs as it has completely emptied itself. The effects of this are that on inspiration, diffusion would take place in between the inspired air and the blood. Due to this the blood pO2 levels would become atmospheric. This means the blood would have 20 Kpa of O2 and 0 Kpa of Co2 as the atmospheric air has a negligible concentration of CO2. As we have such a low concentration of CO2 in the lungs, there is a shift in the carbonic acid equilibrium. Due to this there is less carbonic acid produced as there is no CO2 and the blood becomes more alkaline. This is known as alkalosis. On expiration, as there
Bibliography: ‘Medical Physiology’ by Walter F. Boron, Emile L. Boulpaep ‘Human Physiology’ the basics of medicine. 2nd edition by Pocock G, Richards C. ‘Respiratory Physiology - The Essentials’. 7th edition by West J. http://medical-dictionary.thefreedictionary.com/anatomical+dead+space