Lungs and Breathing

By Alvin Chavez,2014-05-09 20:53
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Lungs and Breathing

    9. Lungs and Breathing

    This material is in Chapter 7 of the text.

    The primary function of the lungs is to supply the O required for the “combustion” 2

    of the body’s fuel and then to remove the resulting CO. We will talk of the exchange 2

    process below. The lungs also act as a heat exchanger for the body by warming and moisturizing the cooler and dryer air we normally inhale. Of course the lungs are also the source of the airflow we use to talk, cough, sneeze, whistle, etc. The lungs are normally

    under the control of involuntary breathing controls. Under that control, adult males

    breathe about 12 times per minute, while women about 20 and infants about 60 times/minute.

    As during sound production, breathing is often under our voluntary control. We inhale more and faster when speaking. While speaking, we spend about 80% of the breathing cycle exhaling and speaking. There is a substantial amount of work done by the lungs in moving air in and out while relatively little is actually converted to sound power. The normal voice produces about 1mW (1/1000 watt). There are many other times when breathing is clearly under voluntary control.

    For the average resting male, the inspiration volume is about ? liter, not to be

    confused with the actual volume of the lungs. With about 12 breaths per minute, the average male breathes about 6 liters of air per minute. The atmospheric air inhaled is

    about 20% O and 80% N while the exhaled air is 80% N, 16% O and 4% CO. The 2222,2

    exhaled air is also saturated with HO. We exhale about 0.5 kg (about 1 lb). of CO 22

    and about 0.5 kg of water each day. More on that later.

    22With about ? liter/breath we take in about 10 molecules of air per inhalation.

    44-22With the total number of molecules in atmosphere at about 10, we take in about 10 of

    22the earth’s atmosphere each time we breathe. So for each ? liter (10) we take in, there

    is on average at least one molecule of any ? liter of the atmosphere at any prior time. If you think about the 150 million breaths taken by Christ, we could expect that each of our breaths has about 150 molecules that Christ breathed in His lifetime. Of course this calculation can be made for just about anyone who lived long ago enough for the Page 77 Edited on 5/9/12 9:01 PM Printed 05/09/12 Dr. Mike Class Notes

    atmosphere to be thoroughly mixed. If you wanted to consider all the molecules that made up any individual that died a long while ago, you could make the case that all of their molecules eventually made their way into the atmosphere and we breathe molecules of any of these individuals also. In fact our own bodies all have some molecules of essentially anyone who lived a long time ago. Talk about recycling!!!

    Air enters the body normally through the noise and the nasal passages. The hairs

    in the noise first filter the air where relatively large dust and dirt are removed. The air is moisturized and warmed before entering the trachea (the windpipe). It is

    therefore a very good idea to breathe through your nose if you really have a choice. We tend to inhale through our mouth when we need a lot of air in a hurry as during strenuous exercise. While the air can enter faster, more irritants can get into the airways as well as the airways drying somewhat by the rapidly moving, relatively dry air that has bypassed the nose and sinus cavities.

    The real business of the lungs occurs in the millions of alveoli at the end of each

    air passage. These sacs are the interface between the air and the blood. The total

    2, about half a tennis court! surface area of these sacs is about 80 m

    Refer to Section 7.5 of the text.

    How the Blood and Air in the Lungs Interact

    The lungs offer little resistance to the flow of blood so the heart doesn’t need

    to develop much pressure to push the blood into and through the lungs. Thus the right side of the heart, that is responsible for pumping blood through the nearby lungs, is smaller and less powerful than the left side that has the huge job of pumping blood through the rest of the body. So while the pressure from the left side is of the order of 120 mm of Hg. the pressure from the right side going to the lungs is of the order of 20 mm Hg or about 1/5 as large.

    A very large amount of blood is in the lungs at any given moment, about 1/5 of the total blood supply. However, while a liter or so might be in the lungs, less than 10% of that, only about 70 ml, is in the capillaries where the gas exchange is taking

    place. With so little of the blood in these capillaries, the gas exchange must take place very rapidly, in fact, on average, a particular chunk of blood is only in the alveoli

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about 1 sec. In order to affect the rapid transfer, the walls of alveoli must be and are

    extremely thin. With only 70 ml of blood spread over the total area of the alveoli area of

    2, the thickness of the film of blood is about 1 m, the thickness of a single red blood 80 m

    cell. What this means is that every red blood cell in the capillaries of the alveoli is in contact with the walls of a capillary in the alveoli. The very short distance over which

    gas molecules must pass from the airside to the blood cell contributes to the very short time over which the necessary gas exchange takes place.

    Two processes are necessary to for the gas exchange. First, there must be a

    good blood supply, a process called perfusion. The second is getting a good airflow

    in and out, a process called ventilation. Fortunately about 90% of the lungs, the alveoli, have both good perfusion and good ventilation. The remaining 10% are areas that only have one of the two necessary processes. As far as the proper functioning of the lungs is concerned, an obstruction in the blood supply or an obstruction in the air supply can cause serious problems. A blood clot, a pulmonary embolism, can block or reduce the blood supply and fluid in the lungs, pneumonia, smoking residue, etc. can reduce the air supply. In short, the blood and the air really have to get very close and do it quickly.

    The actual transfer of O and CO is a diffusion process. On the gas side of the 22

    interface, the capillary wall in the alveoli, the gases must diffuse a small fraction of a

    mm as they cross the air sack of the alveoli. Since diffusion depends upon the speed

    and the mean-free-path of the particles through which they are diffusing, the O and CO 22

    must diffuse in or out respectively through the predominately N gas. Indeed, with the 2

    very small distance to travel, the passage can be made easily during the average time stay of blood in the alveoli. As far as diffusing through the ultra thin capillary wall,

    that takes far less time.

    Under normal breathing, only a fraction of the total air volume in the lungs is exhaled or inhaled in any single breath. Under normal breathing, we expel about 60% of

    the air that was in the lungs and then inhale a similar volume of fresh air. Thus the percentages of O and CO in the alveoli are not the same as normal air. In effect, about 22

    30% old air is being mixed with the newly inhaled air. So expelled air has a slightly

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     and lower CO percentages than are in the alveoli. The expelled air of higher O22

    course is lower in O and higher in CO then the atmosphere. 22

    The diffusion process across the membrane wall is driven by concentration differences of the molecules in question. The actual solubility of O in the blood is 2

    quite small. The O requirements could never be satisfied by this small amount of 2

    gas dissolved in the blood. However, the blood has some really efficient “blood

    carrying freight cars”. These carriers are molecules of hemoglobin that chemically

    combine with oxygen. In fact each red blood cell, which is passing single file through the capillaries, can carry about 1 million molecules of O. This means that about 1 liter of 2

    3thblood can carry about 200 cm of O or about 1/5 of a liter of O. Without the 22

    3hemoglobin, the same one liter of blood could only carry about 2.5 cm of dissolved O at 2

    standard pressure and temperature.

    The name of the game is bring O to the cells where it can participate in the 2

    combustion process of food derivatives. The oxygen, from the oxygen-rich blood reaches the cells by dissociating from the hemoglobin and diffusing into this intercellular fluid that baths both the capillaries and cells. CO, that has diffused out of the cells where it was 2

    produced into the intercellular fluid. As CO diffuses into the blood, the CO stimulates 22

    the release of O from the hemoglobin. Thus the oxygen, stimulated by the CO, is 22

    dumped off where it is needed. The dissociation and diffusion processes depend upon the gas concentrations in and around the cells. When the oxygen levels are very low

    and CO levels high, a larger amount of oxygen is delivered. For non-working 2

    muscles, the fluid around the cells is still quite rich in O since the demand for oxygen by 2

    the cells is low and thus a relatively small amount of oxygen is delivered. The remaining portion continues to circulate and a lesser amount is added into the blood stream by the lungs. When the muscles are active, the local oxygen supply in the muscles is depleted and the circulating blood delivers more. The O depleted blood is returned to the lungs 2

    for a fresh load. The lower O concentrations in and around the cells that cause the 2

    delivery of more O for the working cells is also accompanied by a higher CO level which 22

    diffuses into the blood where a lower concentration exists.

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     from the hemoglobin depends on the local This dissociation process of O2

    concentrations of CO, the acidity and the temperature. These factors increase with 2

    muscle activity thereby increasing the delivery of O to the muscles. Like O, CO is also 222

    transported by coupling itself to chemicals in the blood, and like the oxygen, a certain concentration of CO remains circulating with the blood. This CO serves to regulate 22

    breathing and therefore oxygen levels.

    Carbon monoxide, CO, when inhaled, attaches itself to the O attachment points of 2

    the hemoglobin. In doing so, it dramatically reduces the blood’s ability to carry oxygen and can therefore be fatal.

    Measurement of Lung Volumes

    With normal breathing, the lungs inspire and expire about ? liter of air, the so-

    called tidal volume. Thus there is always about a 2-liter reserve at the beginning or end of a normal breathing cycle. As we become more active, the inspiration and expiration

    volume increases to about a liter, leaving about 1 liter in reserve, an amount that will

    remain even with a maximum effort to expel as much as we possibly can, called the vital

    capacity. Under normal circumstances, the amount of air breathed in 1 minute is called the respiratory minute volume. The maximum volume breathed in 15 seconds is the maximum voluntary ventilation. A normal person can expire about 70% of the vital capacity in ? sec. increasing to almost all of it, 97%, in about 3 seconds.

    The velocity of the expelled air can vary over a very wide range. The velocity

    of a hard sneeze or cough can cause a velocity of the expelled air near the speed of sound! Sometimes that’s what it takes to expel something that should not be in your air passage.

    When measuring these various lung capacities, it is important to provide a system that captures the air expelled without creating a pressure that would stop the airflow. A volume of air like any other mass will not accelerate (change its velocity) unless a force

    ndacts on it. Recall Newton’s 2 Law, F = m a = m (change in velocity per unit time). Thus a volume of air will not move unless a force is acting on the volume. Consider the air in your windpipe as illustrated below.

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    PP1 2 Cross sectional area A

    Volume of Air

    From the definition of pressure, force / area, the force acting on the left side

     A while the force on the right side pushing to the left is p A. The pushing to the right is p12

    net force acting on the volume of air is the difference between these two forces. Thus the air will move from the higher pressure side towards the lower pressure side. Not surprising but we should always keep in mind that it takes a force to change the velocity of a mass, whether it’s stopping a skull in a collision or accelerating air into or out of the lungs.

    Occasionally someone may suggest that we can measure the volume of the lungs by inflating a balloon and then measuring the volume of the balloon. This is not true. Recall our demonstration of how much pressure you could muster by attempting to exhale into a tube that was attached to a vertical water column. Once the water rose to a height of about four feet, the pressure created at the base of the water column by the weight of the water column equaled the maximum pressure of the person exhaling. At that point, no further exhaling was possible regardless of how much air was moved. In fact, with the size of the tube used, a very small volume of air was actually expelled before the maximum pressure point was reached. So if you tried to inflate a balloon as measure of your lungs volume, you would only be measuring the volume of air required in that particular balloon to cause a pressure sufficient to stop the flow of air from your lungs. What is required to measure the lung capacities during breathing cycles is a device that will capture and measure the volume of air moved while maintaining a zero

    gauge pressure in the measuring vessel. Recall a zero gauge pressure means that

    the pressure in the vessel is at atmospheric pressure. Thus either a positive or negative gauge pressure in the lungs is the only issue in driving the air out or in respectively. We can make such a “weightless” piston by providing a counter weight equal in weight to the Page 82 Edited on 5/9/12 9:01 PM Printed 05/09/12 Dr. Mike Class Notes

    piston itself. The counter weight is connected to piston by a rope and pulley system so the piston has an effective zero weight as the volume of air below is changed. The piston must be weightless so that it does not create any pressure on the volume of gas. If we attach a movable pen to the piston so we can record its motion we will have a measure of the volume below the piston. This device is called a spirometer. See figure 7.7 in the text.

    We inhale and exhale by changing the air pressure within our lungs. The

    control of the lungs is usually on “auto-pilot” and takes place without our having to think

    about it. We can over-ride this automatic control and of course we do that all the time

    for a variety of reasons. In order to talk or make other sounds we control the breathing to make the airflow in an appropriate manner. We “hold” our breath to swim underwater. Of course we can only override the autopilot as long as there is no higher priority for automatic control.

    You can only hold your breath as long as the body’s need for oxygen will allow. It is impossible for you to hold your breath to the point of doing any significant damage to yourself. You might perhaps momentarily faint but as soon as you did the autopilot would start breathing and restore you. You probably all have see children try to get attention by attempting this with the result that a parent (who has taken this course) will let nature take its course.

    Another example of the autopilot taking control is the situation of a particle of food “tickling” your throat because of ingestion into the windpipe. The cough reflex will take

    control to expel the potentially dangerous intruder. In a case like this, your lungs can

    generate airflow velocities that are extremely high, a requirement to dislodge and eject the offender. This high velocity is not only generated by a sudden increase in the pressure in the lungs but by a constriction of the air passage. The constriction is caused a decrease in pressure inside the tube because of the higher velocity. That decrease in cross-section of the tube further increases the velocity of the air being forced through.

    The decrease in pressure resulting from a higher velocity of a fluid is called the Bernoulli principle. This is the same principle that allows planes to fly. The velocity of the air flowing over the top of a wing is higher than the air flowing underneath due to the shape of the wing. The higher velocity over the top causes a lower pressure than the Page 83 Edited on 5/9/12 9:01 PM Printed 05/09/12 Dr. Mike Class Notes

    slower air moving under so there is a net upward force that supports the weight of the plane. In the air passage, a significantly higher velocity inside can cause the partial collapse of the somewhat flexible air passage and thus reduce the cross-section that in turn increases the velocity through the passage. Let’s see how this works.

    It’s pretty safe to say that the amount of air flowing into one end of a pipe is

    equal to the amount of air flowing out the other end or in fact past any cross-section of the tube. Thus if a fluid is flowing through a pipe under the influence of a pressure difference at each end, the flow through any cross-section of the pipe will be inversely proportional to the cross-sectional area. So the flow is faster through the narrower portions of the pipe. So a good cough, driven by a sudden, large pressure increase in the lungs, can create a very high air velocity, particularly in a region where a particle might already be constricting the cross-section of the tube. An example of this relationship between the velocity and the cross-section is when you place your thumb over the end of a garden hose to increase the velocity of the exiting water. A more germane example that will be discussed later is the change in the blood’s velocity in regions of blood vessels that are constricted by plaque in the vessels.

    The breathing process In the normal breathing process, a very small pressure

    difference between the lungs and the outside are required to move air in and out, a pressure difference of perhaps 200 Pa (an inch or so of water as might be measured by that water column we had in class). Recall that atmospheric pressure is about 100 KILO Pascals, i.e. about 500 times larger. Atmospheric pressure can push water up about 33 feet if the pressure at the top of the tube is reduced to zero.

    It is interesting to see how one might measure the pressure in the lungs. Recall how blood pressure is measured using Pascal’s principle inside the closed volume

    created by the blood pressure cuff around an arm making the pressure in the cuff equal to the pressure in the vessel. For the closed chest cavity, the pressure in the lungs is equal to the pressure elsewhere in the cavity. That pressure will tend to collapse the rather flexible although muscular tube (the esophagus) passing through it. This tube is normally closed at both ends and therefore becomes a convenient place to measure the pressure in the chest cavity.

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    The largest portion of the lung volume is bounded by the alveoli that are as

    we have seen, extremely thin and elastic air sacks. They are like tiny balloons at the end of each air passageway. Like any inflated balloon, they would deflate if they had the opportunity to do so. The outer boundaries of the lungs are in contact with the inner chest wall that by virtue of being held more or less in place, keep the lungs from deflating. If however the space between the lungs and the chest wall were allowed to be open to the atmosphere say by a puncture of the chest cavity, the lungs would indeed deflate by virtue of the elasticity of all those air sacks. At the same time, the chest cavity would expand as the surrounding muscles relaxed and air filled the space between the lungs and chest cavity.

    Under normal circumstances, the muscles around the chest cavity are maintaining an inward pressure like a spring. The lungs themselves are being “held” inflated by the inability of the sealed chest cavity to collapse. So the breathing is accomplished by changing the volume of the chest cavity by movement of the lower boundary, the diaphragm, and the expansion the chest walls themselves and the lungs. Refer to figure 7.20 page 171 for a somewhat more graphical description of the process. In order to

    exhale, we relax the muscles in the diaphragm so it can bulge upward while the elasticity of the lungs partially deflates the lungs as if it were a balloon. To inhale, the muscles of the diaphragm contract, causing it to flatten thereby increasing the volume of the chest cavity, creating the required negative pressure for inhalation.

    Further we can contract “intercostal muscles in the chest wall that cause the chest walls to expand. Inhalation can therefore take place with either the muscles of the diaphragm or chest walls or both.

    The maximum inspiration and expiration pressures are roughly + or 100 cm. of

    water. That + or one meter is just about what we measured in our simple classroom demonstration! I guess sometimes those demos really work!

    Ref: Pages 160 -161

    The alveoli, with their incredibly thin, “wet” walls act more like soap bubbles than balloons. The elasticity we have been speaking about results from the surface tension of the fluid like membrane. The surface tension of a soap bubble is in fact the real force that Page 85 Edited on 5/9/12 9:01 PM Printed 05/09/12 Dr. Mike Class Notes

    keeps the bubble in tact. It is the force that allows bugs to literally walk on water! If you were to stick a straw in some soapy water and blow a bubble at the end of the straw without allowing it to fly off as a closed volume, the bubble at the end of the straw would collapse if given the opportunity to do so. Thus if you allowed air to escape from the straw, the bubble would just collapse under the surface tension force that is acting to minimize the amount of surface area. The surface tension in its attempt to minimize the surface area is what causes soap bubbles to assume a spherical shape whenever possible. The alveoli are therefore like little soap bubbles at the end of a tube. They

    are trying to collapse but the negative pressure created by the chest cavity keep them from doing so. Actually, the alveoli are coated with a substance called a surfactant

    whose surface tension is not constant. As a result, the alveoli will collapse but as they do, the surface tension decreases to the point that the residual air pressure becomes equal to the pressure created by the lower surface tension. The alveoli therefore collapse to about ? of their original size. Normal soap bubbles have a

    surface tension that is more or less constant with volume and therefore given the chance, will collapse to essentially zero volume. For a bubble, the higher the surface tension the faster it will collapse. Bubbles expelled from lung tissues have been observed to last literally for hours yielding the conclusion that these tiny bubbles must have a very low surface tension.

    Another aspect of the negative pressure in the chest cavity is the help it provides to keep open the major blood vessel.veins, returning blood to the heart, the vena cava.

    The returning blood pressure is only about 50 Pa, so the negative pressure of the chest cavity makes it easier for this low pressure to keep that vein inflated and thereby allow blood to return to the heart more easily.

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