Exchange of Gases in Alveoli and Tissues

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We have now completed our discussion of the lung mechanics that produce alveolar ventilation, but this is only the first step in the respiratory process. Oxygen must move across the alveolar membranes into the pulmonary capillaries, be transported by the blood to the tissues, leave the tissue capillaries and enter the extracellular fluid, and finally cross plasma membranes to gain entry into cells. Carbon dioxide must follow a similar path in reverse.

In the steady state, the volume of oxygen that leaves the tissue capillaries and is consumed by the body cells per unit time is exactly equal to the volume of oxygen added to the blood in the lungs during the same time period. Similarly, in the steady state, the rate at which carbon dioxide is produced by the body cells and enters the systemic blood is identical to the rate at which carbon dioxide leaves the blood in the lungs and is expired. Note that these statements apply to the steady state; transiently, oxygen utilization in the tissues can differ from oxygen uptake in the lungs and carbon dioxide production can differ from elimination in the lungs, but within a short time these imbalances automatically produce changes in diffusion gradients in the lungs and tissues that reestablish steady-state balances.

The amounts of oxygen consumed by cells and carbon dioxide produced, however, are not necessarily identical to each other. The balance depends primarily upon which nutrients are being used for energy. The ratio of CO2 produced/O2 consumed is known as the respiratory quotient (RQ). On a mixed diet, the RQ is approximately 0.8; that is, 8 molecules of CO2 are produced for every 10 molecules of O2 consumed. (The RQ is 1 for carbohydrate, 0.7 for fat, and 0.8 for protein.)

For purposes of illustration, Figure 15-18 presents typical exchange values during 1 min for a person at rest, assuming a cellular oxygen consumption of 250 ml/min, a carbon dioxide production of 200 ml/min, an alveolar ventilation (supply of fresh air to the alveoli) of 4000 ml/min, and a cardiac output of 5000 ml/min.

Since only 21 percent of the atmospheric air is oxygen, the total oxygen entering the alveoli per min in our illustration is 21 percent of 4000 ml, or 840 ml/min. Of this inspired oxygen, 250 ml crosses the alveoli into the pulmonary capillaries, and the rest is subsequently exhaled. Note that blood entering the lungs already contains a large quantity of oxygen, to which the new 250 ml is added. The blood then flows from the lungs to the left heart and is pumped by the left ventricle through the tissue capillaries, where 250 ml of oxygen leaves the blood to be taken up and utilized by cells. Thus, the quantities of oxygen added to the blood in the lungs and removed in the tissues are identical.

The story reads in reverse for carbon dioxide: There is already a good deal of carbon dioxide in systemic arterial blood; to this is added an additional 200 ml, the amount produced by the cells, as blood flows through tissue capillaries. This 200 ml leaves the blood as blood flows through the lungs, and is expired.

Blood pumped by the heart carries oxygen and carbon dioxide between the lungs and tissues by bulk flow, but diffusion is responsible for the net movement of these molecules between the alveoli and blood, and between the blood and the cells of the body. Understanding the mechanisms involved in these diffusional exchanges depends upon some basic chemical and physical properties of gases, which we will now discuss.

Partial Pressures of Gases

Gas molecules undergo continuous random motion. These rapidly moving molecules exert a pressure, the magnitude of which is increased by anything that increases the rate of movement. The pressure a gas exerts is proportional to (1) the temperature (because heat increases the speed at which molecules move) and (2) the concentration of the gas—that is, the number of molecules per unit volume.

As stated by Dalton's law, in a mixture of gases, the pressure exerted by each gas is independent of the pressure exerted by the others. This is because gas molecules are normally so far apart that they do not interfere with each other. Since each gas in a mixture behaves as though no other gases are present, the total pressure of the mixture is simply the sum of the individual pressures. These individual pressures, termed partial pressures, are denoted by a P in front of the symbol for the gas. For example, the partial pressure of oxygen is represented by PO2. The partial pressure of a gas is directly proportional to its concentration. Net diffusion of a gas will occur from a region where its partial pressure is high to a region where it is low.

Atmospheric air consists primarily of nitrogen (approximately 79 percent) and oxygen (approximately 21 percent), with very small quantities of water vapor, carbon dioxide, and inert gases. The sum of the partial pressures of all these gases is termed atmospheric pressure, or barometric pressure. It varies in different parts of the world as a result of differences in altitude (it also varies with local weather conditions), but at sea level it is 760 mmHg. Since the partial pressure of any gas in a mixture is the fractional concentration of that gas times the total pressure of all the gases, the PO2 of atmospheric air is 0.21 X 760 mmHg = 160 mmHg at sea level.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

III. Coordinated Body Functions

15. Respiration

© The McGraw-Hill Companies, 2001

Respiration CHAPTER FIFTEEN

Respiration CHAPTER FIFTEEN

Alveoli

840 ml/min 590 ml/min o2^ n o2

Alveoli

840 ml/min 590 ml/min o2^ n o2

Alveolar ventilation = 4 L/min

Lung capillaries

Alveolar ventilation = 4 L/min

Lung capillaries

Right heart

Right heart

Left heart

Tissue capillaries

Left heart

Tissue capillaries

750 ml O2

1000 ml O2

250 ml O2

Cells

200 ml/min CO,

200 ml/min CO,

2800 ml CO2l ^>2600 ml CO2 Lung capillaries

Cardiac output = 5 L/min

Right heart

2800 ml CO2l ^>2600 ml CO2 Lung capillaries

Right heart

V Left heart

Tissue capillaries

V Left heart

Tissue capillaries

2800 ml CO,

2600 ml CO,

200 ml CO2

Cells

FIGURE 15-18

Summary of typical oxygen and carbon dioxide exchanges between atmosphere, lungs, blood, and tissues during 1 min in a resting individual. Note that the values given in this figure for oxygen and carbon dioxide in blood are not the values per liter of blood but rather the amounts transported per minute in the cardiac output (5 L in this example). The volume of oxygen in 1 L of arterial blood is 200 ml O2/L of blood—that is, 1000 ml O2/5 L of blood.

Diffusion of Gases in Liquids When a liquid is exposed to air containing a particular gas, molecules of the gas will enter the liquid and dissolve in it. Henry's law states that the amount of gas dissolved will be directly proportional to the partial pressure of the gas with which the liquid is in equilibrium. A corollary is that, at equilibrium, the partial pressures of the gas molecules in the liquid and gaseous phases must be identical. Suppose, for example, that a closed container contains both water and gaseous oxygen. Oxygen molecules from the gas phase constantly bombard the surface of the water, some entering the water and dissolving. Since the number of molecules striking the surface is directly proportional to the PO of the gas phase, the number of molecules entering the water and dissolving in it is also directly proportional to the PO2. As long as the PO2 in the gas phase is higher than the PO2 in the liquid, there will be a net diffusion of oxygen into the liquid. Diffusion equilibrium will be reached only when the PO2 in the liquid is equal to the PO2 in the gas phase, and there will be no further net diffusion between the two phases.

Conversely, if a liquid containing a dissolved gas at high partial pressure is exposed to a lower partial pressure of that same gas in a gas phase, there will be a net diffusion of gas molecules out of the liquid into the gas phase until the partial pressures in the two phases become equal.

The exchanges between gas and liquid phases described in the last two paragraphs are precisely the phenomena occurring in the lungs between alveolar air and pulmonary capillary blood. In addition, within a liquid, dissolved gas molecules also diffuse from a region of higher partial pressure to a region of lower partial pressure, an effect that underlies the exchange of gases between cells, extracellular fluid, and capillary blood throughout the body.

Why must the diffusion of gases into or within liquids be presented in terms of partial pressures rather than "concentrations," the values used to deal with the diffusion of all other solutes? The reason is that the concentration of a gas in a liquid is proportional not only to the partial pressure of the gas but also to the solubility of the gas in the liquid; the more soluble the

PART THREE Coordinated Body Functions

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART THREE Coordinated Body Functions

Pulmonary arteries

Systemic veins

Pulmonary arteries

Systemic veins

Gas Exchange Body

Alveoli

Alveoli

Alv Ole Po2 105 Mmhg

Systemic arteries

PO2 < 40 mmHg (mitochondrial PO2 < 5 mmHg) Pco2 > 46 mmHg

FIGURE 15-19

Partial pressures of carbon dioxide and oxygen in inspired air at sea level and various places in the body. The reason that the alveolar PO2 and pulmonary vein PO2 are not exactly the same is described later in the text. Note also that the PO2 in the systemic arteries is shown as identical to that in the pulmonary veins; for reasons involving the anatomy of the blood flow to the lungs, the systemic arterial value is actually slightly less, but we have ignored this for the sake of clarity.

Pulmonary veins

Systemic arteries

PO2 < 40 mmHg (mitochondrial PO2 < 5 mmHg) Pco2 > 46 mmHg

FIGURE 15-19

Partial pressures of carbon dioxide and oxygen in inspired air at sea level and various places in the body. The reason that the alveolar PO2 and pulmonary vein PO2 are not exactly the same is described later in the text. Note also that the PO2 in the systemic arteries is shown as identical to that in the pulmonary veins; for reasons involving the anatomy of the blood flow to the lungs, the systemic arterial value is actually slightly less, but we have ignored this for the sake of clarity.

gas, the greater will be its concentration at any given partial pressure. Thus, if a liquid is exposed to two different gases having the same partial pressures, at equilibrium the partial pressures of the two gases will be identical in the liquid but the concentrations of the gases in the liquid will differ, depending upon their solubilities in that liquid.

With these basic gas properties as the foundation, we can now discuss the diffusion of oxygen and carbon dioxide across alveolar and capillary walls, and plasma membranes. The partial pressures of these gases in air and in various sites of the body are given in Figure 15-19 for a resting person at sea level. We start our discussion with the alveolar gas pressures because their values set those of systemic arterial blood. This fact cannot be emphasized too strongly: The alveolar PO2 and PCO2 determine the systemic arterial PO2 and Pco2.

Alveolar Gas Pressures

Normal alveolar gas pressures are PO2 = 105 mmHg and PCO2 = 40 mmHg. (We do not deal with nitrogen, even though it is the most abundant gas in the alve oli, because nitrogen is biologically inert under normal conditions and does not undergo any net exchange in the alveoli.) Compare these values with the gas pressures in the air being breathed: PO2 = 160 mmHg and PCo2 = 0.3 mmHg, a value so low that we will simply assume it to be zero. The alveolar PO2 is lower than atmospheric PO2 because some of the oxygen in the air entering the alveoli leaves them to enter the pulmonary capillaries. Alveolar PCO2 is higher than atmospheric PCO2 because carbon dioxide enters the alveoli from the pulmonary capillaries.

The factors that determine the precise value of alveolar PO2 are (1) the PO2 of atmospheric air, (2) the rate of alveolar ventilation, and (3) the rate of total-body oxygen consumption. Although there are equations for calculating the alveolar gas pressures from these variables, we will describe the interactions in a qualitative manner (Table 15-5). To start, we will assume that only one of the factors changes at a time.

First, a decrease in the PO2 of the inspired air (at high altitude, for example) will decrease alveolar PO2. A decrease in alveolar ventilation will do the same thing (Figure 15-20) since less fresh air is entering the

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Respiration CHAPTER FIFTEEN

Respiration CHAPTER FIFTEEN

TABLE 15-5 Effects of Various Conditions on Alveolar Gas Pressures

Condition

Alveolar PO2

Alveolar PCO2

Breathing air with low POj

Decreases

No change*

ÎAlveolar ventilation and unchanged metabolism

Increases

Decreases

¿Alveolar ventilation and unchanged metabolism

Decreases

Increases

ÎMetabolism and unchanged alveolar ventilation

Decreases

Increases

¿Metabolism and unchanged alveolar ventilation

Increases

Decreases

Proportional increases in metabolism and alveolar ventilation

No change

No change

*Breathing air with low PO2 has no direct effect on alveolar PCO2. However, as described later in the text, people in this situation will reflexly increase their ventilation, and that will lower PCO2.

*Breathing air with low PO2 has no direct effect on alveolar PCO2. However, as described later in the text, people in this situation will reflexly increase their ventilation, and that will lower PCO2.

ra 50

FIGURE 15-20

Effects of increasing or decreasing alveolar ventilation on alveolar partial pressures in a person having a constant metabolic rate (cellular oxygen consumption and carbon dioxide production). Note that alveolar PO2 approaches zero when alveolar ventilation is about 1 L/min. At this point all the oxygen entering the alveoli crosses into the blood, leaving virtually no oxygen in the alveoli.

alveoli per unit time. Finally, an increase in the cells' consumption of oxygen will also lower alveolar PO2 because a larger fraction of the oxygen in the entering fresh air will leave the alveoli to enter the blood and be used by the tissues (recall that in the steady state, the volume of oxygen entering the blood in the lungs per unit time is always equal to the volume utilized by the tissues). This discussion has been in terms of things that lower alveolar PO2; just reverse the direction of change of the three factors to see how to increase PO2.

The story for PCO2 is analogous, again assuming that only one factor changes at a time. There is normally essentially no carbon dioxide in inspired air and so we can ignore that factor. A decreased alveolar ventilation will increase the alveolar PCO2 (Figure 15-20) because there is less inspired fresh air to dilute the carbon dioxide entering the alveoli from the blood. An increased production of carbon dioxide will also increase the alveolar PCO2 because more carbon dioxide will be entering the alveoli from the blood per unit time (recall that in the steady state the volume of carbon dioxide entering the alveoli per unit time is always equal to the volume produced by the tissues). Just reverse the direction of changes in this paragraph to cause a decrease in alveolar PCO2.

For simplicity we allowed only one factor to change at a time, but if more than one factor changes, the effects will either add to or subtract from each other. For example, if oxygen consumption and alveolar ventilation both increase at the same time, their opposing effects on alveolar PO2 will tend to cancel each other out.

This last example emphasizes that, at any particular atmospheric PO2, it is the ratio of oxygen consumption to alveolar ventilation (that is, O2 consumption/alveolar ventilation) that determines alveolar PO2—the higher the ratio, the lower the alveolar PO2. Similarly, alveolar PCO2 is determined by a ratio, in this case the ratio of carbon dioxide production to alveolar ventilation (that is, CO2 production/alveolar ventilation); the higher the ratio, the higher the alveolar PCO2.

Two terms that denote the adequacy of ventila-tion—that is, the relationship between metabolism and alveolar ventilation—can now be defined. Physiologists state these definitions in terms of carbon dioxide rather than oxygen. Hypoventilation exists when there is an increase in the ratio of carbon dioxide production to alveolar ventilation. In other words, a person is said to be hypoventilating if his alveolar ventilation cannot keep pace with his carbon dioxide production. The result is that alveolar PCO2 increases above the normal value of 40 mmHg. Hyperventilation exists when there is a decrease in the ratio of carbon dioxide production to alveolar ventilation—that is, when alveolar ventilation is actually too great for the amount of carbon dioxide being produced. The result is that alveolar PCO2 decreases below the normal value of 40 mmHg.

ra 50

Alveolar Ventilation And Pco2 Po2

FIGURE 15-20

Effects of increasing or decreasing alveolar ventilation on alveolar partial pressures in a person having a constant metabolic rate (cellular oxygen consumption and carbon dioxide production). Note that alveolar PO2 approaches zero when alveolar ventilation is about 1 L/min. At this point all the oxygen entering the alveoli crosses into the blood, leaving virtually no oxygen in the alveoli.

Alveolar ventilation (L/min)

Hypoventilation Hyperventilation

Alveolar ventilation (L/min)

Hypoventilation Hyperventilation

Vander et al.: Human I III. Coordinated Body I 15. Respiration I I © The McGraw-Hill

Physiology: The Functions Companies, 2001 Mechanism of Body Function, Eighth Edition

PART THREE Coordinated Body Functions

TABLE 15-6 Normal Gas Pressure

Venous

Arterial

Blood

Blood

Alveoli

Atmosphere

po2

40 mmHg

100 mmHg*

105 mmHg*

160 mmHg

Pco2

46 mmHg

40 mmHg

40 mmHg

0.3 mmHg

*The reason that the arterial PO2 and alveolar PO2 are not exactly the same is described later in the text.

*The reason that the arterial PO2 and alveolar PO2 are not exactly the same is described later in the text.

Note that "hyperventilation" is not synonymous with "increased ventilation." Hyperventilation represents increased ventilation relative to metabolism. Thus, for example, the increased ventilation that occurs during moderate exercise is not hyperventilation since, as we shall see, in this situation the increase in production of carbon dioxide is exactly proportional to the increased ventilation.

Alveolar-Blood Gas Exchange

The blood that enters the pulmonary capillaries is, of course, systemic venous blood pumped to the lungs via the pulmonary arteries. Having come from the tissues, it has a relatively high PCO2 (46 mmHg in a normal person at rest) and a relatively low PO2 (40 mmHg) (Figure 15-19 and Table 15-6). The differences in the partial pressures of oxygen and carbon dioxide on the two sides of the alveolar-capillary membrane result in the net diffusion of oxygen from alveoli to blood and of carbon dioxide from blood to alveoli. As this diffusion occurs, the capillary blood PO2 rises and its PCO2 falls. The net diffusion of these gases ceases when the capillary partial pressures become equal to those in the alveoli.

In a normal person, the rates at which oxygen and carbon dioxide diffuse are so rapid and the blood flow through the capillaries so slow that complete equilibrium is reached well before the end of the capillaries (Figure 15-21). Only during the most strenuous exercise, when blood flows through the lung capillaries very rapidly, is there insufficient time for complete equilibration.

Thus, the blood that leaves the pulmonary capillaries to return to the heart and be pumped into the systemic arteries has essentially the same PO2 and PCO2 as alveolar air. (They are not exactly the same, for reasons given later.) Accordingly, the factors described in the previous section—atmospheric PO2, cellular oxygen consumption and carbon dioxide production, and alveolar ventilation—determine the alveolar gas pressures, which then determine the systemic arterial gas pressures.

Given that diffusion between alveoli and pulmonary capillaries normally achieves complete equilibration, the more capillaries participating in this process, the more total oxygen and carbon dioxide can g

Po2 Length Capillary

20 40 60 80

% of capillary length

FIGURE 15-21

Equilibration of blood PO2 with alveolar PO2 along the length of the pulmonary capillaries.

20 40 60 80

% of capillary length

FIGURE 15-21

Equilibration of blood PO2 with alveolar PO2 along the length of the pulmonary capillaries.

be exchanged. Many of the pulmonary capillaries at the apex (top) of each lung are normally closed at rest. During exercise, these capillaries open and receive blood, thereby enhancing gas exchange. The mechanism by which this occurs is a simple physical one; the pulmonary circulation at rest is at such a low blood pressure that the pressure in these apical capillaries is inadequate to keep them open, but the increased cardiac output of exercise raises pulmonary vascular pressures, which opens these capillaries.

The diffusion of gases between alveoli and capillaries may be impaired in a number of ways, resulting in inadequate oxygen diffusion into the blood, particularly during exercise when the time for equilibration is reduced. For one thing, the surface area of the alveoli in contact with pulmonary capillaries may be decreased. In lung infections or pulmonary edema, for example, some of the alveoli may become filled with fluid. Diffusion may also be impaired if the alveolar walls become severely thickened with connective tissue, as, for example, in the disease (of unknown cause) called diffuse interstitial fibrosis. Pure diffusion problems of these types are restricted to oxygen and do not affect elimination of carbon dioxide, which is much more diffusible than oxygen.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Respiration CHAPTER FIFTEEN

Respiration CHAPTER FIFTEEN

Matching of Ventilation and Blood Flow in Alveoli

However, the major disease-induced cause of inadequate oxygen movement between alveoli and pulmonary capillary blood is not diffusion problems but the mismatching of the air supply and blood supply in individual alveoli.

The lungs are composed of approximately 300 million alveoli, each capable of receiving carbon dioxide from, and supplying oxygen to, the pulmonary capillary blood. To be most efficient, the right proportion of alveolar air flow (ventilation) and capillary blood flow (perfusion) should be available to each alveolus. Any mismatching is termed ventilation-perfusion inequality.

The major effect of ventilation-perfusion inequality is to lower the PO2 of systemic arterial blood. Indeed, largely because of gravitational effects on ventilation and perfusion there is enough ventilationperfusion inequality in normal people to lower the arterial PO2 about 5 mmHg. This is the major explanation of the fact, given earlier, that the PO2 of blood in the pulmonary veins and systemic arteries is normally 5 mmHg less than that of average alveolar air.

In disease states, regional changes in lung compliance, airway resistance, and vascular resistance can cause marked ventilation-perfusion inequalities. The extremes of this phenomenon are easy to visualize: (1) There may be ventilated alveoli with no blood supply at all, or (2) there may be blood flowing through areas of lung that have no ventilation (this is termed a shunt). But the inequality need not be all-or-none to be quite significant.

Carbon dioxide elimination is also impaired by ventilation-perfusion inequality but, for complex reasons, not nearly to the same degree as oxygen uptake. Nevertheless, severe ventilation-perfusion inequalities in disease states can lead to some elevation of arterial PCO2.

There are several local homeostatic responses within the lungs to minimize the mismatching of ventilation and blood flow. One of the most important operates on the blood vessels to alter blood-flow distribution. If an alveolus is receiving too little air relative to its blood supply, the PO2 in the alveolus and its surrounding area will be low. This decreased PO2 causes vasoconstriction of the small pulmonary blood vessels. (Note that this local effect of low oxygen on pulmonary blood vessels is precisely the opposite of that exerted on systemic arterioles.) The net adaptive effect is to supply less blood to poorly ventilated areas and more blood to well-ventilated areas.

Gas Exchange in the Tissues

As the systemic arterial blood enters capillaries throughout the body, it is separated from the interstitial fluid by only the thin capillary wall, which is highly permeable to both oxygen and carbon dioxide. The interstitial fluid in turn is separated from intracellular fluid by the plasma membranes of the cells, which are also quite permeable to oxygen and carbon dioxide. Metabolic reactions occurring within cells are constantly consuming oxygen and producing carbon dioxide. Therefore, as shown in Figure 15-19, intracellular PO2 is lower and PCO2 higher than in blood. The lowest PO2 of all—less than 5 mmHg—is in the mitochondria, the site of oxygen utilization. As a result, there is a net diffusion of oxygen from blood into cells (and, within the cells, into the mitochondria), and a net diffusion of carbon dioxide from cells into blood. In this manner, as blood flows through systemic capillaries, its PO2 decreases and its PCO2 increases. This accounts for the systemic venous blood values shown in Figure 15-19 and Table 15-6.

The mechanisms that enhance diffusion of oxygen and carbon dioxide between cells and blood when a tissue increases its metabolic activity were discussed in Chapter 14.

In summary, the supply of new oxygen to the alveoli and the consumption of oxygen in the cells create PO2 gradients that produce net diffusion of oxygen from alveoli to blood in the lungs and from blood to cells in the rest of the body. Conversely, the production of carbon dioxide by cells and its elimination from the alveoli via expiration create PCO2 gradients that produce net diffusion of carbon dioxide from cells to blood in the rest of the body and from blood to alveoli in the lungs

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  • quarto
    What gases enter and leave the capillaries as they flow through other tissues?
    8 years ago
  • Selina Gawkroger
    Can anxiety lower oxygen diffusion?
    8 years ago
  • HILDA SACKVILLE-BAGGINS
    How is it only oxygen leaves the alveoli?
    8 years ago
  • kyle
    Why does co2 enter the blood at the systemic tissues and depart at the alveoli?
    8 years ago
  • rufino romano
    What will be the po2 in the atmospheric air compared to those in alveolar air?
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  • Ayla
    What phenomenon allows gas exchange to occur between the alveoli and capillaries?
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  • elanor
    What enters alveoli through blood?
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    How to improve efficiency of oxygen to leave the alveol and enter the capillary?
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  • Markus
    What percentage of gases are inspired and enter the alveoli?
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  • scott
    Is po2 in blood leaving lungs equal to po2 in alveolar air?
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  • Angelika
    Why is co2 needed in ventilation to speed up oxygen uptake?
    8 years ago
  • mary
    What is the values for po2 and pco2 in atmospheric air?
    8 years ago
  • leah
    What are the effects on respiration when there is an increased alveolar pco2?
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  • sadoc lightfoot
    Why is alveolar po2 lower than atmospheric?
    8 years ago
  • gabriel
    What is the partial pressure of oxygen at sea level if ventilation rate is1000/ml/min?
    8 years ago
  • tapio
    What state is oxygen in tissues?
    8 years ago
  • venla
    What is the pco2 in mitochondria?
    8 years ago
  • sanelma
    How oxygen leaves the alveoli and enters the capillary?
    8 years ago
  • michelle
    Why is po2 of atmosphere higher than alveoli?
    8 years ago
  • hannu
    What gases are exchanged between the blood supply and the body cells?
    8 years ago
  • HAMISH
    Is oxygen or nitrogen net diffusion from body tissues to blood?
    8 years ago
  • JODI
    Why is alveolar PCO2 greater than atmospheric PCO2?
    7 years ago
  • Taneli
    Where is co2 during its movement from the tissues of the body to the lungs?
    7 years ago
  • Nairn Robertson
    Which of the gases in air is the most important to the normal functions of our body cells?
    7 years ago
  • Maisy
    What determines which direction carbon dioxide and oxygen diffuse in the lungs and the tissues?
    7 years ago
  • goldilocks
    Is PCO2 high in the alveoli and low in the capillaries?
    7 years ago
  • Celedor
    How does po2 gradients allow gas exchange in lungs and tissues?
    7 years ago
  • bellisima
    How gas is exchanged in the lungs and in the systemic tissues?
    7 years ago
  • VIRGINIA
    How pO2 and pCO2 gradients allow gas exchange in the lungs and tissues?
    7 years ago
  • MARTIN
    Where and how is carbon dioxide and oxygen exchanged in the lungs and systemic tissues?
    7 years ago
  • dominik
    What type of tissue are the pulmonary capillaries of the alveoli?
    7 years ago
  • priamus
    Why is there a decrease in alveolar pco2 during exercise?
    7 years ago
  • tori
    How is oxygen transported from alveoli to the tissues?
    7 years ago
  • arsenio
    Which process by which gases move between the alveoli and blood and blood and cells?
    7 years ago
  • Belinda
    What tissues do oxygen molecules diffuse in?
    7 years ago
  • gale counts
    Which is greater pO2 in blood leaving lungs or in alveolar air quizlet?
    7 years ago
  • AMBROSINO
    What determine the exchange of gases in the tissue near the alveoli?
    7 years ago
  • brian
    IS PCO2 in the interstitial fluid lower than in the tissue capillaries?
    7 years ago
  • prospero
    Are lungs and alveoli the same thing?
    7 years ago
  • mareta
    What cause oxygen to leave the alveoli and enter capillary?
    7 years ago
  • KENZIE
    When oxygen increase in alveoli?
    7 years ago
  • Fernanda Manna
    Does nitrogen diffusion from alveoli to blood?
    7 years ago
  • Furuta Awet
    What is the mechanism of gas exchange between alveoli and pulmonary capillaries?
    7 years ago
  • innes mackay
    What gases are exchanged across the alveoli?
    7 years ago
  • Estella
    Why the po2 that has left the alveolus and is same in the plasma?
    7 years ago
  • rhoda gamgee
    WHICH INSTANCE DOES PO2 DECREASE AVEOLI AIR AND ARTERIAL BLOOD AND BODY TISSUE?
    7 years ago
  • marigold
    What determines venous po2?
    6 years ago
  • Kristen
    How are gases exchanged in the body tissues?
    6 years ago
  • Kauko
    What gases enter and leave the capillaries as they flow through the lungs?
    6 years ago
  • Reilly Johnstone
    What determine in which direction carbon dioxide and oxygen will diffuse in the lungs and tissues?
    6 years ago
  • Nina
    What is the amount of po2 in the body tissues?
    6 years ago
  • brigitte
    How oxygen and carbondioxid exchange in alveoli and tissue?
    6 years ago
  • Phillipp
    Which tissues allow for diffusion of o2 and co2 in the alveoli?
    6 years ago
  • giacinta
    What determines alveolar ventilation?
    6 years ago
  • fnan
    HOW IS OXYGEN TRANSPORTED FROM ALVEOLI TO SYSTEMIC TISSUE?
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  • James
    How oxygen transport from alveoli to tissue?
    5 years ago
  • Kibra
    What determines the direction carbon dioxide and oxegen will diffuse in the lungs and tissue?
    5 years ago
  • bibiana
    How is oxygen transported to Alveolus?
    5 years ago
  • alix
    When does nitrogen leaves the body tissues?
    5 years ago
  • Jai McDonald
    What direction do the co2 molecules move if the pulmonary capillary is 50mmhg?
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  • JADEN
    Does gas exchange occur between the capillaries and the tissues until partial pressures are equal?
    5 years ago
  • GIRMA BISRAT
    How co2enter to alveoli and out of the alveoli?
    5 years ago
  • MELAMPUS
    Does nitrogen perfuse from alveoli to capillary at atmospheric pressure?
    5 years ago
  • Stig
    Are pO2 and pCO2 inversely proportional in the alveoli?
    4 years ago
  • meghan wright
    What is the role of the gas pressure of oxygen and carbon dioxide in the tissue and capillaries?
    4 years ago
  • Guido Took
    How is oxygsen transported from the alveoli to the body tissues?
    4 years ago
  • michael
    What determines the direction of carbin dioxide will diffuse in the lungs?
    4 years ago
  • Genoveffa
    What is the direction of diffusion of gases at the alveoli of the lungs?
    4 years ago
  • EDITTA
    Is oxygen and co2 move in same direction?
    4 years ago
  • ramsay
    What process do gases move from alveoli to blood and from blood to interstitial fluid?
    4 years ago
  • natalie gibson
    How is oxygen transported from the alveolus to the tissue?
    4 years ago
  • Nikola
    Which blood vessels allows for gas and nutrition exchangeto occur between the blood and the body?
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  • ronja
    Why is po2 of tissue fluid lower than blood?
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  • diamond
    How is o2 carried from the alveoli to the tissues?
    4 years ago
  • noora laitinen
    Why nitrogen is not used in exchange between alveoli?
    4 years ago
  • Amanuel
    Is nitrogen exchanged in the alveoli?
    4 years ago
  • bladud
    How is pressure suposed to be in the pulmonary capillaries for blood flow to occur in the alveolus?
    4 years ago
  • gaudenzia
    What is the phenomenon which involves the exchange of gas in the alveoli?
    4 years ago
  • kaarlo
    How does partial pressure effect body tissue?
    4 years ago
  • Tina
    Which direction does CO2 move in the capillaries of the lungs and those in the body tissues?
    3 years ago
  • oreste ferrari
    What causes oxygen to enter pulmonary capillaries from alveoli?
    3 years ago
  • Isabella
    What is exchange of gases is tissues?
    3 years ago
  • willie
    Why o2 with less % diffuse more efficiently in the alveoli than nitrogen?
    3 years ago
  • j
    How the exchange takes in artery oxygeningaseous state and carbondioxide in liqide state?
    3 years ago
  • nebyat
    How does exchange of gas occur between alveoli and tissue?
    3 years ago
  • Jessika
    What tissues separate the air of the alveoli and blood of pulmonary cappollaries?
    3 years ago
  • Bowman
    What tissue separate the air of the alveoli and blood of pulmonary capillaries?
    3 years ago
  • gherardo
    Why is alveolar Po2 is higher than venous Po2?
    3 years ago
  • leon
    How do gases moves between lungs ,blood and tissues?
    3 years ago
  • simone
    Which gradient allows oxygen to leave the blood and enter body tissue?
    2 years ago
  • Gabriel
    Which phenomenon helpful exchange of gases between lungs alveoli and capillaries?
    2 years ago
  • aapo jokihaara
    How to calculate the amount of oxygen entering the blood vessels per minute?
    2 years ago
  • archie
    Why nitrogen is not used during gaseous exchange?
    2 years ago
  • Lobelia
    What is the PO2 in the systemic arteries at rest?
    2 years ago
  • aamos
    Where is the PCO2 the highest: ( )a. alveoli ( )c. tissues?
    2 years ago
  • katja
    How is oxygen transported from alveolus to the tissues?
    2 years ago
  • HABTE HAILE
    How gaseous exchange occur cross the alveolar?
    2 years ago
  • AWET
    Why is co2 in the alveoli higher than the co2 of fresh air?
    2 years ago
  • giacinto
    Why alveoli are used for rapid gaseous exchange?
    2 years ago
  • CORNELIA
    What is the exchange of gases between the blood and tissue cells?
    2 years ago
  • NUNZIA
    Which two gases diffuse in the alveoli?
    1 year ago
  • BISIRAT
    Does exchange of gases takes place in tissues in the human body?
    1 year ago
  • Ky
    Why is po2 lower in alveoli than in atmosphere?
    10 months ago
  • ronni
    How are gases move from the alveoli to the tissues?
    9 months ago
  • Yvonne
    What is the lung tissue that moves gases?
    8 months ago
  • Gerardina
    How does pco2 compare in the atmosphere and in the lungs?
    8 months ago
  • Shona Thompson
    How is oxygen and carbondioxide exchanged in the tissues of the body?
    6 months ago
  • vihtori sundstedt
    Why is the PCO2 higher in the interstitial fluid surrounding cells?
    6 months ago
  • adonay kinfe
    What is gaseous exchange in the alveoli and tissues?
    6 months ago

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