Physical Principles of Respiratory Care

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Physical Principles of Respiratory Care
Egan Chapter 6

Physical Principles of Respiratory Care
States of Matter Change of State Gas Behavior Under Changing Conditions Fluid Dynamics

III. Gas Behavior Under Changing Conditions
Gas Laws Effect of Water Vapor Corrected Pressure Computations Correction Factors Properties of Gases at Extremes of Temperature and Pressure Critical Temperature and Pressure

A. Gas Laws Boyle’s Law Charles’ Law Gay-Lussac’s Law

Pressure-volume relationship
Boyle’s Law Robert Boyle British chemist Pressure-volume relationship

Boyle’s Law If temperature (T) remains constant, pressure (P) will vary inversely to volume (V) P1V1 = P2V2

Boyle’s Law If temperature (T) remains constant, pressure (P) will vary inversely to volume (V) If gas P , the V occupied by the gas  If gas V , the P exerted by the gas 

Boyle’s Law P T V

Ventilation is an example of Boyle’s Law
As the diaphragm drops at the beginning of inspiration The volume of the lungs increases As lung volume increases what happens to the pressure inside the lungs? It _________ And air flows into the lungs v=q6-oyxnkZC0

Boyle’s Law A snorkel diver is preparing to descend a freshwater pond to a depth of 66 feet. At sea level, his lungs contain about 3000 mL of air. As he descends below the surface of the water, what will happen to the gas volume in his lungs as he descends to 66 feet below the surface of the pond? P1V1 = P2V2 P1V1 = V2 P2 33.9 ftH2O x 3000 mL = V2 99.9 ftH2O mL = V2 = 1018 mL 99.9

Volume- temperature relationship
Charles’ Law Jacques Charles French chemist Volume- temperature relationship

Charles’ Law If pressure (P) remains constant, the volume (V) will vary directly with temperature (T) V1 = V2 T1 T2

Charles’ Law If gas T , the V occupied by the gas also 
If pressure (P) remains constant, the volume (V) will vary directly with temperature (T) If gas T , the V occupied by the gas also  As T , kinetic energy , the gas molecules move more vigorously and the gas expands If gas T , the V occupied by the gas also  As T , kinetic energy , the gas molecules slow down and the gas contracts

Tire volume is an example of Charles’ Law

Gay-Lussac’s Law An oxygen cylinder has a pressure of 2200 psi at a temperature of 25 C, what will its pressure be if it is heated to 50 C? (Must first convert temperature to Kelvin) P1 = P2 T1 T2 P1 T2 = P2 T1 2200 psi x 323 K = P2 298 K psi = P2 298 2384 psi

Expanded on Charles’ work Pressure-temperature relationship
Gay-Lussac’s Law Joseph Gay-Lussac Expanded on Charles’ work Pressure-temperature relationship

Gay-Lussac’s Law If volume (V) remains constant, pressure (P) will vary directly with temperature (T) P1 = P2 T1 T2

Gay-Lussac’s Law If volume (V) remains constant, pressure (P) will vary directly with temperature (T) If gas T , the P exerted by the gas also  If gas T , the P exerted by the gas also 

Gas Cylinder Pressures and Temp is an example of Gay-Lussac’s Law
An alarm signals that there is a fire in the basement of the hospital. Although the fire is confined to an area that is approximately 300 feet from the room where the compressed- gas cylinders are stored, you are asked to move the cylinders to a safer location. Why is it necessary to move the cylinders? Keep cylinder temperature below 125°F (51.7°C) j0

Charles’ Law Given: 100 mL of an unknown gas measured at 28° C and 760 mmHg. Find the gas volume measured at 37° C and 760 mmHg.(Must first convert temperature to Kelvin) V1 = V2 T1 T2 V1T2 = V2 T1 100 mL x 310 K = V2 301 K 31000 mL = V2 301 = 103 mL

III. Gas Behavior Under Changing Conditions
Gas Laws Effect of Water Vapor In clinical practice, most gas law calculations must take into account the presence of water vapor. Water vapor, similar to any gas, occupies space. The dry volume of a gas at a constant pressure and temperature is always smaller than its saturated volume. The opposite is also true. Correcting from the dry state to the saturated state always yields a larger gas volume. The addition of water vapor to a gas mixture always lowers the partial pressures of the other gases present. This fact becomes relevant when discussing the partial pressure of gases in the lung where the gases are saturated with water vapor at body temperature.

III. Gas Behavior Under Changing Conditions
Gas Laws Effect of Water Vapor The addition of water vapor to a gas mixture always lowers the partial pressures of the other gases present. This fact becomes relevant when discussing the partial pressure of gases in the lung where the gases are saturated with water vapor at body temperature.

III. Gas Behavior Under Changing Conditions
Gas Laws Effect of Water Vapor Corrected Pressure Computations PC = (PT – PH20)

III. Gas Behavior Under Changing Conditions
Gas Laws Effect of Water Vapor Correction Factors Correction from ATPS to BTPS Correction from ATPS to STPD Correction from STPD to BTPS

Table 6-3

Clinical Example: Spirometry
Spirometry: laboratory evaluation of lung function using a spirometer Spirometer: a device for measuring lung volumes and/or flows When a patient exhales into a spirometer the gas is measured at room temperature About 25°C ATPS: Ambient Temperature Pressure Saturated But inside the lungs the gas is at body temperature About 37°C BTPS: Body Temperature Pressure Saturated

Clinical Example: Spirometry
As the patient exhales and the temperature of the gas decreases to room temperature What will happen to the volume of gas? Whose law tells us this? It will DECREASE

III. Gas Behavior Under Changing Conditions
Gas Laws Properties of Gases at Extremes of Temperature and Pressure See Mini Clini: Variations from Ideal Gas Behavior: Expansion Cooling and Adiabatic Compression page 121.

III. Gas Behavior Under Changing Conditions
Gas Laws Critical Temperature and Pressure Critical temperature: the highest temperature at which a substance can exist as a liquid. The temperature above which the kinetic activity of its molecules is so great that the attractive forces cannot keep them in a liquid state. Critical pressure: the pressure needed to maintain equilibrium between the liquid and gas phases of a substance at this critical temperature. Together, the critical temperature and pressure represent the critical point of a substance.

2. Critical Temperature and Pressure
Liquid oxygen is produced by separating it from a liquefied air mixture at a temperature below its boiling point (−183° C or −297° F). After it is separated from air, the oxygen must be maintained as a liquid by being stored in insulated containers below its boiling point (-183 ° C) If higher temperatures are needed, higher pressures must be used. If at any time the liquid oxygen exceeds its critical temperature of −118.8° C, it converts immediately to a gas.