Gas Exchange

Page 1.  Introduction

Oxygen and carbon dioxide diffuse between the alveoli and pulmonary capillaries in the lungs, and between the systemic capillaries and cells throughout the body.

The diffusion of these gases, moving in opposite directions, is called gas exchange.

Page 2.  Goals

To apply gas law relationships - between partial pressure, solubility, and concentration - to gas exchange.

To explore the factors which affect external and internal respiration.

Page 3.  Dalton's Law of Partial Pressures

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In order to understand gas exchange, we must first understand the air we breathe.  The atmosphere is a mixture of gases, including oxygen, carbon dioxide, nitrogen, and water.

The combined pressure of these gases equals atmospheric pressure.

At sea level, atmospheric pressure is 760 mm Hg, which means that the atmosphere pushes a column of mercury to a height of 760 millimeters.  Each gas within the atmosphere is responsible for part of that pressure in proportion to its percentage in the atmosphere.

Oxygen comprises 20.9% of the atmosphere.  The pressure exerted by oxygen is 20.9% of the total pressure of 760 millimeters of mercury, which equals 159 millimeters of mercury.  This value is known as the partial pressure of oxygen, and is written as "P" with the subscript "O2".

Notice that the partial pressures of the four gases add up to 760 millimeters of mercury, the total atmospheric pressure.  This demonstrates Dalton's Law of Partial Pressures, which states that in a mixture of gases, the total pressure equals the sum of the partial pressures exerted by each gas.  The partial pressure of each gas is directly proportional to its percentage in the total gas mixture.

Page 4.  Effect of High Altitude on Partial Pressures

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Atmospheric pressure decreases with increasing altitude. For example, on the top of Mt. Whitney, atmospheric pressure drops to approximately 440 millimeters of mercury.

Oxygen still makes up 20.9% of the atmosphere, but the PO2 is 20.9% of 440 millimeters of mercury, or about 92 millimeters of mercury. Compare that to the PO2 at sea level of 159 millimeters of mercury. Lower atmospheric pressure means fewer gas molecules, and therefore fewer oxygen molecules, are available.  That explains why you may gasp for breath at high altitudes.

As you can see, at high altitudes the partial pressures of all gases are lower than at sea level.

Page 5.  Henry's Law

Within the lungs, oxygen and carbon dioxide diffuse between the air in the alveoli and the blood, that is between a gas and a liquid.

This movement is governed by Henry's Law, which states that the amount of gas which dissolves in a liquid is proportional to:

1. the partial pressure of the gas

2. the solubility of the gas

In this container, the oxygen in the air is at equilibrium with the oxygen in the liquid.  At equilibrium, the pressure of the oxygen in the air is the same as in the liquid, with the gas molecules diffusing at the same rate in both directions. 

If you increase the pressure in the container more oxygen molecules dissolve in the liquid, moving from a region of high pressure to a region of low pressure.  Diffusion continues until a new equilibrium is reached.  This is what happens when oxygen moves from the alveoli into the blood.

Now let's look at the diffusion of carbon dioxide.  Although both gases are at the same pressure, far more carbon dioxide dissolves in the liquid than oxygen.  This occurs because carbon dioxide is much more soluble than oxygen. As stated in Henry's Law, the amount of oxygen and carbon dioxide which dissolves is proportional to the partial pressure and the solubility of each gas.

Page 6.  Sites of Gas Exchange

Sites of gas exchange in the body:  

External Respiration. 

Blood that is low in oxygen is pumped from the right side of the heart, through the pulmonary arteries to the lungs.

External respiration occurs within the lungs, as carbon dioxide diffuses from the pulmonary capillaries into the alveoli, and oxygen diffuses from the alveoli into the pulmonary capillaries.

Oxygen-rich blood leaves the lungs and is transported through the pulmonary veins to the left side of the heart.

Internal Respiration. 

From there it is pumped through the systemic circuit to tissues throughout the body.

Internal respiration occurs within tissues, as oxygen diffuses from the systemic capillaries into the cells, and carbon dioxide diffuses from the cells into the systemic capillaries.

Page 7.  Factors Influencing External Respiration

Efficient external respiration depends on three main factors:

1. The surface area and structure of the respiratory membrane. The 300 million alveoli, covered with a dense network of pulmonary capillaries, provide an enormous surface area for efficient gas exchange. In addition, the thinness of the respiratory membrane increases efficiency.

2. The partial pressure gradients between the alveoli and capillaries.

3. Efficient gas exchange requires matching alveolar airflow to pulmonary capillary blood flow.

 

Page 8.  External Respiration: Partial Pressures

Let's see how partial pressure gradients affect gas exchange between the alveoli and the pulmonary capillaries.

Notice that the partial pressures in the alveoli differ from those in the atmosphere. This difference is caused by a combination of several factors:

1. Humidification of inhaled air.  As it travels through the respiratory passageways to the alveoli, air is humidified, picking up water molecules. This greatly increases the partial pressure of water.

2. Gas exchange between the alveoli and pulmonary capillaries.  A continuous exchange of oxygen and carbon dioxide occurs between the alveoli and pulmonary capillaries, changing the partial pressures of both gases. Oxygen diffuses out of the alveoli into the pulmonary capillaries and carbon dioxide diffuses from the pulmonary capillaries into the alveoli.

3. Mixing of new and old air.  Since the alveoli do not completely empty between breaths, the air in the alveoli is a mixture of new air and air remaining from previous breaths.

Label this diagram:

 

Page 9.  External Respiration: Loading O2

Let's first look at the loading of oxygen into the blood.  Each alveolus is surrounded by a network of capillaries.  This diagram shows just one alveolus and one capillary.

The PO2 of the alveolar air is 104 mm Hg.  At rest, the oxygen-poor blood entering the pulmonary capillaries has a PO2 of 40 mm Hg.

As blood flows past the alveolus, the PO2 increases.

Notice that there is a net diffusion of oxygen along its partial pressure gradient, from the alveolus into the blood, until equilibrium is reached.  The PO2 of the oxygen-rich blood has increased to 104 mm Hg.

As indicated in the graph, equilibrium is reached rapidly, within the first third of the pulmonary capillary.

Label this diagram:

Page 10.  External Respiration: Unloading CO2

Now let's look at the unloading of carbon dioxide from the blood into the alveolus.

The PCO2 of the alveolar air is 40 millimeters of mercury. At rest, the PCO2 of the blood entering the pulmonary capillaries is 45 millimeters of mercury.

As blood flows past the alveolus, the PCO2 decreases.  Carbon dioxide diffuses along its partial pressure gradient, from the blood into the alveolus, until equilibrium is reached.  The PCO2 of the blood has decreases to 40 millimeters of mercury.

Equilibrium is reached rapidly, within the first four tenths of the pulmonary capillary.

Label this diagram:

Page 11. External Respiration O2 and CO2 Exchange

Loading oxygen and unloading carbon dioxide occur simultaneously.  As you inhale, you replenish oxygen, and as you exhale, you eliminate carbon dioxide.

Notice how much smaller carbon dioxide's partial pressure gradient is than oxygen's.  As Henry's law states, the number of molecules which dissolve in a liquid is proportional to both the partial pressure and the gas solubility.  Since carbon dioxide is very soluble in blood, a large number of molecules diffuse along this small partial pressure gradient.  Oxygen, which is less soluble, requires a much larger concentration gradient to provide adequate oxygen to the body.

 Page 12.  Ventilation-Perfusion Coupling: Effect of PO2

The third factor in external respiration is ventilation-perfusion coupling, which facilitates efficient gas exchange.  It does this by maintaining alveolar airflow that is proportional to the pulmonary capillary blood flow.

When airflow through a bronchiole is restricted, as when blocked by mucus, the resulting low PO2 causes the local arterioles to vasoconstrict.  This response redirects the blood to other alveoli which have a higher airflow, and therefore have more oxygen available to be picked up by the blood.

In regions with high airflow compared to their blood supply, the resulting high PO2 causes the local arterioles to vasodilate.  This brings more blood to the alveoli, allowing the blood to pick up the abundant oxygen.

Page 13.  Ventilation-Perfusion Coupling: Effect of PCO2

We've seen that during ventilation-perfusion coupling, the arterioles respond to changes in PO2.  The bronchioles, on the other hand, respond to changes in PCO2.

When airflow through a bronchiole is lower than normal, the PCO2 rises.  The bronchioles respond by dilating, thereby eliminating the excess carbon dioxide from the alveoli. 

When airflow through a bronchiole is high compared to its blood supply, the PCO2 drops.  The bronchioles then constrict, reducing the airflow so it is proportional to the local blood flow.

Page 14.  Predict the Effect of PO2 and PCO2

Assume that ventilation to an alveolar sac is low, due to a small tumor growing in the bronchiole. The PO2 decreases because oxygen is not replenished, and the PCO2 increases, because the carbon dioxide is not eliminated. See if you can predict the response of the arterioles and bronchioles.

The low PO2 causes the arterioles to constrict, and the high PCO2 causes the bronchioles to dilate. The airflow and blood flow are now in the proper proportions for optimum gas exchange. Notice that both the arterioles and bronchioles respond simultaneously.

Page 15.  Internal Respiration

During internal respiration:

Oxygen diffuses from the systemic capillaries into the cells.

Carbon dioxide diffuses from the cells into the systemic capillaries.

Factors affecting the exchange of oxygen and carbon dioxide during internal respiration:

1. The available surface area, which varies in different tissues throughout the body.

2. Gases diffuse along their partial pressure gradients.

3. The rate of blood flow in a specific tissue.

Blood flow in a tissue varies for many reasons, including the tissue's metabolic rate. Recall that during metabolism, cells use oxygen and produce carbon dioxide.

  Page 16.  Internal Respiration O2 and CO2 Exchange

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The exchange of oxygen and carbon dioxide during internal respiration:

In relatively inactive organs, the tissue cells have a PO2 of 40 millimeters of mercury, and a PCO2 of 45 millimeters of mercury.

As the blood enters the systemic capillaries, it has a PO2 of 100 millimeters of mercury, and a PCO2 of 40 millimeters of mercury.

Notice that the PO2 of blood entering the systemic capillaries is lower than the alveolar PO2 of 104 millimeters of mercury.  This small decrease is due primarily to imperfect ventilation-perfusion coupling in the lungs.

Gas exchange continues until equilibrium is reached.  At equilibrium, the blood in the systemic capillaries has a PO2 of 40 millimeters of mercury, and a PCO2 of 45 millimeters of mercury.

The oxygen-poor blood now returns, through the systemic veins, to the right side of the heart.

 

 

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