Principles of Gas Exchange in Lungs
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Notes on Pulmonology
Principles of Gas Exchange in Lungs
What is Dalton’s Law? What is its effect on atmospheric air?
Dalton’s Law states that the total pressure exerted by a mixture of gases equals the sum total of the individual partial pressures of all the gases. Partial pressures of gases in atmospheric air govern the movement of O2 and CO2 between air and lungs.
What are the gases that make up the air we breathe? What is their proportion? What are the partial pressures of the gases in the air?
Atmospheric air is also a mixture of various gases and total pressure air pressure is: Partial pressures of nitrogen, oxygen, water vapor, carbon dioxide, and other gases.
N: 597.4 mm Hg
O2: 158.8 mm Hg
H20: 3.0 mm Hg
CO2: 0.3 mm Hg
Other: 0.5 mm Hg
What are the partial pressures of these gases in the alveoli? How, and why, do these partial pressures differ from their values in air?
The partial pressures of these gases are lower in the alveoli compared to their partial pressures in the atmosphere. This is to let air diffuse from the atmosphere into the alveoli. For example, PO2 (air) = 159 mm Hg, and PO2 (alveoli) = 105 mm Hg.
What causes oxygen to enter pulmonary capillaries from the alveoli?
This too depends on pressure. The partial pressure of oxygen in pulmonary capillaries is lower than that of oxygen in the alveoli. This drives the gas from a higher partial pressure to a lower partial pressure.
What causes oxygen to enter tissue cells from systemic capillaries?
The partial pressure of oxygen in tissue cells is lower than that of oxygen in systemic capillaries. This drives the gas from a higher partial pressure to a lower partial pressure.
List the factors that affect the movement of gases across a sheet of tissue, i.e., a membrane.
Partial pressures of gases involved
Blood pressure
Resistance to the flow by: vessel diameter, and blood viscosity
Having answered Question 5, explain how oxygen will diffuse across the alveolar capillary membrane, how carbon dioxide will diffuse across the alveolar capillary membrane, and how the diffusion of the two may differ.
Diffusion of O2 and CO2 across the alveolar capillary membrane depends on a series of chemical reactions. First, the partial pressure of oxygen is lower (40 mm Hg) in the alveolar capillary membrane compared to that in the air (105 mm Hg), so O2 diffuses from the higher partial pressure to the lower partial pressure. On the other hand, the partial pressure of CO2 is lower in the alveolar capillary membrane (40 mm Hg) compared to the partial pressure of CO2 in pulmonary arteries (45 mm Hg), so CO2 flows in the other direction.
A medical student forces herself to breathe at a rate of 40 breaths per minute, and begins to feel lightheaded. She believes her light-headedness is caused by all the extra oxygen she has accumulated from breathing fast. Her friend disagrees, and believes it has to do with carbon dioxide concentration. Explain what is happening.
The medical student is hyperventilating herself. Hyperventilation is followed by:
Temporary apnea (stop to breathing)
Alkalemia (blood pH above normal)
PCO2 (in blood) is below normal, meaning, more CO2 is floating in tissue cells, causing the light headedness.
Explain how oxygen is normally transported in blood.
Four oxygen molecules are carried on a hemoglobin molecule.
Explain how carbon dioxide is normally transported in blood.
Some CO2 is dissolved in blood, some reacts with hemoglobin to form carbamino hemoglobin. The rest is converted to bicarbonate and hydrogen ions. Most CO2 is transported in the blood as bicarbonate ions.
What is high altitude sickness? Explain its cause.
High altitude sickness is the leakage of fluid from capillaries then building up in lungs and the brain. The body has to adjust to breathing in less oxygen molecules per breath. This, in addition to higher altitude and lower air pressure causes high altitude sickness. The body goes into hypoxic hypoxia.
Pulmonary and systemic circulations differ in terms of resistance to flow, as was covered in the learning issue on the difference between the two circulations. How do the circulations differ in regard to their response to hypoxia? Explain the physiology of this difference.
Pulmonary vessels constrict in response to hypoxia whereas systemic vessels dilate. In lungs this mechanism will help to direct blood away from poorly oxygenated alveoli to better oxygenated portions of the lungs and facilitate oxy. of the blood thus improving the match bet. ventilation and perfusion. In the tissues, dilation of blood vessels in response to hypoxia will help deliver oxygenated blood to tissues in need of it.
Additional Reading:
Basic Pulmonology
1. Lung Mechanics
2. Alveolar-Blood Gas Exchange
3. Gas Transport and Regulation of Respiration
4. Four Causes of Hypoxemia
5. Control of Respiration
6. Systemic vs Pulmonary Circulation FAQ
7. Principles of Gas Exchange in Lungs
8. Hypoxia, Hypoxemia & Hemoglobin-Oxygen Saturation Curve
9. FAQ on Mechanics of Breathing
10. FAQ on Control of Breathing
11. Criteria for Transudate Pleural Effusion
12. Light's Criteria for Exudate Pleural Effusion
13. Notes on Lung Sounds
14. Patient with Acute Respiratory Distress Syndrome (ARDS)
15. Management of Acute Deep Venous Thrombosis
16. Notes on Asthma Treatment
Pulmonology Videos
1. Video of Pulmonology Examination in a Clinical Setting
Related Topics
1. Histology of the Respiratory System
2. Upper and Lower Respiratory Disorders
3. Pulmonary Examination for Internal Medicine
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