25 th April Physiology #2 Pulmonary volumes & capacities Dr. Nayef AL-Gharaibeh KHALED MOUSA BACHA
We will start this lecture by explaining an important concept from the previous one: Intrapleural pressure is equal to -5 cmh 2 O at resting state, while during inspiration phase it will be around -8 cmh 2 O; so why there is an increase in this pressure negativity? There are two forces that affect the pleural space which are: 1- The chest expansion and diaphragm contraction which will pull the parietal layer "out and down" creating more negativity in the pleura (the red thick arrow). 2-Air filling force in the lungs that would push the visceral layer in the same direction which will decrease pleural suction (the blue thin arrow). If both forces have the same rate, there won't be a change in the pleural resting pressure, but the rate of chest expansion is more than air filling rate in the lungs which will validate the creation of more negativity in this cavity (e.g. the net pressure is equal to the negative pressure -suction forcecreated by air filling in the that is created by chest expansion minus the positive pressure that is lungs), Like if we close the glottis and try to expand the chest by doing forceful inspiration, the parietal layer will move while there is no air filling in the lungs which means that the visceral layer won't move, and that will create much more negativity in the pleura (-15 or -16 cmh 2 O). Now let's join this biophysical concept with another one: As this curve illustrates, intra-alveolar pressure starts from zero, then decreases to reach -1 cmh 2 O, after that it starts to increase in the middle of inspiratory phase to reach zero again, while intrapleural pressure keeps decreasing through the whole phase. Why? At the beginning of inspiratory phase -which is a suction force-, a negative pressure will be created due to inspiratory muscles contraction, and this negative pressure will appear at the curve (from zero to -1) but in the middle of inspiration there is another force which comes from the air filling of the alveoli which is positive pressure, now the airflow filling the alveoli force is more than the suction P a g e 1
force of the chest muscles, so the pressure will go up instead of keep decreasing, this is similar somehow to intrapleural pressure changes, but the pleural cavity is enclosed compartment which means that there is no air filling inside the pleura whereas the pushing force comes from the lungs!!! Now let's explain an another concept from this curve, the "Transpulmonary Pressure" Obviously it s the difference between the intra-alveolar pressure and the intraplural pressure. At the resting state the glottis is opened and before the inspiration - = 0 (-5) = 5 cmh 2 O At the middle of inspiration phase = (-1) (-6.5) = 5.5 cmh 2 O At the end of inspiration = 0 (-8) = 8 cmh 2 O When the transpulmonary pressure is equal to 5 cmh 2 O, the lungs are holding "x" volume of air, and as the transpulmonary pressure increases (from 5 to 5.5 then to 8 cmh 2 O), this indicates an increment in the air volume in the lungs "more than x" Because "transpulmonary pressure" definition: is the pressure which holds a certain volume of air in the lungs. The opposite scenario will happen during expiratory phase (from 8 to 5.5 then to 5 cmh 2 O), so the volume inside the lungs will decrease. And whenever the air volume in the lungs got fixed, the transpulmonary pressure won't change by its definition-. Pulmonary volumes and capacities This is another field of respiratory physiology and it is important for the evaluation of the respiratory functions. As the tests like ECG - which is done to evaluate cardiovascular functions-, blood pressure, temperature and chemicals concentration in the plasma. There is a "pulmonary function test; PFT" which we can explore the function of respiratory system through it. P a g e 2
The machine used in such test is called a spirometer, the classical one looks like the one in this picture, but we are going to use a high-tech, electrical and computerized one in the lab! How does this machine work? Spirometer consists of a chamber that is filled partially -at the edges- by water, and there is another chamber (floating drum) that is floating on the water on the top of the first chamber creating a cavity of air, at the base of it, there are two openings for tubes that the air will move through them (in & out), if the air increased in this cavity (by expiration) the floating drum will go up, while if it decreased (by inspiration) the drum will go down, and there is a connection to this drum with a pen that will move against a paper that is wrapped around rotating cylinder, this pen is going to draw lines on the paper according to the movement of the drum. Patient should close his nose and breathe from his mouth via the spirometer tube, during expiration, the volume of the air in the cavity will increase leading to raise the drum and drawing a line up, and the opposite thing will happen during inspiration, this lines will be proportional to the air volume inside the cavity, and because the cylinder is rotating, the lines will be drawn as waves making a "Spirograph" (while they would appear as overlapped lines if the cylinder was a static one) First of all, the lungs are floating in the chest with a certain volume of air that s always present in the lungs -the first breathe outside the uterus after the delivery is the hardest one in our entire life (excluding pathological situations) because it inflates the lungs, after that a fixed amount of air will remain in the lungs making the breathing process much easier!- Under a resting condition, we breathe normally and quietly because we do not need too much oxygen, while upon an exposure to any activity or stressful conditions we will start to breathe forcefully. Actually, we are able to breathe forcefully in a voluntary way, without a stress condition or doing exercises! And by that we can measure the different volumes inside the lung. TV = Tidal volume (500ml) IRV = Inspiratory reserve volume (3,000 ml) IC = Inspiratory capacity (3,500 ml) ERV = Expiratory reserve volume (1,000 ml) RV = Residual volume (1,200 ml) FRC = Functional residual capacity (2,200 ml) VC = Vital capacity (4,500 ml) TLC = Total lung capacity (5,700 ml) P a g e 3
Now let's talk about these different pulmonary volumes: Tidal Volume The volume of air that we inspire or expire during a resting state with a normal quiet breathing is called the Tidal Volume, we can approach it by asking the patient to breathe normally in the spirometer and that will be reflected as a small wave on Spirograph, the height of this wave represents the TV. The average value of TV is 0.5 L, and for females it's less than that! Inspiratory reserve volume Now, can we take a forceful inspiration after a normal one? Yes, because you are trying to do it now and it really works! actually that s done by sending signals by an action potential from the brain to all the muscles of inspiration (external intercostal muscles, diaphragm and accessory muscles) to contract, at first we inspire the half liter which is the TV, after that by increasing the contraction, chest expansion and the suction around the lungs, we will inspire much more than the TV, this inspired volume which entered after the TV is called the Inspiratory reserve volume. Its value is 3 L, which is a gift from god to take the amount of the oxygen that we need by increasing the volume of air in our lungs up to 3 liters on the top of TV upon demand! So physiologically, the human body is able to accommodate oxygen demand 6 times more than that required amount at resting state, which is a safety factor. Expiratory reserve volume We can approach it by doing a forceful expiration after we expel the TV by quiet expiration, and that is done by the contraction of internal intercostal muscles and abdominal recti. This extra amount of volume that's been expelled after the TV is called the Expiratory reserve volume and it reaches up to 1000 ml (1 L) it equals to 1100 milliliters according to Guyton which is our reference- Residual volume Which is the volume that remains in the lungs after the maximum ability for expiratory muscles contraction and reaching the point that we cannot expel more air; it equals to 1.2 L Humans lose this volume after a death or in the pathological situations like penetration of the lung layer or the chest layer which is called in medicine a "pneumothorax" (air inside the chest); like when a sharp object penetrates the chest cavity (pleural cavity) turning intrapleural pressure into zero, which means that there is no air inside the lung. P a g e 4
Now let's explain these volumes in physiological terms: Figure A represents the lungs after a forceful expiration (Minimal lung volume -residual volume- at maximum deflation). A The continuous line in Figure B represents the volume of air inside the lungs after a normal expiration (which is RV+ERV; 1000+1200 ml) it's called the resting condition volume, and during normal breathing the volume will start to go up and down between the continuous line and the dashed one (± TV) B Figure C illustrates the volume inside the lungs after a forceful inspiration, which is the maximum inflation of the lungs (RV; 1200 ml + ERV; 1000 ml + TV; 500 ml + IRV; 3000 ml = 5700 ml) Why is that important? Because sometimes we need more oxygen and we are able to take it from IRV, other times we need to expel more CO 2 from the lungs and we are able to get rid of more CO 2 through expelling the ERV from lungs; and by that we get rid of the "contaminated" air and get a fresh air as an ERV, TV and IRV leading to renewal of the air in our lungs. C Also having this amount of air in the lungs which is the "Functional residual capacity" (RV+ERV) gives us beauty to our chests; the chest won't be a good looking if it's fully collapsed after the expiration and fully inflated after the inspiration! And that meets the function also, since the blood is continuously flowing through the lungs, there should be an amount of air inside them to maintain the gas exchange function, so the blood can take up oxygen gas even during the expiratory phase -from ERV and RV-. To sum up: Residual volume: the minimum amount of air remaining in our lungs after the maximum forceful expiration. Expiratory reserve volume: the maximum amount of air a person can expire by forceful expiration after a normal expiration. Tidal volume: the amount of air a person inspires or expires under a resting condition. Inspiratory reserve volume: the maximum amount of air a person can inspire from the atmosphere by forceful inspiration after a normal inspiration. P a g e 5
Pulmonary Capacities A capacity is a value that results from the summation of two or more volumes; like: Inspiratory capacity Mathematically is TV + IRV (3.5 L) Physiologically, it is the maximum amount of air a person can inspire by forceful inspiration after a normal expiration. Functional residual capacity Equals to ERV + RV (2.2 L) And it is the maximum amount of air remaining inside the lungs after a normal expiration. Vital capacity Equals to IRV + TV +ERV (4.5 L) It is the maximum amount of air a person can expire forcefully after a forceful inspiration, or the maximum amount of air a person can inspire forcefully after a forceful expiration. And it's called a "vital" because it is vital for our life, since it is responsible for the renewal of the air in the lungs by increasing the oxygen and decreasing carbon dioxide contents. Total lung capacity The total value of all respiratory volumes; IRV + TV + ERV + RV (5.7 L) All these capacities are measurable by spirometry except the FRC and TLC since both of them contain the residual volume, which is unmeasurable by spirometer because it stays in the lungs and could not be expelled out or inspired! But there is a simple method called "Helium dilution method" to calculate FRC, RV and TLC. Helium dilution method Helium is a nontoxic gas to be inspired, so we can use it. A spirometer of known volume is filled with atmospheric air mixed with helium at a known concentration (this is called the initial concentration of helium; Ci He ). Before breathing from the spirometer, the person expires normally. At the end of this expiration, the remaining volume in the lungs is equal to the functional residual capacity. At this point, the subject immediately begins to breathe from the spirometer (with a closed nose), and the gases of the spirometer mix with the gases of the lungs. P a g e 6
As a result, the helium becomes diluted by the functional residual capacity gases and evenly distributed between FRC and the spirometer, and the volume of the functional residual capacity can be calculated from the degree of dilution of the helium, using the following formula: where FRC is functional residual capacity, Ci He is initial concentration of helium in the spirometer, Cf He is final concentration of helium in the spirometer, and Vi Spir is initial volume of the spirometer. FRC = ( CiHe CfHe 1) ViSpir Once FRC is determined, we can calculate the residual volume by this equation: RV = FRC ERV Also the total lung capacity could be calculated through this one: TLC = FRC + IC *Actually, we can predict the FRC without these calculations, like if we try this method on a child female, and an adult male, the final concentration of the helium in spirometer would be much less for the adult male because the FRC is much larger, and the opposite thing for the child female. Finally, the dead space It s a volume on top of all the previously discussed ones, that is cannot be measured by simple spirometry (it needs a special method to be measured by spirometer). It is defined as the amount of air located in the air conducting channels; trachea, bronchi and bronchioles, where no gas exchange can happen (so called "dead space"), because gas exchange can occur only in the respiratory spaces; respiratory bronchioles, ducts and alveoli. there are two types of the dead space: the anatomical dead space: is the volume of air located in the bronchial tree (150 ml) it can be measured by a simple method as following: as we know, atmospheric air contains an oxygen, nitrogen, and a small amount of carbon dioxode, and the lungs contains the same gases! P a g e 7
at first the object has to expire forceflly then take a deep breath from a pure oxygen, allowing this oxygen to fill the lungs and bronchial tree (pulmonary spaces as well as the dead space), after that he have to expel the air in the spirometer that is connected with a device to measure the nitrogrn concentration, the first portion will be a pure oxygen (which was in the bronchial tree dead space- since there is no gas exchange there), after that the next portion of air will contains carbon dioxide and nitrogen due to gas exchage in the pulmonary spaces and mixing of the pure oxygen with pre-existing gases in the lungs. and the nitrogen concentration in this portion will be detected by the device that records and plotts nitrogin concentration to make a curve, and by that we can calculate the dead space volume (the first portion which consists of pure oxygen). The physiological dead space: some area in the respiratory spaces which has a ventilation but doesn t have a blood flow, it's considerd as a dead space because it doesn't exhibit any gas exchange, this space doesn't exist in healthy people, it might exist in tall people or those who have low blood pressure at the upper parts of their lungs (just 10-15 ml), but it becomes a significant volume in abnormal pathological situations. THE END! Edited by: Cyrine katanani. Done by: Khaled Mousa Bacha. P a g e 8