1、9. Lungs and BreathingThis material is in Chapter 7 of the text.The primary function of the lungs is to supply the O2 required for the “combustion” of the bodys fuel and then to remove the resulting CO2. We will talk of the exchange process below. The lungs also act as a heat exchanger for the body

2、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 ti

3、mes 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 o

4、f 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 abou

5、t 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% O2 and 80% N2 while the exhaled air is 80% N2, 16% O2, and 4% CO2. The exhaled air is also saturated

6、 with H2O. We exhale about 0.5 kg (about 1 lb). of CO2 and about 0.5 kg of water each day. More on that later.With about liter/breath we take in about 1022 molecules of air per inhalation. With the total number of molecules in atmosphere at about 1044, we take in about 10-22 of the earths atmosphere

7、 each time we breathe. So for each liter (1022) 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

8、 His lifetime. Of course this calculation can be made for just about anyone who lived long ago enough for the 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 eve

9、ntually 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

10、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 nee

11、d 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

12、 millions of alveoli at the end of each air passage. These sacs are the interface between the air and the blood. The total surface area of these sacs is about 80 m2, about half a tennis court!Refer to Section 7.5 of the text.How the Blood and Air in the Lungs InteractThe lungs offer little resistanc

13、e to the flow of blood so the heart doesnt 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 bl

14、ood 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 b

15、lood 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 c

16、hunk of blood is only in the alveoli 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 80 m2, the thickness of the film of blood is about 1 mm, the thickness of a single

17、red blood 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 n

18、ecessary 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

19、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 embo

20、lism, 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 O2 and CO2 is a diffusion process. On the gas side of the interface, t

21、he 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 O2 and CO2 must diffuse in or out respectively through the

22、 predominately N2 gas. Indeed, with the 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 vol

23、ume 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 O2 and CO2 in the alveoli are not the same as normal air. In effect, about 30% old air is be

24、ing mixed with the newly inhaled air. So expelled air has a slightly higher O2 and lower CO2 percentages than are in the alveoli. The expelled air of course is lower in O2 and higher in CO2 then the atmosphere.The diffusion process across the membrane wall is driven by concentration differences of t

25、he molecules in question. The actual solubility of O2 in the blood is quite small. The O2 requirements could never be satisfied by this small amount of gas dissolved in the blood. However, the blood has some really efficient “blood carrying freight cars”. These carriers are molecules of hemoglobin t

26、hat 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 O2. This means that about 1 liter of blood can carry about 200 cm3 of O2 or about 1/5th of a liter of O2. Without the hemoglobin, the same one

27、 liter of blood could only carry about 2.5 cm3 of dissolved O2 at standard pressure and temperature.The name of the game is bring O2 to the cells where it can participate in the combustion process of food derivatives. The oxygen, from the oxygen-rich blood reaches the cells by dissociating from the

28、hemoglobin and diffusing into this intercellular fluid that baths both the capillaries and cells. CO2, that has diffused out of the cells where it was produced into the intercellular fluid. As CO2 diffuses into the blood, the CO2 stimulates the release of O2 from the hemoglobin. Thus the oxygen, sti

29、mulated by the CO2, is 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 CO2 levels high, a larger amount of oxygen is delivered. For non-working muscles, the fluid around the ce

30、lls is still quite rich in O2 since the demand for oxygen by 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

31、in the muscles is depleted and the circulating blood delivers more. The O2 depleted blood is returned to the lungs for a fresh load. The lower O2 concentrations in and around the cells that cause the delivery of more O2 for the working cells is also accompanied by a higher CO2 level which diffuses i

32、nto the blood where a lower concentration exists.This dissociation process of O2 from the hemoglobin depends on the local concentrations of CO2, the acidity and the temperature. These factors increase with muscle activity thereby increasing the delivery of O2 to the muscles. Like O2, CO2 is also tra

33、nsported by coupling itself to chemicals in the blood, and like the oxygen, a certain concentration of CO2 remains circulating with the blood. This CO2 serves to regulate breathing and therefore oxygen levels.Carbon monoxide, CO, when inhaled, attaches itself to the O2 attachment points of the hemog

34、lobin. In doing so, it dramatically reduces the bloods ability to carry oxygen and can therefore be fatal.Measurement of Lung VolumesWith 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 en

35、d 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 circumstance

36、s, 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 o

37、f 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 thats what it takes to expel something that should not be in your air passage.When measuring these various lung capacities, it is imp

38、ortant 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 acts on it. Recall Newtons 2nd Law, F = m a = m (change in velocity per unit time). Thus a v

39、olume of air will not move unless a force is acting on the volume. Consider the air in your windpipe as illustrated below.P1P2Cross sectional area AVolume of AirFrom the definition of pressure, force / area, the force acting on the left side pushing to the right is p1 A while the force on the right

40、side pushing to the left is p2 A. The 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

41、of a mass, whether its 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

42、 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.

43、 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 wo

44、uld 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 maintain

45、ing 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

46、” piston by providing a counter weight equal in weight to the 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 o

47、n 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

48、 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 “h

49、old” 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 bodys need for oxygen will allow. It is impossible for you to hold your breath to the point of doing any signific

50、ant 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

51、.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

52、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.

53、 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 ove

54、r 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 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 veloc

55、ity 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. Lets see how this works.Its pretty safe to say that the amount of air flowing into one end of a pipe is equal to the amount of air

56、 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 fast

57、er 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

58、 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 bloods velocity in regions of blood vessels that are constricted by plaque in the ves

59、sels.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 la