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Gas Exchange: Understanding Partial Pressures and Henry's Law, Study notes of Physiology

The concept of gas exchange in the human body, focusing on the application of gas laws, specifically Dalton's Law of Partial Pressures and Henry's Law. the effects of altitude on partial pressures, the role of Henry's Law in gas dissolution, and the sites and mechanisms of gas exchange. Students will gain insights into the importance of partial pressure gradients, efficient gas exchange, and the factors influencing the exchange of oxygen and carbon dioxide during both external and internal respiration.

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Gas Exchange
Graphics are used with permission of:
Pearson Education Inc., publishing as Benjamin Cummings (http://www.aw-bc.com)
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
• Fill out this chart as you work through this page:
• 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.
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Gas Exchange

Graphics are used with permission of: Pearson Education Inc., publishing as Benjamin Cummings (http://www.aw-bc.com)

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

  • Fill out this chart as you work through this page:
  • 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

  • Fill out this chart as you work through this page:
  • 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.

** Now is a good time to go to quiz questions 1-3:

  • Click the Quiz button on the left side of the screen..
  • Work through questions 1-3.
  • After answering question 3, click the Back to Topic button on the left side of the screen.
  • To get back to where you left off, click on the scrolling page list at the top of the screen and choose "6. Sites of Gas Exchange".

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.
  • 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 CO

  • 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 PO

  • 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 PO 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 PCO

  • 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.
  • 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.

Page 17. Summary

  • Gas laws show the relationship between partial pressure, solubility, and concentration of gases.
  • Gases diffuse along their partial pressure gradients, from regions of high partial pressure to regions of low partial pressure.
  • During external respiration, oxygen loads from alveoli into pulmonary capillaries and carbon dioxide unloads from pulmonary capillaries into alveoli.
  • During internal respiration, oxygen unloads from systemic capillaries into cells and carbon dioxide loads from cells into systemic capillaries.
  • Efficient gas exchange depends on several factors including surface area, partial pressure gradients, blood flow and airflow.
  • During external respiration, ventilation-perfusion coupling maintains airflow and blood flow in proper proportions for efficient gas exchange.

** Now is a good time to go to quiz questions 4-8:

  • Click the Quiz button on the left side of the screen.
  • Click on the scrolling page list at the top of the screen and choose "4. External Respiration".
  • Work through quiz questions 4-8.

Notes on Quiz Questions: Quiz Question #1a:

  • This question asks you to calculate the partial pressure of oxygen gas in an atmosphere in a fictitious situation.

Quiz Question #1b:

  • This question asks you to determine the relative amount of carbon dioxide and oxygen dissolved in the blood in a fictitious situation.

Quiz Question #2a:

  • This question asks you to determine gas solubility differences.

Quiz Question #2b:

  • This question asks you to find ways to increase the solubility of nitrogen in the blood.

Quiz Question #3:

  • This question asks you to predict what happens to the concentration of nitrogen gas in the blood of divers who are under pressure.

Quiz Question #4:

  • This question asks you to recall the partial pressures of carbon dioxide and oxygen gas during external respiration.

Quiz Question #5:

  • This question asks you to recall the partial pressures of carbon dioxide and oxygen gas during internal respiration.

Quiz Question #6:

  • This question asks you to label a graph of partial pressures vs. time during external respiration.

Quiz Question #7:

  • This question asks you to identify the factors that could increase the partial pressure of oxygen in the pulmonary capillaries.

Quiz Question #8:

  • This question asks you to predict what happens during an asthma attack.

Study Questions on Gas Exchange:

  1. (Page 1.) What four gases are found in the atmosphere?
  2. (Page 1.) Each of these gases exerts a pressure, what is the total pressure of all the gases in the atmosphere called?
  3. (Page 1.) What is a typical atmospheric pressure at sea level in millimeters of Hg?
  4. (Page 1.) What is Dalton's Law of Partial Pressures?
  5. (Page 2.) As altitude increases, what happens to the atmospheric pressure?
  6. (Page 2.) Oxygen gas makes up 20.9% of the atmosphere at sea level where the atmospheric pressure is 760 mm Hg. a. What percentage of oxygen gas is there at a high altitude, where the atmospheric pressure is 440 mm Hg? b. Explain what happens to the partial pressure of oxygen gas at the high altitude.
  7. (Page 3.) Henry's Law states that the amount of gas which dissolves in a liquid is proportional to what two factors?
  8. (Page 4.) In a container containing water and oxygen gas, some of the oxygen dissolves in the water. When equilibrium is reached, the pressure of the oxygen gas above the water ____________ the pressure of oxygen in the liquid. a. is greater than b. is less than c. is equal to
  9. (Page 4.) In a container containing water and oxygen gas, some of the oxygen dissolves in the water. When equilibrium is reached, the rate of oxygen gas diffusing into the water ____________ the rate of oxygen gas diffusing out of the water. a. is greater than b. is less than c. is equal to
  1. (Page 10.) 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 _____________. a. increases b. decreases

  1. (Page 10.) 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. During external respiration carbon dioxide diffuses along its partial pressure gradient, from the blood into the alveolus, until equilibrium is reached. As blood flows past the alveolus, the PCO2 _____________. a. increases to 45 mm Hg b. decreases to 40 mm Hg

  1. (Page 10.) During external respiration, carbon dioxide equilibrium is reached _______________ of the pulmonary capillary. a. at the end b. within the last half c. within the first four-tenths
  2. (Page 10.) Fill out this graph to show what happens to the partial pressure of carbon dioxide in the pulmonary arteries during external respiration:
  3. (Page 11.) Why does carbon dioxide have a smaller partial pressure gradient than oxygen?
  4. (Page 12.) Explain how ventilation-perfusion coupling facilitates efficient gas exchange.
  5. (Page 12.) What factor causes vasoconstriction and vasodilation associated with ventilation-perfusion coupling?
  6. (Page 13.) How do bronchioles respond to levels of blood gases?
  7. (Page 13.) What would cause the PCO2 in the bronchioles to rise?
  8. (Page 13.) During ventilation-perfusion coupling, the arterioles respond to changes in _________ and the bronchioles respond to changes in _____________. a. PO2 b. PCO
  9. (Page 14.) Match the following: a. Arterioles constrict b. Arterioles dilate c. Bronchioles constrict d. Bronchioles dilate

Low PCO Low PO High PCO High PO

  1. (Page 15.) On this diagram, indicate where both internal and external respiration occurs.
  2. (Page 15.) On this diagram, indicate where there would be a net movement of oxygen into the blood and carbon dioxide out of the blood.
  3. (Page 15.) On this diagram, indicate where there would be a net movement of oxygen out of the blood and carbon dioxide into the blood.
  4. (Page 15.) What three factors affect the exchange of oxygen and carbon dioxide during internal respiration?
  5. (Page 16.) Why would the rate of blood flow vary within a tissue?
  6. (Page 16.) As gases are exchanged between the tissues and systemic capillaries, what happens to the partial pressures of both gases?
  7. (Page 16.) The PO2 of the blood entering the

systemic capillaries is 100 mm Hg. As blood flows through the systemic capillaries, the PO2 ___________. a. increases b. decreases

  1. (Page 16.) The PCO2 of the blood entering the

systemic capillaries is 40 mm Hg. As blood flows through the systemic capillaries, the PCO2 ___________. a. increases b. decreases

  1. (Page 16.) The PO2 of the blood entering the systemic capillaries is 100 mm Hg. During internal

respiration there is a net diffusion of oxygen along its partial pressure gradient, from the blood into the tissues, until equilibrium is reached. As this occurs, the PO2 of the blood ______________. a. increases to 104 mm Hg b. decreases to about 40 mm Hg

  1. (Page 16.) The PCO2 of the blood entering the systemic capillaries is about 40 millimeters of mercury.

At rest, the PCO2 of the blood leaving the systemic capillaries is about 45 millimeters of mercury. As blood flow through the systemic capillaries, the PCO2 _____________. a. increases b. decreases

  1. (Page 16.) During internal respiration carbon dioxide diffuses along its partial pressure gradient until equilibrium is reached. As blood flows through the systemic capillaries, the PCO2 _____________. a. increases to about 45 mm Hg b. decreases to about 40 mm Hg